mendel's experiments and heredity

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21 Mendel’s Experiments

By the end of this section, you will be able to:

• Explain the scientific reasons for the success of Mendel’s experimental work
• Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles

Johann Gregor Mendel (1822–1884) (Figure 1) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics that is used to study a specific biological phenomenon to gain understanding to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local natural history society. He demonstrated that traits are transmitted faithfully from parents to offspring in specific patterns. In 1866, he published his work, Experiments in Plant Hybridization, 1 in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community, which incorrectly believed that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring. This hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation is the range of small differences we see among individuals in a characteristic like human height. It does appear that offspring are a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. Mendel worked instead with traits that show discontinuous variation . Discontinuous variation is the variation seen among individuals when each individual shows one of two—or a very few—easily distinguishable traits, such as violet or white flowers. Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring as would have been expected at the time, but that they were inherited as distinct traits. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime; in fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel’s Crosses

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum , to study inheritance. This species naturally self-fertilizes, meaning that pollen encounters ova within the same flower. The flower petals remain sealed tightly until pollination is completed to prevent the pollination of other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true-breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendel performed hybridizations , which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety.

Plants used in first-generation crosses were called P , or parental generation, plants (Figure 2). Mendel collected the seeds produced by the P plants that resulted from each cross and grew them the following season. These offspring were called the F 1 , or the first filial (filial = daughter or son), generation. Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendel’s experiments extended beyond the F 2 generation to the F 3 generation, F 4 generation, and so on, but it was the ratio of characteristics in the P, F 1 , and F 2 generations that were the most intriguing and became the basis of Mendel’s postulates.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea-pod size, pea-pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants and reported results from thousands of F 2 plants.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he was using plants that bred true for white or violet flower color. Irrespective of the number of generations that Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical. This was an important check to make sure that the two varieties of pea plants only differed with respect to one trait, flower color.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait had completely disappeared in the F 1 generation.

Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that 705 plants in the F 2 generation had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers to one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained approximately the same ratio irrespective of which parent—male or female—contributed which trait. This is called a reciprocal cross —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics that Mendel examined, the F 1 and F 2 generations behaved in the same way that they behaved for flower color. One of the two traits would disappear completely from the F 1 generation, only to reappear in the F 2 generation at a ratio of roughly 3:1 (Figure 3).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these dominant and recessive traits, respectively. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-colored flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate (and were not blended) in the plants of the F 1 generation. Mendel proposed that this was because the plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of their two copies to their offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic, or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

CONCEPTS IN ACTION

For an excellent review of Mendel’s experiments and to perform your own crosses and identify patterns of inheritance, visit the Mendel’s Peas web lab .

Also, check out the following video as review

• Johann Gregor Mendel, “Versuche über Pflanzenhybriden.” Verhandlungen des naturforschenden Vereines in Brünn , Bd. IV für das Jahr, 1865 Abhandlungen (1866):3–47. [for English translation, see http://www.mendelweb.org/Mendel.plain.html]

Introductory Biology: Evolutionary and Ecological Perspectives Copyright © by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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NOTIFICATIONS

Mendel’s experiments.

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Mendel is known as the father of genetics because of his ground-breaking work on inheritance in pea plants 150 years ago.

Gregor Johann Mendel was a monk and teacher with interests in astronomy and plant breeding. He was born in 1822, and at 21, he joined a monastery in Brünn (now in the Czech Republic). The monastery had a botanical garden and library and was a centre for science, religion and culture . In 1856, Mendel began a series of experiments at the monastery to find out how traits are passed from generation to generation. At the time, it was thought that parents’ traits were blended together in their progeny .

Studying traits in peas

Mendel studied inheritance in peas ( Pisum sativum ). He chose peas because they had been used for similar studies, are easy to grow and can be sown each year. Pea flowers contain both male and female parts, called stamen and stigma , and usually self-pollinate. Self-pollination happens before the flowers open, so progeny are produced from a single plant.

Peas can also be cross-pollinated by hand, simply by opening the flower buds to remove their pollen-producing stamen (and prevent self-pollination) and dusting pollen from one plant onto the stigma of another.

Traits in pea plants

Mendel followed the inheritance of 7 traits in pea plants, and each trait had 2 forms. He identified pure-breeding pea plants that consistently showed 1 form of a trait after generations of self-pollination.

Mendel then crossed these pure-breeding lines of plants and recorded the traits of the hybrid progeny. He found that all of the first-generation (F1) hybrids looked like 1 of the parent plants. For example, all the progeny of a purple and white flower cross were purple (not pink, as blending would have predicted). However, when he allowed the hybrid plants to self-pollinate, the hidden traits would reappear in the second-generation (F2) hybrid plants.

Dominant and recessive traits

Mendel described each of the trait variants as dominant or recessive Dominant traits, like purple flower colour, appeared in the F1 hybrids, whereas recessive traits, like white flower colour, did not.

Mendel did thousands of cross-breeding experiments. His key finding was that there were 3 times as many dominant as recessive traits in F2 pea plants (3:1 ratio).

Traits are inherited independently

Mendel also experimented to see what would happen if plants with 2 or more pure-bred traits were cross-bred. He found that each trait was inherited independently of the other and produced its own 3:1 ratio. This is the principle of independent assortment.

Find out more about Mendel’s principles of inheritance .

The next generations

Mendel didn’t stop there – he continued to allow the peas to self-pollinate over several years whilst meticulously recording the characteristics of the progeny. He may have grown as many as 30,000 pea plants over 7 years.

Mendel’s findings were ignored

In 1866, Mendel published the paper Experiments in plant hybridisation ( Versuche über plflanzenhybriden ). In it, he proposed that heredity is the result of each parent passing along 1 factor for every trait. If the factor is dominant , it will be expressed in the progeny. If the factor is recessive, it will not show up but will continue to be passed along to the next generation. Each factor works independently from the others, and they do not blend.

The science community ignored the paper, possibly because it was ahead of the ideas of heredity and variation accepted at the time. In the early 1900s, 3 plant biologists finally acknowledged Mendel’s work. Unfortunately, Mendel was not around to receive the recognition as he had died in 1884.

Useful links

Download a translated version of Mendel’s 1866 paper Experiments in plant hybridisation from Electronic Scholarly Publishing.

This apple cross-pollination video shows scientists at Plant & Food Research cross-pollinating apple plants.

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Gregor Mendel

By Sam Wong

FLHC 52 / Alamy

Gregor Mendel discovered the basic principles of heredity through experiments with pea plants, long before the discovery of DNA and genes. Mendel was an Augustinian monk at St Thomas’s Abbey near Brünn (now Brno, in the Czech Republic). He studied natural sciences and mathematics at the University of Vienna, Austria, but twice failed to obtain a teaching certificate, instead becoming a part-time assistant teacher and carrying out research in plant breeding.

His most famous experiments were done between 1857 and 1864, during which time he grew some 10,000 pea plants. Pea plants are hermaphroditic, meaning they have both male and female sex cells and usually fertilise themselves. Mendel was able to cross-breed the plants by transferring pollen with a paintbrush. He meticulously recorded a range of characteristics for each plant, including its height, pod shape, pea shape and pea colour. When plants self-fertilised, these characteristics remained consistent in the offspring.

At the time, it was widely believed that heredity worked by blending the characteristics of parents, producing offspring that were in some way diluted. Mendel showed that when two varieties of purebred plants cross-breed, the offspring resembled one or other of the parents, not a blend of the two. He found that some traits are dominant and would always be expressed in a first generation cross, while others are recessive and would not appear in this generation. However, these recessive traits re-appear in the next generation if these first-generation plants self-fertilise.

Mendel hypothesised that parents contribute some particulate substance to the offspring which determine its heritable characteristics. We now know that these particles correspond to genes made of DNA. Without any knowledge of the molecules involved, Mendel was able to infer that heritable particles are separated into gametes – eggs and sperm – and that offspring inherit one particle from each parent.

Mendel was far ahead of his time, and his work was largely ignored for the next 35 years. In 1868 he was appointed as an abbot and, overwhelmed with administrative duties, had little time left to continue his research. Late in his career, he wrote: “My scientific work brought me such satisfaction, and I am convinced the entire world will recognise the results of these studies.” He died in 1884, aged 62.

In 1900, three scientists independently confirmed his work, but it was another 30 years before his conclusions were widely accepted. Then evolutionary biologists such as Ronald Fisher realised that Mendel’s laws of inheritance could explain how natural selection could make beneficial traits become more prevalent and eliminate negative ones. His work formed part of “the modern synthesis”, a reformulation of Darwin’s ideas based on the new understanding of genetics .

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Mendel’s 3 Laws (Segregation, Independent Assortment, Dominance)

• In the 1860s, an Austrian monk named Gregor Mendel introduced a new theory of inheritance based on his experimental work with pea plants.
• Mendel believed that heredity is the result of discrete units of inheritance, and every single unit (or gene) was independent in its actions in an individual’s genome.
• According to this Mendelian concept, the inheritance of a trait depended on the passing-on of these units.
• For any given trait, an individual inherits one gene from each parent so that the individual has a pairing of two genes. We now understand the alternate forms of these units as ‘alleles’.
• If the two alleles that form the pair for a trait are identical, then the individual is said to be homozygous and if the two genes are different, then the individual is heterozygous for the trait.
• The breeding experiments of the monk in the mid‐1800s laid the groundwork for the science of genetics.
• He studied peas plant for 7 years and published his results in 1866 which was ignored until 1900 when three separate botanists, who also were theorizing about heredity in plants, independently cited the work.
• In appreciation of his work he was considered as the “Father of Genetics”.
• A new stream of genetics was established after his name as Mendelian genetics which involves the study of heredity of both qualitative (monogenic) and quantitative (polygenic) traits and the influence of environment on their expressions.
• Mendelian inheritance while is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900.

Table of Contents

Interesting Science Videos

Mendel’s Experiment

Mendel carried out breeding experiments in his monastery’s garden to test inheritance patterns. He selectively cross-bred common pea plants ( Pisum sativum ) with selected traits over several generations.  After crossing two plants which differed in a single trait (tall stems vs. short stems, round peas vs. wrinkled peas, purple flowers vs. white flowers, etc), Mendel discovered that the next generation, the “F1” (first filial generation), was comprised entirely of individuals exhibiting only one of the traits.  However, when this generation was interbred, its offspring, the “F2” (second filial generation), showed a 3:1 ratio- three individuals had the same trait as one parent and one individual had the other parent’s trait.

Mendel’s Laws

I. Mendel’s Law of Segregation of genes (the “First Law”)

Image Source:  Encyclopædia Britannica .

• The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles.
• An offspring thus receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms: one allele for each trait from each parent.
• Hence, according to the law, two members of a gene pair segregate from each other during meiosis; each gamete has an equal probability of obtaining either member of the gene.

II. Mendel’s Law of Independent Assortment (the “Second Law”)

• Mendel’s second law. The law of independent assortment; unlinked or distantly linked segregating genes pairs behave independently.
• The Law of Independent Assortment states that alleles for separate traits are passed independently of one another.
• That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait.
• Mendel found support for this law in his dihybrid cross experiments. In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios.
• This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each.

III. Mendel’s Law of Dominance (the “Third Law”)

• The genotype of an individual is made up of the many alleles it possesses.
• An individual’s physical appearance, or phenotype, is determined by its alleles as well as by its environment.
• The presence of an allele does not mean that the trait will be expressed in the individual that possesses it.
• If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism’s appearance and is called the dominant allele; the other has no noticeable effect on the organism’s appearance and is called the recessive allele.
• Thus, the dominant allele will hide the phenotypic effects of the recessive allele.
• This is known as the Law of Dominance but it is not a transmission law: it concerns the expression of the genotype.
• The upper case letters are used to represent dominant alleles whereas the lowercase letters are used to represent recessive alleles.
• Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
• Gardner, E. J., Simmons, M. J., & Snustad, D. P. (1991). Principles of genetics. New York: J. Wiley.
• https://www.cliffsnotes.com/study-guides/biology/plant-biology/genetics/mendelian-genetics
• http://kmbiology.weebly.com/mendel-and-genetics—notes.html
• http://knowgenetics.org/mendelian-genetics/
• https://en.wikipedia.org/wiki/Mendelian_inheritance
• https://www.acpsd.net/site/handlers/filedownload.ashx?moduleinstanceid=40851&dataid=33888&FileName=Mendelian%20Genetics.pdf

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2 thoughts on “Mendel’s 3 Laws (Segregation, Independent Assortment, Dominance)”

Good to know when one works with plants like me.

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• Review Article
• Published: 12 April 2022

Demystifying the mythical Mendel: a biographical review

• Daniel J. Fairbanks   ORCID: orcid.org/0000-0001-7422-0549 1  

Heredity volume  129 ,  pages 4–11 ( 2022 ) Cite this article

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• Plant sciences

Gregor Mendel is widely recognised as the founder of genetics. His experiments led him to devise an enduring theory, often distilled into what are now known as the principles of segregation and independent assortment. Although he clearly articulated these principles, his theory is considerably richer, encompassing the nature of fertilisation, the role of hybridisation in evolution, and aspects often considered as exceptions or extensions, such as pleiotropy, incomplete dominance, and epistasis. In an admirable attempt to formulate a more expansive theory, he researched hybridisation in at least twenty plant genera, intentionally choosing some species whose inheritance he knew would deviate from the patterns he observed in the garden pea ( Pisum sativum ). Regrettably, he published the results of only a few of these additional experiments; evidence of them is largely confined to letters he wrote to Carl von Nägeli. Because most original documentation is lost or destroyed, scholars have attempted to reconstruct his history and achievements from fragmentary evidence, a situation that has led to unfortunate omissions, errors, and speculations. These range from historical uncertainties, such as what motivated his experiments, to unfounded suppositions regarding his discoveries, including assertions that he never articulated the principles ascribed to him, staunchly opposed Darwinism, fictitiously recounted experiments, and falsified data to better accord with his theory. In this review, I have integrated historical and scientific evidence within a biographical framework to dispel misconceptions and provide a clearer and more complete view of who Mendel was and what he accomplished.

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Introduction.

The year 2022 marks the bicentennial of Gregor Mendel’s birth. He rose from an impoverished childhood in a small village to become a successful teacher, scientist, priest, and ultimately prelate and abbot. His discoveries and interpretations of elegant symmetrical patterns of inheritance in the garden pea ( Pisum sativum L.) led him to develop a theory of inheritance that has endured with little change. No one, including Mendel, recognised the importance of his theory in his day; it languished mostly unnoticed until its dramatic rediscovery founded the science of genetics at the beginning of the twentieth century. The fragmentary evidence of Mendel’s history has left much room for speculation and conjecture. Inevitably, misunderstandings, myths, omissions, and rumours have become part of popular and scholarly accounts of his accomplishments and history. Some mysteries may never be resolved due to the absence of sufficient evidence. In this review, I examine within a biographical framework the scientific and historical evidence to clarify some of the most important Mendelian misconceptions.

Mendel’s youth and early education

Johann Mendel was born in July 1822 in the village of Heinzendorf (Hynčice) Footnote 1 in Austrian Silesia (currently in the Czech Republic); his parents were Rosina Schwirtlich and Anton Mendel. When he became a friar in 1843, he took on the monastic name Gregor. The first of several historical misconceptions is the day of his birth, disputed as July 20 or 22. Some authors, mostly in popular online biographies, have attempted to resolve this discrepancy by speculating that he was born on July 20 and baptised on July 22. However, the evidence contradicts this presumption. The parish birth register lists the date of his birth and baptism as July 20 (Moravian Museum 1965 ). After pointing out several discrepancies in the birth register, Klein and Klein ( 2013 ) noted, “Another peculiarity of the register is that all seven children born in 1822 were baptized on the day of their birth” (p. 123), suggesting that the dates may be incorrect because, at the time, infants were rarely born and baptised on the same day. Mendel himself consistently listed his birthdate as July 22 on all known documents. His nephew, Alois Schindler, wrote that his uncle Gregor and his mother Theresia insisted that the correct birthdate was July 22, the feastday of St. Mary Magdalene. Schindler further reasoned, “Perhaps the parish dates were recorded belatedly and incorrectly” (translated from the original 1902 German in Kříženecký 1965 , p. 80).

Although writers often state that Johann had two siblings, probably based on Iltis ( 1924 , 1966 ), in fact he was the second of five children in the family. Two of his sisters, Veronika and Theresia, lived to adulthood. Two other sisters, both named Rosina, died as children, one as a toddler the other as an infant (Klein and Klein 2013 ). Theresia lived to see her late brother Gregor attain fame as the founder of genetics in the early twentieth century. She provided some of the most important information of his early history based on her recollections and documents she retained. Two of her sons, Alois and Ferdinand Schindler, recorded her reminiscences along with their own (Kříženecký 1965 ).

Johann and his sisters attended classes in a small schoolhouse a short walk from their home. His teachers arranged for him to attend boarding school for gifted children in Leipnik (Lipník) where he studied for one academic year (1833–34). He received admission to the Troppau (Opava) Gymnasium where he would continue his schooling for six academic years, graduating in 1840. He then attended the Olmütz (Olomouc) Philosophical Institute, graduating in 1843. Once while he was in Troppau and again while he was in Olmütz, he suffered episodes of nervous illness so severe that he had to retreat home for months to recuperate, the second time costing him a year of his schooling. Despite these prolonged illnesses, his performance was outstanding at all three schools.

In the time preceding the summer of 1841, he faced a pivotal decision. His father, crippled by an accident three years earlier, could no longer manage the farm. Johann, now at his nineteenth birthday, had to decide whether to take over the family farm or continue his education. A document from the time makes it clear that by the end of that summer he had decided to enter the priesthood. Respecting that decision, his father sold the farmstead to Alois Sturm, Veronika’s husband, with the following provision: “The purchaser shall pay to the son of the seller, Johann by name, if the latter as he now designs should enter the priesthood, or should he in any other way begin to earn an independent livelihood, the sum of 100 fl. … and shall also defray all expenses connected with the first mass” (Iltis 1966 , p. 39). Johann’s physics professor in Olmütz, Friedrich Franz, highly recommended him for admission to the Augustinian order in the St. Thomas monastery in Brünn (Brno), the capital of Moravia. Mendel was officially admitted to the order on October 9, 1843 (Iltis 1966 , p. 43).

Friar, scientist, and teacher

The group of Augustinians Mendel joined in October 1843 was extraordinary. Although often identified as a monk, he was a friar, which is an important distinction. The mendicant orders, including Augustinians, consist of friars in that their members openly serve the community, leading much less cloistered lives than traditional monks. Several of the St. Thomas friars were highly educated, serving as teachers and professors, conducting scholarly research in the sciences, arts, and humanities, and holding prestigious administrative positions in commerce and academic societies. They were especially dedicated to secular academic teaching and research, a situation that often placed them in conflict with their ecclesiastical superiors beyond the monastery.

The abbot, Cyrill Franz Napp, was a highly respected scholar characterised by a fellow friar as “a famous prelate, scientist, secret freethinker, and patriot, and expert in state affairs and economy” (Matalová 1973 , p. 252). Prior to his abbacy, Napp taught at the Brünn Theological Institute. The monastery had large agricultural holdings, and Napp was committed to implementing scientific advances in agriculture. He was an influential member of the Moravian-Silesian Agricultural Society, especially in the society’s sheep breeding and pomological associations. With Napp’s encouragement, Mendel took classes in scientific agriculture at the Brünn Philosophical Institute and was elected to membership in the Agricultural Society in 1851 (Matalová and Matalová 2022 ).

Mendel’s close friend and mentor during these early years was his fellow friar Matouš František Klácel, a philosopher specialising in the writings of Georg Wilhelm Friedrich Hegel and a self-described freethinker who was constantly at odds with church authorities beyond the monastery. Shortly after Mendel arrived, Bishop Anton Ernst Schaffgotsch dismissed Klácel from his teaching position at the Brünn Theological Institute for teaching “pantheism and other heresies related to Hegelianism” (Peaslee and Orel 2007 , p. 152).

Revolutionary sentiment swept much of Europe in 1848 and was especially forceful in Vienna, spilling over into Brünn. The St. Thomas friars supported revolutionary reforms, with Napp’s enthusiastic encouragement. Klácel seized the opportunity to compose a petition demanding greater freedom for friars from religious duties, allowing them to devote themselves more fully to secular research and teaching. The wording of the petition was scathing, its content overflowing with hyperbole. It concluded, “the undersigned professors and pastoral workers in the Order of St. Augustine in Altbrünn take the liberty of appealing to the imperial parliament to grant them constitutional civil rights , and request to be allowed to devote their entire efforts, according to their abilities and past services, to public teaching institutions and to free, united, and indivisible citizenship … [and] make it respectfully their missions to promote science and humanity…” ( underlining in the original , Klein and Klein 2013 , p. 281). Mendel was one of six friars who signed the petition.

Several premature deaths in the 1840s created a shortage of parish priests, leading Napp to recommend Mendel’s ordination at the earliest possible date. Napp assigned Mendel to serve as a parish priest but soon discovered that he was poorly suited to this role. In a letter, Napp informed Schaffgotsch that he had relieved Mendel of his ecclesiastical duties because he was “much less fitted for work as a parish priest, the reason being that he is seized with an unconquerable timidity when he has to visit a sick-bed or to see anyone ill and in pain” (Iltis 1966 , p. 58). Napp, as administrator over Moravian schools, arranged for Mendel to instead assume a teaching position at the Znaim (Znojmo) Gymnasium, southwest of Brünn.

Mendel immediately proved to be an exemplary teacher, loved by his students, and praised by his colleagues. A newly implemented law, however, required that teachers be certified through a gruelling series of examinations. Accordingly, Mendel applied in 1850 to be certified in physics and natural history. He received the first part of the examination, a homework portion that he was to complete by writing two essays in response to questions, one on physics and the other on natural history. His essay on natural history contains his first known allusion to evolution, a part of which reads, “The vegetable and animal life developed more and more richly; its oldest forms disappeared in part to make way for new and more perfect ones” (Fairbanks 2020 ).

The examiner for physics, Andreas von Baumgartner, found Mendel’s essay on this topic to be informed and well written. However, Rudolf Kner, the examiner for natural history, determined that Mendel’s essay on this subject was deficient. Both examiners, however, recommended him for the next part known as the Klausurprüfung , an on-site written examination in a locked room at the University of Vienna with no access to resources. Mendel’s written answers this time were less than favourable. His examiners, nonetheless, allowed him to proceed to the viva voce (oral) portion. Here he faced a commission, among them the famed physicist Christian Doppler, after whom the Doppler effect is named. His physics examiners evaluated him as “unqualified to teach physics….” and Kner wrote that “he is not yet competent to become a teacher” (Iltis 1966 , p. 72). The written report languished in bureaucracy as it bounced from one administrative office to another, finally reaching Napp and Mendel in August 1851, almost a year after the examination in Vienna.

The University of Vienna and the motivation for Mendel’s experiments

By the time the examination report finally arrived, Napp was already arranging for Mendel to study at the University of Vienna in preparation for a teaching career. The wheels of bureaucracy again turned slowly, and when Mendel finally departed for Vienna, he was five weeks late for the beginning of the 1851 fall term. Serendipitously, due to delays in renovation of the physics laboratory, the experimental physics course began at the same time as Mendel’s arrival. This was his only course that fall term, and it was influential, taught by Doppler to thirteen students. For the 1852 spring term, Mendel again enrolled in Doppler’s course, with additional courses in other subjects. Doppler departed that summer for Italy to recuperate from an illness and died soon thereafter, so Mendel was one of his last students. Although Mendel was originally scheduled to spend a year at university, he remained for almost two years, taking advanced courses in physics, mathematics, chemistry, botany, zoology, and palaeontology, and assisting with entomological research in an extracurricular setting.

Some have argued that Mendel was a staunch anti-evolutionist and adherent of the doctrine of special creation (Callender 1988 ; Bishop 1996 ). There is ample evidence, however, to contradict these views, beginning with Mendel’s studies at the University of Vienna. Pre-Darwinian evolutionary theory was prominent at the time, and Mendel studied it in courses on botany, zoology, and palaeontology. One of his most influential professors was Franz Unger, a botanist and palaeontologist. Unger popularised evolution for the public through a series of newspaper articles later compiled as a book (Unger 1852 ). He also published a popular book with hand-tinted lithographs of geological periods dating from the present to hundreds of millions of years ago (Unger 1851 ). Unger’s conception of evolution was remarkably like Darwin’s, even though Origin of Species was still eight years from publication. Gliboff ( 1998 ) thoroughly reviewed Unger’s evolutionary theory, titling it the “theory of universal common descent” (p. 223). Unger’s development of this theory reached its peak while Mendel was studying with him in Vienna.

At the time Mendel was attending Unger’s lectures, he witnessed first-hand a series of anti-evolutionary attacks pitting Catholicism against evolution. Sebastian Brunner was a prominent Catholic priest, a prolific author and orator, purveyor of religious orthodoxy, and anti-Semite, known by the epithet Malleus episcoporum , the bishop’s hammer (Gliboff 1998 ). Brunner publicly singled out Unger in his attacks, which began two days before Mendel’s arrival in Vienna in October of 1851. These attacks persisted unabated until the spring of 1856, approximately a year and a half after Mendel had returned to the monastery. Brunner named Unger in a newspaper headline as “Isis Priest and Philistine” and in another article as “a man who openly denied the creation and the Creator” (Olby 1985 , pp. 202–203). In his most sarcastic article, Brunner wrote that Vienna’s botanists “do everything they can to make themselves into plants of botanical learning that can be smelt from afar—and place themselves voluntarily into the eternally stinking dung-bed of the pantheistic world view, which nevertheless fosters a certain richness of blossoms” (Fairbanks 2020 , p. 265).

By the time Brunner wrote these words, Mendel had been officially appointed as one of these Viennese botanists. In 1853, his professors and colleagues elected him to full membership in the Imperial-Royal Zoological-Botanical Society in Vienna. Some have erroneously surmised that Mendel’s classic 1866 paper was his first scientific publication when, in fact, it was the third of eight (Mendel 1853 , 1854 , 1866 , 1870 , 1871 , 1879a , 1879b , 1882 ). Much of his focus was on physics, which led him to pursue meteorology as one of his principal research activities throughout the remainder of his life. If his published compilations of meteorological data are added to the list, the number of his scientific journal publications totals fourteen. Mendel presented a scientific paper to the Imperial-Royal Zoological-Botanical Society in Vienna in 1853 on lepidopteran predation in radishes. This paper became his first scientific publication when it appeared in the society’s journal (Mendel 1853 ). In 1854, he submitted another paper based on microscopic examination of the pea weevil and its infestation of pea seeds, which Vincenz Kollar, one of his professors, presented to the society in Mendel’s absence. It too was published in the society’s journal (Mendel 1854 ).

Mendel returned from the University of Vienna to the monastery in the summer of 1853. By then, Pope Pius IX had issued an edict that Austrian monasteries be investigated for secularism and neglect of religious piety. Cardinal Schwartzenberg in Prague appointed Bishop Schaffgotsch in Brünn to investigate the St. Thomas monastery. The investigation concluded with a formal visitation in early June 1854. At the time, Mendel had recently accepted a teaching appointment at the Realschule, a school focused on training students in their adolescent years in science, mathematics, and technical subjects. This teaching assignment prompted Schaffgotsch to accuse Mendel of studying “profane sciences at a worldly establishment in Vienna at the expense of the monastery to become a professor of said sciences at a state institution” (Klein and Klein 2013 , p. 295). At the conclusion of his report, Schaffgotsch recommended dissolution of the order, determining that “any hopes that the spirit could be exorcized and the order returned to a conscientious observance of its rules and constitutions must be given up” (Klein and Klein 2013 , p. 295). The report made its way to the Vatican. Although no actions were taken, and Mendel’s monasterial community remained intact, the friars lived under a cloud knowing that dissolution could be imminent.

This threat coincided with Mendel’s earliest known pea experiments (Mendel 1854 , 1866 ; Stern and Sherwood 1966 ; Orel 1996 ; Klein and Klein 2013 ). There is little evidence, however, to indicate the extent to which this threat had any influence on his experimental approach. Some have speculated that this and later threats from ecclesiastical superiors led Mendel to carefully avoid naming controversial evolutionary biologists, such as Darwin and Unger, in his printed publications, but nonetheless showing how his research contributed to evolutionary theory (Klein and Klein 2013 ; Fairbanks 2020 ). Mendel more overtly expressed his Darwinian views in his private correspondence than in his published writings (Iltis 1966 ; Fairbanks 2020 ).

In 1855, Mendel arranged to retake his teacher certification examination. He completed the homework portion at an unknown date then during the first week in May 1856 he travelled to Vienna for the on-site written and oral portions. Fragmentary accounts of what transpired have provoked exaggerated myths regarding Mendel and his motivations for his famous experiments.

In the early part of the twentieth century, Hugo Iltis ( 1924 , 1966 ) interviewed one of Mendel’s school colleagues who recalled that when Mendel returned from the examination, he was “very much out of humour” because “he had a very sharp difference of opinion with the examiner in botany, and had stubbornly maintained his own point of view” (Iltis 1966 , p. 95). This account has morphed into the notion that the unnamed examiner was Eduard Fenzl, one of Mendel’s botany professors. Mendel purportedly insisted during the examination that heredity was biparental whereas Fenzl authoritatively proclaimed that it was purely paternal, the female parent serving merely as a nurse to the pollen (Wunderlich 1982 ; Olby 1985 ; Orel 1996 ; Klein and Klein 2013 ). According to Iltis ( 1966 ), Mendel’s school colleague believed that “this dispute with the examiner led Mendel to begin his experiments” (p. 95).

A letter from Klácel, written immediately after Mendel’s return from the fateful examination, provides a contemporary and much more accurate account of what transpired:

Although he [Mendel] drew easy questions, he fell ill during the first Klausurprüfung and as a consequence was unable to write. He seems to have problems with his nerves generally since he endured several such insidious attacks already and they say that in his youth he suffered from epilepsy. The day passed and nothing was achieved. One has to feel sorry for him, since his homework etc. was graded as excellent. But formalities are formalities; in this case it was not possible to continue. Afraid that further attacks might continue, he returned home without accomplishing anything. (Klein and Klein 2013 , p. 364)

This account makes it clear that Mendel had performed well in the homework portion, but he experienced yet another nervous attack early during the Klausurprüfung (locked-room, written portion) and “was unable to write”. Because he abandoned the examination before the oral portion, he could not have confronted Fenzl. Mendel then rescheduled the examination for August but there is no record that he travelled to Vienna for it.

Further evidence shows that the abandoned examination could not have motivated Mendel’s experiments. Although he began his pea hybridisations that same spring in 1856, he probably planted the parental varieties at least a month earlier. Importantly, he already had his experiments in mind two years earlier, having conducted essential preliminary experiments with the commercial pea varieties during the summers of 1854 and 1855 to ensure that they were true-breeding and to determine which of them were most suitable for his hybridisation experiments.

Although anachronisms dispel the notion that the abandoned examination motivated Mendel’s experiments, an earlier dispute between Unger and Fenzl may have played a role (Olby 1985 ). Cell theory was a rapidly developing discipline at the time, and Unger and Fenzl were two of its leading researchers. They debated the nature of fertilisation, based in part on their interpretations of competing hypotheses of Matthias Jakob Schleiden and Giovanni Battista Amici (Olby 1985 ; Orel 1996 ; Klein and Klein 2013 ). Mendel was undoubtedly familiar with the Unger-Fenzl dispute long before this examination. Several aspects of his experimental design directly addressed this dispute and conclusively resolved it.

Mendel’s experiments and theory

Mendel carried out his hybridisation experiments over eight years (1856–63), then presented them as two lectures in 1865 and published them in his classic paper the following year (Mendel 1866 ). Two recent English translations are freely available online, one by Abbott and Fairbanks ( 2016 ) and the other by Müller-Wille and Hall (Mendel 2016 ). My focus here is on misconceptions, myths, controversies, and omissions shrouding his experiments, discoveries, and theory.

One of Mendel’s most important contributions, often omitted from accounts in textbooks and articles, is his definitive resolution of the Unger-Fenzl dispute. At the time, competing hypotheses regarding fertilisation and inheritance included strict uniparental inheritance, some form of unequal biparental inheritance, or strict biparental equality. Mendel’s definitive resolution of the issue in terms of cell theory is evident in a passage that Sekerák ( 2017 ) highlighted as the place where “Mendel reveals the generally valid essence of the reproduction of living organisms” (p. 65). Here Mendel concluded that “one germ cell and one pollen cell unite into a single cell that is able to develop into an independent organism through the uptake of matter and the formation of new cells. This development takes place according to a constant law that is founded in the material nature and arrangement of the elements” (Abbott and Fairbanks 2016 , p. 420).

To the term “single cell” in this passage, Mendel appended a footnote that unambiguously addressed the dispute between Unger and Fenzl, albeit without naming either:

With Pisum it is shown without doubt that there must be a complete union of the elements of both fertilising cells for the formation of the new embryo. How could one otherwise explain that among the progeny of hybrids both original forms reappear in equal number and with all their peculiarities? If the influence of the germ cell on the pollen cell were only external, if it were given only the role of a nurse, then the result of every artificial fertilisation could be only that the developed hybrid was exclusively like the pollen plant or was very similar to it. In no manner have experiments until now confirmed that. Fundamental evidence for the complete union of the contents of both cells lies in the universally confirmed experience that it is unimportant for the form of the hybrid which of the original forms was the seed or the pollen plant. (Abbott and Fairbanks 2016 , p. 420)

A few years later, in 1869, while reading the chapter on pangenesis in a German translation of Darwin’s Variation of Animals and Plants Under Domestication (Darwin 1868b ), Mendel encountered Darwin’s supposition that fertilisation of a single germ cell requires more than one pollen grain. Mendel annotated a passage (Fairbanks 2020 ), which reads in Darwin’s original English:

The pollen grains of Mirabilis are extraordinarily large, and the ovarium contains only a single ovule; and these circumstances led Naudin to make the following interesting experiments: a flower was fertilised by three grains and succeeded perfectly; twelve flowers were fertilised by two grains, and seventeen flowers by a single grain, and of these one flower alone in each lot perfected its seed; and it deserves especial notice that the plants produced by these two seeds never attained their proper dimensions, and bore flowers of remarkably small size. (Darwin 1868a , p. 364)

This passage compelled Mendel to carry out an experiment, the importance of which is evident in his description of it in an 1870 letter to Carl von Nägeli:

But one experiment seemed to me to be so important that I could not bring myself to postpone it to some later date. It concerns the opinion of Naudin and Darwin that a single pollen grain does not suffice for fertilization of the ovule. I used Mirabilis jalappa for an experimental plant, as Naudin had done; the result of my experiment, however, is completely different. From fertilization with single pollen grains, I obtained 18 well developed seeds, and from these an equal number of plants, of which 10 are already in bloom. … According to Naudin, at least three [pollen grains] are needed! (Stern and Sherwood 1966 , pp. 92–93)

Later observations by microscopists solidified the fundamental concept that two gametes unite at fertilisation to form a zygote. Rarely, however, is Mendel credited with the definitive experimental confirmation of this concept, or the fact that he viewed this discovery as one of his most important achievements.

Of the many misunderstandings and myths obscuring Mendel’s experimental approach are assertions that his description of his experiments was fictitious, that he never articulated the laws of segregation and independent assortment, and that his data were falsified to more closely approximate expectation. Moreover, some phenomena Mendel addressed in his paper are not attributed to him, instead considered as extensions or exceptions to his laws. I will briefly address these issues here. For extensive reviews of them, see Sapp ( 1990 ), Hartl and Orel ( 1992 ), Orel ( 1996 ), Fairbanks and Rytting ( 2001 ), Westerlund and Fairbanks ( 2004 ), Hartl and Fairbanks ( 2007 ), and Franklin et al. ( 2008 ).

The claim that Mendel’s description of his experiments was fictitious dates to Bateson ( 1902 ), who speculated that “it is very unlikely that Mendel could have had seven pairs of varieties such that the members of each pair differed from each other in only one considerable character” (p. 59). Fisher ( 1936 ) quoted Bateson’s claim and dismissed it: “there can, I believe, be no doubt whatever that his report is to be taken entirely literally, and that his experiments were carried out in just the way and in much the order that they are recounted” (p. 132). Corcos and Monaghan ( 1984 ) resurrected Bateson’s claim, then di Trocchio ( 1991 ) amplified it, proposing that Mendel hybridised the 22 parental pea varieties he had chosen as parents in all possible combinations then disaggregated the data into fictitious experiments to make his presentations more understandable. Such assertions, however, directly contradict the words Mendel chose to succinctly describe his monohybrid experiments: “[parental] plants were used that differed in only one essential character” (Abbott and Fairbanks 2016 , p. 412). After examining published characteristics of nineteenth century pea varieties, Fairbanks and Rytting ( 2001 ) determined that “the nature of variation in pea varieties (both old and modern) facilitates, rather than prevents, the construction of monohybrid experiments” (p. 744) and “Mendel’s account describes a well-conceived experimental design that would not have been difficult for him to perform” (p. 745).

Claims that Mendel did not conceive the laws of segregation and independent assortment date at least to Callender ( 1988 ) who referred to “the myth of ‘Mendel’s Law of Segregation’; a law not to be found in either of Mendel’s papers, nor in his scientific correspondence, nor in any statement that can be unambiguously attributed to him” (pp. 41–42), and Monaghan and Corcos ( 1990 ) who contended that “the traditional Mendelian laws of segregation and independent assortment are not given in the paper” (p. 268). Although Mendel did not directly articulate segregation and independent assortment as distinct and separate laws, they are evident in the theory he derived as a “constant law that is founded in the material nature and arrangement of the elements” (Abbott and Fairbanks 2016, p. 420). In a passage appearing shortly after introducing this theory, he lucidly articulated what we can now phrase in modern terms as the pairing of differing alleles of a gene in heterozygotes and their segregation during meiosis:

In relation to those hybrids whose progeny are variable, one might perhaps assume that there is an intervention between the differing elements of the germ and pollen cells so that the formation of a cell as the foundation of the hybrid becomes possible; however, the counterbalance of opposing elements is only temporary and does not extend beyond the life of the hybrid plant. Because no changes are perceptible in the general appearance of the plant throughout the vegetative period, we must further infer that the differing elements succeed in emerging from their compulsory association only during development of the reproductive cells. In the formation of these cells, all existing elements act in a completely free and uniform arrangement in which only the differing ones reciprocally segregate themselves. In this manner the production of as many germ and pollen cells would be allowed as there are combinations of formative elements. (Abbott and Fairbanks 2016 , p. 420)

A key phrase in this passage is “reciprocally segregate themselves” from Mendel’s “ sich gegenseitig ausschliessen ”. This phrase was translated by Müller-Wille and Hall (Mendel 2016 ) as “mutually exclude each other” (p. 42), by Stern and Sherwood ( 1966 ) as “separate from each other” (p. 43), and by Druery and Bateson (Bateson 1902 ) as “mutually separate themselves” (p. 89). Mendel’s explanation of “differing elements” paired in “compulsory association” that “reciprocally segregate themselves” “only during the development of the reproductive cells” clearly reflects the modern concept of paired allelic segregation during meiosis.

Independent assortment, implied by Mendel in the last sentence of this passage, is more fully clarified in other passages, such as the following: “the behaviour of each pair of differing characters in hybrid union is independent of the other differences between the two original plants and, further, that the hybrid produces as many types of germ and pollen cells as there are possible constant combination forms” (Abbott and Fairbanks 2016 , p. 421).

Aspects that Mendel included in his paper, often stated as extensions or exceptions to his laws, include pleiotropy, incomplete dominance, and epistasis. He described a case of pleiotropy for seed coat colour, flower colour, and axillary pigmentation as follows: “The difference in the colour of the seed coat … is either coloured white, a character consistently associated with white flower colour, or it is grey, grey-brown, or leather brown with or without violet spots, in which case the colour of the standard petal appears violet, that of the wings purple, and the stem at the base of the leaf axils is tinged reddish” (Abbott and Fairbanks 2016 , p. 408). As reviewed by Hartl and Fairbanks ( 2007 ), this pleiotropic association clarifies some perplexing questions about Mendel’s experimental design, such as his reason for choosing seed-coat colour as the third character in his trihybrid experiment.

Mendel’s comparison of full and incomplete dominance is evident in the following sentences:

The experiments conducted with ornamental plants in past years already produced evidence that hybrids, as a rule, do not represent the precise intermediate form between the original parents. With individual characters that are particularly noticeable, like those related to the form and size of the leaves and to the pubescence of the individual parts, the intermediate form is in fact almost always apparent; in other cases, however, one of the two original parental characters possesses such an overwhelming dominance that it is difficult or quite impossible to find the other in the hybrid. (Abbott and Fairbanks 2016 , p. 409)

Mendel’s inference of what is now known as epistasis is near the end of his paper in an experiment with flower colour in the common bean ( Phaseolus ). From an interspecific cross between P. nanus L. (with white flowers) and P. multifloris W. (with coloured flowers), he noted partial dominance for flower colour and reduced fertility in the F 1 hybrids. Of the 31 F 2 plants that flowered, one had white flowers, and 30 displayed varying shades of coloured flowers. He attempted to interpret this result in the context of what he had observed in Pisum , speculating that if two “independent characters” (as he put it) influenced flower colour, a 15:1 ratio is expected, whereas if three did so, a 63:1 ratio is expected. He astutely added the caveat, “It must not be forgotten, however, that the explanation proposed here is based only on a mere supposition that has no other support than the very imperfect result of the experiment just discussed” (Abbott and Fairbanks 2016 , p. 418). The ratios he proposed reflect what is now designated as recessive epistasis.

No Mendelian controversy has generated as much debate as the accusation that Mendel’s data were falsified to more closely approximate expectation. Weldon was the first to raise questions, privately writing to Pearson in 1901 that Mendel had “cooked his figures, but that he was substantially right” (Mangello 2004 , p. 23, italics in original). After applying Pearson’s newly developed chi-squared test to Mendel’s data, Weldon ( 1902 ) did not overtly claim in print that Mendel manipulated the data but dangled the possibility in several statements, one of which reads, “the odds against a result as good as this or better are 20 to 1” (p. 235). Fisher, probably influenced by Weldon’s paper, famously stated in a 1911 lecture, “It may just have been luck, or it may be that the worthy German abbot, in his ignorance of probable error, unconsciously placed doubtful plants on the side which favoured his hypothesis” (Norton and Pearson 1976 , p. 160). The controversy, now known as the Mendel-Fisher controversy, is based largely on an article by Fisher ( 1936 ) wherein he famously wrote, “the data of most, if not all, of the experiments have been falsified so as to agree closely with Mendel’s expectations” (p. 132).

This assertion is, in fact, less incriminatory than it may seem when viewed in context of Fisher’s overall paper. Fisher presumed that an assistant, rather than Mendel, must have manipulated the data, and he dedicated only a relatively small part of the paper to evidence of questionable data. Fisher’s admiration for Mendel is evident in the conclusion where he referred to Mendel’s paper as “experimental researches conclusive in their results, faultlessly lucid in presentation, and vital to the understanding not of one problem of current interest, but of many” (Fisher 1936 , p. 137).

After its publication, Fisher’s paper received little attention until the centennial of Mendel’s lectures in 1965 when the controversy began in earnest. It lasted for more than forty years in numerous articles and books whose authors drew a wide range of conclusions based on analyses examining essentially every conceivable aspect of Mendel’s experiments. Allan Franklin’s introductory essay in Franklin et al. ( 2008 ) is the most exhaustive and definitive review of the Mendel-Fisher controversy. After evaluating the complex statistical, historical, and botanical aspects of the many published analyses of Mendel’s data, Franklin concluded that “the experiments that had initially triggered Fisher’s suspicions can be explained without any fraud,” but “the issue of the ‘too-good-to-be-true’ aspect of Mendel’s data found by Fisher still stands”. Finally, he urged, “It is time to end the controversy” (Franklin et al., 2008 , p. 68). Fortunately, most scholars have heeded this plea.

Mendel and Darwin

Mendel became well acquainted with biological evolution from his university studies years before he learned of Darwin. Although Mendel and Darwin were contemporaries, it is unlikely that Mendel learned of Darwin until 1863, the final year of his Pisum experiments. Darwin published Origin of Species in 1859, the fourth year of Mendel’s experiments, but Mendel obtained his German translation of the book in 1863. It contains his hand annotations, published by Fairbanks and Rytting ( 2001 ) as an online supplement. By the time Mendel presented his lectures in 1865, Darwin’s Origin of Species was widely known and popular. In the January 1865 monthly meeting of the Natural Science Society in Brünn, Mendel’s friend and fellow teacher, Alexander Makowsky lectured on Origin of Species, addressing some of the same topics that Mendel addressed in the next two monthly meetings in February and March.

The existing evidence of Mendel’s acquaintance with Darwin’s theory and books, as well as Mendel’s statements referencing Darwin, strongly counter claims that Mendel was “in favor of the orthodox doctrine of special creation” (Bishop 1996 , p. 212) and “an opponent of descent with modification” (Callender 1988 , p. 41). The cumulative evidence suggests that Mendel had strong interest in Darwin’s writings and their relevance to his research, but that he did not become an avid promoter of Darwinism (Fairbanks 2020 ). Those who knew Mendel who lived into the twentieth century to share their recollections, independently confirmed this impression (Iltis 1966 ; Coleman 1967 ).

Although Mendel was thoroughly acquainted with Darwin’s writings, there is no evidence that Darwin knew anything about Mendel. A common rumour purports that Darwin owned an offprint of Mendel’s 1866 paper but that it was uncut. For example, Hennig ( 2000 ) wrote, “Another uncut reprint was found in the library of Charles Darwin, so Mendel must have sent him a copy, too” (p. 143). Despite several similar claims, there is no evidence that Darwin owned an offprint by Mendel (Lorenzano 2011 ). In fact, there is evidence to dispel the common notion that Mendel sent uncut offprints. The offprints contain several typesetter errors, which are hand-corrected in the same places and in the same manner in the offprints Mendel sent, evidence that Mendel made the corrections rather than later readers, which he could do only if the offprints were cut (Müller-Wille and Hall 2016 ; Fairbanks 2022 ).

Darwin owned two books with brief references to Mendel’s experiments. One is a book by Hoffmann ( 1869 ), which contains short and essentially uninformative references, not likely to lead Darwin to seek Mendel’s paper (Olby 1985 ). The other is a book by Focke ( 1881 ), published the year before Darwin’s death, which Darwin loaned to a friend. The pages in this book with references to Mendel remain uncut to this day, possibly the source of the rumour of uncut offprints (Lorenzano 2011 ).

Mendel’s subsequent experiments and letters to Nägeli

Mendel sent an offprint to Carl von Nägeli, a renowned botanist whom Unger often praised, on December 31, 1866 with a detailed accompanying letter. Fortunately, Nägeli retained Mendel’s letters, although at least one is missing, and a page from another may also be missing (van Dijk and Ellis 2016 ). Mendel’s letters to Nägeli provide important and detailed information of his research after 1866. Cautious about drawing sweeping conclusions, Mendel conducted hybridisation experiments in other plant species. These experiments were much more extensive than is often portrayed. Mendel recounted experiments with numerous plant genera, among them Hieracium , Circium , Geum , Linaria , Calceolaria , Zea , Ipomoea , Cheiranthus , Antirrhinum , Tropaeolum , Veronica , Viola , Potentilla , Carex , Verbascum , Mirabilis , Aquilegia , Lychnis , and Matthiola . The letters contain detailed results for several of these genera, especially Hieracium , Circium , Geum , Linaria , Verbascum , Mirabilis , Matthiola , and Zea . Mendel noted that the progeny from hybrids in Matthiola , Zea , and Mirabilis “behave exactly like those of Pisum ” (Stern and Sherwood 1966 , p. 93).

In his classic 1866 paper, Mendel classified hybrids into two types: those that produce variable progeny (as was the case with Pisum and Phaseolus ), and those that produce constant progeny, meaning that all the progeny uniformly and consistently retain the characters of the hybrid parent through repeated generations of self-fertilisation. In his experiments with other plant species, he intentionally included genera that he expected to be variable and others that he expected to be constant. For example, he wrote to Nägeli that Geum “belongs to the few known hybrids that produce nonvariable progeny as long as they remain self-pollinated” (Stern and Sherwood 1966 , pp. 58–59). By researching both types, Mendel hoped to develop a more expansive theory to explain inheritance and speciation in the progeny of hybrids.

Mendel’s choice to research Hieracium is often portrayed as disastrous, as is evident in the following excerpts: “the worst possible choice” (Sturtevant 1965 , p. 11), “shattered the hopes he had entertained of finding a confirmation” (Iltis 1966 , p. 174), “a completely misguided choice” (Hennig 2000 , p. 159), and “the results were a mess” (Mukherjee 2016 , p. 55). However, a detailed examination of Mendel’s Hieracium research in his letters to Nägeli, and in the paper he published on Hieracium (Mendel 1870 ), reveals extensive and productive research. Orel ( 1996 ) characterised Mendel’s choice as “in no way unfortunate”, and “a logical step forward” (p. 184). Disparagement of Mendel’s choice is based on the misguided presumption that all species of Hieracium reproduce exclusively through apomixis, seemingly ensuring uniparental-maternal inheritance and preventing artificial hybridisation. In fact, the genus Hieracium is extraordinarily diverse (one of the reasons Mendel chose it), and its reproductive mechanisms include varying degrees of apomixis, self-fertilisation, self-incompatibility, and cross-fertilisation, as well as a powerful influence of polyploidy on apomixis (Bicknell et al. 2016 ; Mráz and Zdvořák 2019 ; Underwood et al. 2022 ). Mendel’s accounts make it clear that he, like other researchers, obtained true Hieracium hybrids, albeit not without considerable effort. He speculated that the progeny of Hieracium hybrids might remain constant, as in Geum , but he was not initially sure. His decision to choose genera that he suspected would behave differently than Pisum is admirable; it was his intentional attempt to better understand the complexity of hybridisation in nature.

In his brief paper on Hieracium , Mendel ( 1870 ) determined that “we do not possess a complete theory of hybridisation and we may be led into erroneous conclusions if we take rules deduced from observations of certain other hybrids to be Laws of hybridisation and try to apply them to Hieracium without further consideration” (Stern and Sherwood 1966 , p. 52). Mendel observed that the F 1 hybrid plants obtained from apparently true-breeding parents tended to vary among themselves, but that their F 2 progeny from apparent self-fertilisation remained constant. He clearly stated the inevitable conclusion: “In Pisum the hybrids, obtained from the immediate crossing of two forms, have the same type, but their posterity, on the contrary, are variable and follow a definite law in their variations. In Hieracium according to the present experiments exactly the opposite phenomenon seems to be exhibited” (Stern and Sherwood 1966 , p. 55). He then noted that Hieracium was not the only genus to display such behaviour, citing the research of Wichura indicating that Salix behaved similarly.

Mendel’s observations were probably due to natural heterozygosity and polyploidy in the parental plants, which appeared to him to breed true due to apomixis. When he successfully hybridised them, the F 1 progeny displayed variability due to parental heterozygosity and possible variations in ploidy, then the F 2 progeny remained constant, resembling the original F 1 parents, due again to apomixis (Bicknell et al. 2016 ; Mráz and Zdvořák 2019 ). These observations revealed “exactly the opposite” of his observations in Pisum . The fact that he observed concordance with Pisum in several genera and a range of patterns in Hieracium and other genera neither surprised nor misled him. The only true misfortune is that he published only a fraction of what he had discovered.

Mendel’s abbacy and death

After Napp’s death, Mendel was elected abbot in 1868. This change in status did not initially deter him from research; his letters to Nägeli reveal extensive hybridisation research for the next five years (1868–73). However, in his last letter to Nägeli, Mendel lamented that “I am really unhappy about having to neglect my plants and my bees so completely” (Stern and Sherwood 1966 , p. 97). By then, a bitter dispute over monastery taxation was overwhelming him. He sent his Hieracium plants and herbarium specimens to Nägeli, essentially bringing his hybridisation research to a close.

Mendel died on January 6, 1884. Had he published the enormous data he collected on plant hybridisation, his work might have been more broadly known. Why he did not do so has been a matter of speculation. One of the young friars in the monastery, Prior Alphonsus Tkadlec, recalled years later that Mendel “was even attacked and his theory suspected of being contrary to the revealed truths of the Christian religion…. In bitterness he burned everything which reminded him of his previous activity” (Orel 1996 , p. 195). Mendel’s nephew, Ferdinand Schindler, provided a contradictory account: “He often said to us nephews, that we shall find at his heritage, papers for publication, that he could not publish in his life. But we did not receive anything from the cloister, not even a thing for remembrance” (Coleman 1967 , p. 10). Antonín Doupovec, who attended to the aging abbot with his mother, remembered, “thousands of sheets of paper covered with scientific notes and data were found after his death” (Orel, 1971 , p. 270). Another young friar, Pater Clemens Janetschek claimed that most of Mendel’s papers were burned after his death, only the bound books retained (Iltis 1966 , p. 281). It is fortunate that Nägeli and his heirs preserved Mendel’s letters. Otherwise, much of his extensive research after 1866 would have remained unknown.

Mendel’s classic 1866 paper remains one of the finest examples of the nature of science, a detailed and lucid presentation of extensive data exemplifying careful experimental design, hypothesis testing, and the development of an enduring theory of heredity. His paper, as the founding document for the science of genetics, is much enhanced when viewed in the context of his life, his choices, and those who influenced him at one the most extraordinary times in the history of science. In this review, I have attempted to demystify key events in his history and scientific approach to hopefully provide a clearer view of who he was and what he accomplished as we commemorate the bicentennial of his birth.

When naming cities and places, I have used the Anglicised name if it is available (for example Moravia instead of Mähren or Morava). Because many of the places associated with Mendel are now in the Czech Republic and bear Czech names, but were known by both their German and Czech names in his day, and he typically used their German names, I have included the German name first in each instance, followed by the Czech name in parentheses, and used only the German name for each subsequent use.

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Acknowledgements

Some of the historical information in this article is based on research for a recently completed book-length biography of Mendel (Fairbanks 2022 ). I am much indebted to Jiří Sekerák, Anna Matalová, Eva Matalová, and Eva Janečková for generously sharing documentary information and offering critique, and to Peter van Dijk for sharing recent discoveries on apomixis in Hieracium . I am grateful to Barbara Mable whose editorial recommendations substantially improved the manuscript, and to three anonymous referees for their helpful and constructive comments.

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Fairbanks, D.J. Demystifying the mythical Mendel: a biographical review. Heredity 129 , 4–11 (2022). https://doi.org/10.1038/s41437-022-00526-0

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Gregor Mendel

(1822-1884)

Who Was Gregor Mendel?

Gregor Mendel, known as the "father of modern genetics," was born in Austria in 1822. A monk, Mendel discovered the basic principles of heredity through experiments in his monastery's garden. His experiments showed that the inheritance of certain traits in pea plants follows particular patterns, subsequently becoming the foundation of modern genetics and leading to the study of heredity.

Gregor Johann Mendel was born Johann Mendel on July 20, 1822, to Anton and Rosine Mendel, on his family’s farm, in what was then Heinzendorf, Austria. He spent his early youth in that rural setting, until age 11, when a local schoolmaster who was impressed with his aptitude for learning recommended that he be sent to secondary school in Troppau to continue his education. The move was a financial strain on his family, and often a difficult experience for Mendel, but he excelled in his studies, and in 1840, he graduated from the school with honors.

Following his graduation, Mendel enrolled in a two-year program at the Philosophical Institute of the University of Olmütz. There, he again distinguished himself academically, particularly in the subjects of physics and math, and tutored in his spare time to make ends meet. Despite suffering from deep bouts of depression that, more than once, caused him to temporarily abandon his studies, Mendel graduated from the program in 1843.

That same year, against the wishes of his father, who expected him to take over the family farm, Mendel began studying to be a monk: He joined the Augustinian order at the St. Thomas Monastery in Brno, and was given the name Gregor. At that time, the monastery was a cultural center for the region, and Mendel was immediately exposed to the research and teaching of its members, and also gained access to the monastery’s extensive library and experimental facilities.

In 1849, when his work in the community in Brno exhausted him to the point of illness, Mendel was sent to fill a temporary teaching position in Znaim. However, he failed a teaching-certification exam the following year, and in 1851, he was sent to the University of Vienna, at the monastery’s expense, to continue his studies in the sciences. While there, Mendel studied mathematics and physics under Christian Doppler, after whom the Doppler effect of wave frequency is named; he studied botany under Franz Unger, who had begun using a microscope in his studies, and who was a proponent of a pre-Darwinian version of evolutionary theory.

In 1853, upon completing his studies at the University of Vienna, Mendel returned to the monastery in Brno and was given a teaching position at a secondary school, where he would stay for more than a decade. It was during this time that he began the experiments for which he is best known.

Experiments and Theories

Around 1854, Mendel began to research the transmission of hereditary traits in plant hybrids. At the time of Mendel’s studies, it was a generally accepted fact that the hereditary traits of the offspring of any species were merely the diluted blending of whatever traits were present in the “parents.” It was also commonly accepted that, over generations, a hybrid would revert to its original form, the implication of which suggested that a hybrid could not create new forms. However, the results of such studies were often skewed by the relatively short period of time during which the experiments were conducted, whereas Mendel’s research continued over as many as eight years (between 1856 and 1863), and involved tens of thousands of individual plants.

Mendel chose to use peas for his experiments due to their many distinct varieties, and because offspring could be quickly and easily produced. He cross-fertilized pea plants that had clearly opposite characteristics—tall with short, smooth with wrinkled, those containing green seeds with those containing yellow seeds, etc.—and, after analyzing his results, reached two of his most important conclusions: the Law of Segregation, which established that there are dominant and recessive traits passed on randomly from parents to offspring (and provided an alternative to blending inheritance, the dominant theory of the time), and the Law of Independent Assortment, which established that traits were passed on independently of other traits from parent to offspring. He also proposed that this heredity followed basic statistical laws. Though Mendel’s experiments had been conducted with pea plants, he put forth the theory that all living things had such traits.

In 1865, Mendel delivered two lectures on his findings to the Natural Science Society in Brno, who published the results of his studies in their journal the following year, under the title Experiments on Plant Hybrids . Mendel did little to promote his work, however, and the few references to his work from that time period indicated that much of it had been misunderstood. It was generally thought that Mendel had shown only what was already commonly known at the time—that hybrids eventually revert to their original form. The importance of variability and its evolutionary implications were largely overlooked. Furthermore, Mendel's findings were not viewed as being generally applicable, even by Mendel himself, who surmised that they only applied to certain species or types of traits. Of course, his system eventually proved to be of general application and is one of the foundational principles of biology.

Later Life, Death and Legacy

In 1868, Mendel was elected abbot of the school where he had been teaching for the previous 14 years, and both his resulting administrative duties and his gradually failing eyesight kept him from continuing any extensive scientific work. He traveled little during this time and was further isolated from his contemporaries as the result of his public opposition to an 1874 taxation law that increased the tax on the monasteries to cover Church expenses.

Gregor Mendel died on January 6, 1884, at the age of 61. He was laid to rest in the monastery’s burial plot and his funeral was well attended. His work, however, was still largely unknown.

It was not until decades later, when Mendel’s research informed the work of several noted geneticists, botanists and biologists conducting research on heredity, that its significance was more fully appreciated, and his studies began to be referred to as Mendel’s Laws. Hugo de Vries, Carl Correns and Erich von Tschermak-Seysenegg each independently duplicated Mendel's experiments and results in 1900, finding out after the fact, allegedly, that both the data and the general theory had been published in 1866 by Mendel. Questions arose about the validity of the claims that the trio of botanists were not aware of Mendel's previous results, but they soon did credit Mendel with priority. Even then, however, his work was often marginalized by Darwinians, who claimed that his findings were irrelevant to a theory of evolution. As genetic theory continued to develop, the relevance of Mendel’s work fell in and out of favor, but his research and theories are considered fundamental to any understanding of the field, and he is thus considered the "father of modern genetics."

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• Name: Gregor Mendel
• Birth Year: 1822
• Birth date: July 20, 1822
• Birth City: Heinzendorf
• Birth Country: Austria
• Gender: Male
• Best Known For: Gregor Mendel was an Austrian monk who discovered the basic principles of heredity through experiments in his garden. Mendel's observations became the foundation of modern genetics and the study of heredity, and he is widely considered a pioneer in the field of genetics.
• Science and Medicine
• Astrological Sign: Cancer
• University of Vienna
• University of Olmütz
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• Death Year: 1884
• Death date: January 6, 1884
• Death City: Brno
• Death Country: Austria

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• Original Published Date: April 2, 2014
• My scientific studies have afforded me great gratification; and I am convinced that it will not be long before the whole world acknowledges the results of my work.

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• Biology Article
• Mendel Laws Of Inheritance

Mendel's Laws of Inheritance

Inheritance can be defined as the process of how a child receives genetic information from the parent. The whole process of heredity is dependent upon inheritance and it is the reason that the offsprings are similar to the parents. This simply means that due to inheritance, the members of the same family possess similar characteristics.

It was only during the mid 19th century that people started to understand inheritance in a proper way. This understanding of inheritance was made possible by a scientist named Gregor Mendel, who formulated certain laws to understand inheritance known as Mendel’s laws of inheritance.

Table of Contents

Mendel’s Laws of Inheritance

Why was pea plant selected for mendel’s experiments, mendel’s experiments, conclusions from mendel’s experiments, mendel’s laws, key points on mendel’s laws.

Between 1856-1863, Mendel conducted the hybridization experiments on the garden peas. During that period, he chose some distinct characteristics of the peas and conducted some cross-pollination/ artificial pollination on the pea lines that showed stable trait inheritance and underwent continuous self-pollination. Such pea lines are called true-breeding pea lines.

Also Refer:   Mendel’s Laws of Inheritance: Mendel’s Contribution

He selected a pea plant for his experiments for the following reasons:

• The pea plant can be easily grown and maintained.
• They are naturally self-pollinating but can also be cross-pollinated.
• It is an annual plant, therefore, many generations can be studied within a short period of time.
• It has several contrasting characters.

Mendel conducted 2 main experiments to determine the laws of inheritance. These experiments were:

Monohybrid Cross

Dihybrid cross.

While experimenting, Mendel found that certain factors were always being transferred down to the offspring in a stable way. Those factors are now called genes i.e. genes can be called the units of inheritance.

Mendel experimented on a pea plant and considered 7 main contrasting traits in the plants. Then, he conducted both experiments to determine the inheritance laws. A brief explanation of the two experiments is given below.

In this experiment, Mendel took two pea plants of opposite traits (one short and one tall) and crossed them. He found the first generation offspring were tall and called it F1 progeny. Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1. To know more about this experiment, visit Monohybrid Cross – Inheritance Of One Gene .

Mendel even conducted this experiment with other contrasting traits like green peas vs yellow peas, round vs wrinkled, etc. In all the cases, he found that the results were similar. From this, he formulated the laws of Segregation And Dominance .

In a dihybrid cross experiment, Mendel considered two traits, each having two alleles. He crossed wrinkled-green seed and round-yellow seeds and observed that all the first generation progeny (F1 progeny) were round-yellow. This meant that dominant traits were the round shape and yellow colour.

He then self-pollinated the F1 progeny and obtained 4 different traits: round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds in the ratio 9:3:3:1.

Check Dihybrid Cross and Inheritance of Two Genes to know more about this cross.

After conducting research for other traits, the results were found to be similar. From this experiment, Mendel formulated his second law of inheritance i.e. law of Independent Assortment.

• The genetic makeup of the plant is known as the genotype. On the contrary, the physical appearance of the plant is known as phenotype.
• The genes are transferred from parents to the offspring in pairs known as alleles.
• During gametogenesis when the chromosomes are halved, there is a 50% chance of one of the two alleles to fuse with the allele of the gamete of the other parent.
• When the alleles are the same, they are known as homozygous alleles and when the alleles are different they are known as heterozygous alleles.

Also Refer:   Mendelian Genetics

The two experiments lead to the formulation of Mendel’s laws known as laws of inheritance which are:

• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

This is also called Mendel’s first law of inheritance. According to the law of dominance, hybrid offspring will only inherit the dominant trait in the phenotype. The alleles that are suppressed are called the recessive traits while the alleles that determine the trait are known as the dominant traits.

The law of segregation states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. In other words, allele (alternative form of the gene) pairs segregate during the formation of gamete and re-unite randomly during fertilization. This is also known as Mendel’s third law of inheritance.

Also known as Mendel’s second law of inheritance, the law of independent assortment states that a pair of traits segregates independently of another pair during gamete formation. As the individual heredity factors assort independently, different traits get equal opportunity to occur together.

• The law of inheritance was proposed by Gregor Mendel after conducting experiments on pea plants for seven years.
• Mendel’s laws of inheritance include law of dominance, law of segregation and law of independent assortment.
• The law of segregation states that every individual possesses two alleles and only one allele is passed on to the offspring.
• The law of independent assortment states that the inheritance of one pair of genes is independent of inheritance of another pair.

Also Read:   Non-Mendelian Inheritance

Stay tuned with BYJU’S to learn more about Mendel’s Laws of Inheritance. You can also download the BYJU’S app for further reference on Mendel’s laws.

Frequently Asked Questions

What are the three laws of inheritance proposed by mendel.

The three laws of inheritance proposed by Mendel include:

Which is the universally accepted law of inheritance?

Law of segregation is the universally accepted law of inheritance. It is the only law without any exceptions. It states that each trait consists of two alleles which segregate during the formation of gametes and one allele from each parent combines during fertilization.

Why is the law of segregation known as the law of purity of gametes?

The law of segregation is known as the law of purity of gametes because a gamete carries only a recessive or a dominant allele but not both the alleles.

Why was the pea plant used in Mendel’s experiments?

Mendel picked pea plants in his experiments because the pea plant has different observable traits. It can be grown easily in large numbers and its reproduction can be manipulated. Also, pea has both male and female reproductive organs, so they can self-pollinate as well as cross-pollinate.

What was the main aim of Mendel’s experiments?

The main aim of Mendel’s experiments was:

• To determine whether the traits would always be recessive.
• Whether traits affect each other as they are inherited.
• Whether traits could be transformed by DNA.

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very nice. it is the best to study about genetics

Genetic inheritance is so interesting

It helped me a lot Thanks

It is so amazing thanks a lot

Superb, it’s interesting.

It is very useful becoz all details explain in simple manner with examples

AWESOME, the above notes are fabulous

well that helped me a lot

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It helped me alot

If Mendel gave three law the what is the law of unit of characters and who proposed this law . Please clear my doubt a little bit faster , it is little important for me.

The Law of unit characters was proposed by Mendel. He explained that the inheritance of a trait is controlled by unit characters or factors, which are passed from parents to offspring through the gametes. These factors are now known as genes. Each factor exists in pairs, which are known as alleles.

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Module 11: Trait Inheritance

Mendel’s experiments and heredity, learning outcomes.

• Describe Mendel’s study of garden peas and hereditary

Figure 1. Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics. (credit: modification of work by Jerry Kirkhart)

Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel selected a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because of Mendel’s work, the fundamental principles of heredity were revealed. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the capability to be replicated, expressed, or mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel’s experiments serve as an excellent starting point for thinking about inheritance.

Mendel’s Experiments and the Laws of Probability

Figure 2. Johann Gregor Mendel is considered the father of genetics.

Johann Gregor Mendel (1822–1884) (Figure 2) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary  model system (a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community that believed, incorrectly, that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring; this hypothetical process appeared to be correct because of what we know now as continuous variation.  Continuous variation results from the action of many genes to determine a characteristic like human height. Offspring appear to be a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. The blending theory of inheritance asserted that the original parental traits were lost or absorbed by the blending in the offspring, but we now know that this is not the case. Mendel was the first researcher to see it. Instead of continuous characteristics, Mendel worked with traits that were inherited in distinct classes (specifically, violet versus white flowers); this is referred to as discontinuous variation . Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring, nor were they absorbed, but rather that they kept their distinctness and could be passed on. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel’s Model System

Mendel’s seminal work was accomplished using the garden pea,  Pisum sativum , to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendelian Crosses

Mendel performed  hybridizations , which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. In plants, pollen carries the male gametes (sperm) to the stigma, a sticky organ that traps pollen and allows the sperm to move down the pistil to the female gametes (ova) below. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant’s flowers before they had a chance to mature.

Plants used in first-generation crosses were called P 0 , or parental generation one, plants (Figure 3). Mendel collected the seeds belonging to the P 0 plants that resulted from each cross and grew them the following season. These offspring were called the  F 1 , or the first filial ( filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendel’s experiments extended beyond the F 2 generation to the F 3 and F 4 generations, and so on, but it was the ratio of characteristics in the P 0 −F 1 −F 2 generations that were the most intriguing and became the basis for Mendel’s postulates.

Figure 3. In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P 0 generation). The resulting hybrids in the F 1 generation all had violet flowers. In the F 2 generation, approximately three quarters of the plants had violet flowers, and one quarter had white flowers.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A  trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants, reporting results from 19,959 F 2 plants alone. His findings were consistent.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait in the F 1 generation had completely disappeared.

Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that, of F 2 -generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers per one white flower, or approximately 3:1 . When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of which parent, male or female, contributed which trait. This is called a  reciprocal cross —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics Mendel examined, the F 1 and F 2 generations behaved in the same way as they had for flower color. One of the two traits would disappear completely from the F 1 generation only to reappear in the F 2 generation at a ratio of approximately 3:1 (Table 1).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these, respectively, dominant and recessive traits.  Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear, in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate (not blended) in the plants of the F 1 generation. Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together . Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

In Summary: Mendel’s Experiments and Heredity

Working with garden pea plants, Mendel found that crosses between parents that differed by one trait produced F 1 offspring that all expressed the traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the F 2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P 0 parent. Reciprocal crosses generated identical F 1 and F 2 offspring ratios. By examining large sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that the traits were inherited as independent events.

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8 Chapter 8: Mendel’s Experiments and Heredity

Chapter outline.

• 8.1 Mendelian Genetics

8.2 Characteristics and Traits

8.3 laws of inheritance.

• 8.4 Chromosomal Theory and Genetic Linkage
• 8.5 Chromosomal Basics of Inherited Disorders

Figure 8.1 The species of pea plant the Gregor Mendel used in his experiments to discover patterns of inheritance.

Learning Objectives

 You will be able to describe how traits are passed to offspring:

•  Identify the relationship between chromosomes, genes and alleles.
• Understand the principles of simple inheritance
• Identify the terms homozygous, heterozygous, dominant and recessive
• Use a Punnett square to predict the inheritance of a simple trait
• Recognize that most human traits have complex inheritance patterns.
• Understand the genetics of sex determination
• Describe some examples of genetic disorders
• Understand how chromosomes contribute to disorders

8.1 | Mendelian Genetics

Figure 8.2 Johann Gregor Mendel is considered the father of genetics. 1.Johann Gregor Mendel, Versuche über Pflanzenhybriden Verhandlungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr, 1865 Abhandlungen,3–47. [for English translation see http://www.mendelweb.org/Mendel.plain.html]

Mendel’s Model System

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum , to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendelian Crosses

Mendel performed hybridizations, which involve mating two true-breeding individuals that have different traits. In the pea,which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant ofone variety to the stigma of a separate mature pea plant of the second variety. In plants, pollen carries the male gametes(sperm) to the stigma, a sticky organ that traps pollen and allows the sperm to move down the pistil to the female gametes(ova) below. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendelpainstakingly removed all of the anthers from the plant’s flowers before they had a chance to mature.Plants used in first-generation crosses were called P0, or parental generation one, plants (Figure 12.3). Mendel collectedthe seeds belonging to the P0 plants that resulted from each cross and grew them the following season. These offspring werecalled the F1, or the first filial (filial = offspring, daughter or son), generation. Once Mendel examined the characteristicsin the F1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F1plants to produce the F2, or second filial, generation. Mendel’s experiments extended beyond the F2 generation to the F3 andF4 generations, and so on, but it was the ratio of characteristics in the P0−F1−F2 generations that were the most intriguingand became the basis for Mendel’s postulates.

Chapter 12 | Mendel’s Experiments and Heredity 315

Figure 8.3 In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P generation). The resulting hybrids in the F1generation all had violet flowers. In the F2 generation, approximately three quarters of the plants had violet flowers, and one quarter had white flowers.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F1 and F2 plants, reporting results from 19,959 F2 plants alone. His findings were consistent.What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of theF1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait in the F1 generation had completely disappeared.

Importantly, Mendel did not stop his experimentation there. He allowed the F1 plants to self-fertilize and found that, of F2-generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers per one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of which parent, male or female, contributed which trait. This is called a reciprocal cross—a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics Mendel examined, the F1 and F2 generations behaved in the same way as they had for flower color. One of the two traits would disappear completely from the F1 generation only to reappear in the F2 generation at a ratio of approximately 3:1 (Table 12.1).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these, respectively, dominant and recessive traits. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear, in the offspring of a hybridization.The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F2 generation meant that the traits remained separate (not blended) in the plants of the F1 generation.

Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

So why did Mendel repeatedly obtain 3:1 ratios in his crosses? To understand how Mendel deduced the basic mechanisms of inheritance that lead to such ratios, we must first review the laws of probability.

Probability Basics

Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur. It is also possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur by the number of times that it could occur. Empirical probabilities come from observations, like those of Mendel. Theoretical probabilities come from knowing how the events are produced and assuming that the probabilities of individual outcomes are equal. A probability of one for some event indicates that it is guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur.

8.2 | Characteristics and Traits

By the end of this section, you will be able to:• Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems• Develop a Punnett square to calculate the expected proportions of genotypes and phenotypes in a monohybrid cross• Explain the purpose and methods of a test cross• Identify non-Mendelian inheritance patterns such as incomplete dominance, co-dominance, recessive lethals, multiple alleles, and sex linkage.

For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles. Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population.

Phenotypes and Genotypes

Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F2 offspring. Therefore, theF1 plants must have been genotypically different from the parent with yellow pods.The P1 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes.Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P1 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined.

Dominant and Recessive Alleles

Our discussion of homozygous and heterozygous organisms brings us to why the F1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals (Table 8.4).

Several conventions exist for referring to genes and alleles. For the purposes of this chapter, we will abbreviate genes using the first letter of the gene’s corresponding dominant trait. For example, violet is the dominant trait for a pea plant’s flower color, so the flower-color gene would be abbreviated as V (note that it is customary to italicize gene designations). Furthermore, we will use uppercase and lowercase letters to represent dominant and recessive alleles, respectively. Therefore, we would refer to the genotype of a homozygous dominant pea plant with violet flowers as VV, a homozygous recessive pea plant with white flowers as vv, and a heterozygous pea plant with violet flowers as Vv.

The Punnett Square Approach for a Monohybrid Cross

When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, and the resulting offspring are monohybrids. Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F1 and F2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely. To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds, respectively.

A Punnett square, devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies. To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow

seeds ( Figure 8.4 ).

Figure 8.4 In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype.

Punnett square analysis can be used to predict the genotypes of the F2 generation.A self-cross of one of the Yy heterozygous offspring can be represented in a  Punnett square because each parent can donate one of two different alleles. Therefore, the offspring can potentially have one of four allele combinations: YY,Yy, yY, or yy ( Figure 8.4 ). Notice that there are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm. Both of these possibilities must be counted. Recall that Mendel’s pea plant characteristics behaved in the same way in reciprocal crosses. Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents. They are grouped together. Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1 ( Figure 8.4 ). Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green. Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.Mendel validated these results by performing an F3 cross in which he self-crossed the dominant- and recessive-expressingF2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous(Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross.

The Test Cross Distinguishes the Dominant Phenotype

Figure 8.6 Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal allele, so their genotype gets the “A?” designation.What are the genotypes of the individuals labeled 1, 2 and 3?

Alternatives to Dominance and Recessiveness

Mendel’s experiments with pea plants suggested that: (1) two “units” or alleles exist for every gene; (2) alleles maintain their integrity in each generation (no blending); and (3) in the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype. Therefore, recessive alleles can be “carried” and not expressed by individuals.Such heterozygous individuals are sometimes referred to as “carriers.” Further genetic studies in other plants and animals have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelism. If Mendel had chosen an experimental system that exhibited these genetic complexities, it’s possible that he would not have understood what his results meant.

Incomplete Dominance

Mendel’s results, that traits are inherited as dominant and recessive pairs, contradicted the view at that time that offspring exhibited a blend of their parents’ traits. However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents. For example, in the snapdragon, Antirrhinum majus ( Figure 8.7 ), a cross between a homozygous parent with white flowers (CWCW) and a homozygous parent with red flowers (CRCR) will produce offspring with pink flowers (CRCW). (Note that different genotypic abbreviations are used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.) This pattern of inheritance is described as incomplete dominance, denoting the expression of two contrasting alleles such that the individual displays an intermediate phenotype. The allele for redCRCW:1 CWCW, and the phenotypic ratio would be 1:2:1 for red:pink:white.

Figure 8.7 These pink flowers of a heterozygote snapdragon result from incomplete dominance. (credit:“storebukkebruse”/Flickr)

Codominance

A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance is the MN blood groups of humans. The M and N alleles are expressed in the form of an M or N antigen present on the surface of red blood cells. Homozygotes (LMLM andLNLN) express either the M or the N allele, and heterozygotes (LMLN) express both alleles equally. In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct. However, the 1:2:1 genotypic ratio characteristic of a Mendelian monohybrid cross still applies.

Multiple Alleles

Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed. Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype among wild animals as the wild type (often abbreviated “+”); this is considered the standard or norm. All other phenotypes or genotypes are considered variants of this standard, meaning that they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.An example of multiple alleles is coat color in rabbits ( Figure 8.8 ). Here, four alleles exist for the c gene. The wild-type version, C+C+, is expressed as brown fur. The chinchilla phenotype, cchcch, is expressed as black-tipped white fur. TheHimalayan phenotype, chch, has black fur on the extremities and white fur elsewhere. Finally, the albino, or “colorless”phenotype, cc, is expressed as white fur. In cases of multiple alleles, dominance hierarchies can exist. In this case, the wild type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino. This hierarchy, or allelic series, was revealed by observing the phenotypes of each possible heterozygote offspring.

Figure 8.8 Four different alleles exist for the rabbit coat color (C) gene.The complete dominance of a wild-type phenotype over all other mutants often occurs as an effect of “dosage” of a specific gene product, such that the wild-type allele supplies the correct amount of gene product whereas the mutant alleles cannot.For the allelic series in rabbits, the wild-type allele may supply a given dosage of fur pigment, whereas the mutants supply a lesser dosage or none at all. Interestingly, the Himalayan phenotype is the result of an allele that produces a temperature sensitive gene product that only produces pigment in the cooler extremities of the rabbit’s body.Alternatively, one mutant allele can be dominant over all other phenotypes, including the wild type. This may occur when the mutant allele somehow interferes with the genetic message so that even a heterozygote with one wild-type allele copy expresses the mutant phenotype. One way in which the mutant allele can interfere is by enhancing the function of the wild type gene product or changing its distribution in the body. One example of this is the Antennapedia mutation in Drosophila(Figure 12.9). In this case, the mutant allele expands the distribution of the gene product, and as a result, the Antennapedia heterozygote develops legs on its head where its antennae should be.

Figure 8.9 As seen in comparing the wild-type Drosophila (left) and the Antennapedia mutant (right), the Antennapedia mutant has legs on its head in place of antennae.

Multiple Alleles Confer Drug Resistance in the Malaria Parasite

Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae (Figure 12.10a), and is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia. Plasmodium falciparum and P. vivax are the most common causative agents of malaria, and P. falciparum is the most deadly (Figure 12.10b). When promptly and correctly treated, P. falciparum malaria has a mortality rate of 0.1 percent. However, in some parts of the world, the parasite has evolved resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by geographic region.

Figure 8.10

2. Sumiti Vinayak, et al., “Origin and Evolution of Sulfadoxine Resistant Plasmodium falciparum,” Public Library of Science Pathogens 6, no. 3 (2010):e1000830, doi:10.1371/journal.ppat.1000830.

X-Linked Traits

In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes.The sex chromosomes are one pair of non-homologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair.

Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes. When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. Eye color in Drosophila was one of the first X-linked traits to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in 1910. Like humans, Drosophila males have an XY chromosome pair, and females are XX. In flies, the wild-type eye color is red (XW) and it is dominant to white eye color (Xw) (Figure 12.11). Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous, because they have only one allele for any X-linked characteristic. Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males. Drosophila males lack a second allele copy on the Y chromosome; that is, their genotype can only be XWY or XwY. In contrast, females have two allele copies of this gene and can be XWXW, XWXw, or XwXw.

Figure 8.11 In Drosophila, several genes determine eye color.

The genes for white and vermilion eye colors are located on the X chromosome. Others are located on the autosomes. Clockwise from top left are brown, cinnabar, sepia, vermilion, white, and red. Red eye color is wild-type and is dominant to white eye color.In an X-linked cross, the genotypes of F1 and F2 offspring depend on whether the recessive trait was expressed by the male or the female in the P1 generation. With regard to Drosophila eye color, when the P1 male expresses the white-eye phenotype and the female is homozygous red-eyed, all members of the F1 generation exhibit red eyes (Figure 12.12).The F1 females are heterozygous (XWXw), and the males are all XWY, having received their X chromosome from the homozygous dominant P1 female and their Y chromosome from the P1 male. A subsequent cross between the XWXw female and the XWY male would produce only red-eyed females (with XWXW or XWXw genotypes) and both red- and white-eyed males (with XWY or XwY genotypes). Now, consider a cross between a homozygous white-eyed female and a male with red eyes. The F1 generation would exhibit only heterozygous red-eyed females (XWXw) and only white-eyed males (XwY).Half of the F2 females would be red-eyed (XWXw) and half would be white-eyed (XwXw). Similarly, half of the F2 males would be red-eyed (XWY) and half would be white-eyed (XwY).

Figure 8.12

Discoveries in fruit fly genetics can be applied to human genetics. When a female parent is homozygous for a recessive X linked trait, she will pass the trait on to 100 percent of her offspring. Her male offspring are, therefore, destined to express the trait, as they will inherit their father’s Y chromosome. In humans, the alleles for certain conditions (some forms of colorblindness, hemophilia, and muscular dystrophy) are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, recessive X-linked traits appear more frequently in males than females.In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous. Human Sex-linked DisordersSex-linkage studies in Morgan’s laboratory provided the fundamentals for understanding X-linked recessive disorders in humans, which include red-green color blindness, and Types A and B hemophilia. Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. When they inherit one recessiveX-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected. Carrier females can manifest mild forms of the trait due to the inactivation of the dominant allele located on one of the X chromosomes. However, female carriers can contribute the trait to their sons, resulting in the son exhibiting the trait, or they can contribute the recessive allele to their daughters, resulting in the daughters being carriers of the trait (Figure 12.13).Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted to subsequent generations.

Figure 8.13 The son of a woman who is a carrier of a recessive X-linked disorder will have a 50 percent chance of being affected. A daughter will not be affected, but she will have a 50 percent chance of being a carrier like her mother.

Watch this video ( http://openstaxcollege.org/l/sex-linked_trts ) to learn more about sex-linked traits.

A large proportion of genes in an individual’s genome are essential for survival. Occasionally, a nonfunctional allele for an essential gene can arise by mutation and be transmitted in a population as long as individuals with this allele also have a wild-type, functional copy. The wild-type allele functions at a capacity sufficient to sustain life and is therefore considered to be dominant over the nonfunctional allele. However, consider two heterozygous parents that have a genotype of wild type/nonfunctional mutant for a hypothetical essential gene. In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the nonfunctional allele. Because the gene is essential, these individuals might fail to develop past fertilization, die in utero, or die later in life, depending on what life stage requires this gene.

Dominant lethal alleles are very rare because, as you might expect, the allele only lasts one generation and is not transmitted. However, just as the recessive lethal allele might not immediately manifest the phenotype of death, dominant lethal alleles also might not be expressed until adulthood. Once the individual reaches reproductive age, the allele may be unknowingly passed on, resulting in a delayed death in both generations. An example of this in humans is Huntington’s disease, in which the nervous system gradually wastes away (Figure 12.14). People who are heterozygous for the dominant Huntington allele (Hh) will inevitably develop the fatal disease. However, the onset of Huntington’s disease may not occur until age 40, at which point the afflicted persons may have already passed the allele to 50 percent of their offspring.

Figure 8.14 The neuron in the center of this micrograph (yellow) has nuclear inclusions characteristic of Huntington’sdisease (orange area in the center of the neuron). Huntington’s disease occurs when an abnormal dominant allele forthe Huntington gene is present. (credit: Dr. Steven Finkbeiner, Gladstone Institute of Neurological Disease, The Taube-Koret Center for Huntington’s Disease Research, and the University of California San Francisco/Wikimedia)

8.3 | Laws of Inheritance

By the end of this section, you will be able to:

• Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of meiosis
• Use the forked-line method and the probability rules to calculate the probability of genotypes and phenotypes from multiple gene crosses
• Explain the effect of linkage and recombination on gamete genotypes
• Explain the phenotypic outcomes of epistatic effects between genes

Mendel generalized the results of his pea-plant experiments into four postulates, some of which are sometimes called“laws,” that describe the basis of dominant and recessive inheritance in diploid organisms. As you have learned, more complex extensions of Mendelism exist that do not exhibit the same F2 phenotypic ratios (3:1). Nevertheless, these laws summarize the basics of classical genetics.

Pairs of Unit Factors, or Genes

Mendel proposed first that paired unit factors of heredity were transmitted faithfully from generation to generation by the dissociation and reassociation of paired factors during gametogenesis and fertilization, respectively. After he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F2 generation, Mendel deduced that hereditary factors must be inherited as discrete units. This finding contradicted the belief at that time that parental traits were blended in the offspring.

Alleles Can Be Dominant or Recessive

Mendel’s law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively. The recessive allele will remain “latent” but will be transmitted to offspring by the same manner in which the dominant allele is transmitted. The recessive trait will only be expressed by offspring that have two copies of this allele (Figure 12.15), and these offspring will breed true when self-crossed.

Since Mendel’s experiments with pea plants, other researchers have found that the law of dominance does not always hold true. Instead, several different patterns of inheritance have been found to exist.

Figure 8.15 The child in the photo expresses albinism, a recessive trait.

Equal Segregation of Alleles

Observing that true-breeding pea plants with contrasting traits gave rise to F1 generations that all expressed the dominant trait and F2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation. This law states that paired unit factors (genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor. For the F2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive. Because heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and homozygous dominant individuals are phenotypically identical, the law supports Mendel’s observed 3:1 phenotypic ratio. The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes. The physical basis of Mendel’s law of segregation is the first division of meiosis, in which the homologous chromosomes with their different versions of each gene are segregated into daughter nuclei. The role of the meiotic segregation of chromosomes in sexual reproduction was not understood by the scientific community during Mendel’s lifetime.

Independent Assortment

Mendel’s law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur. The independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics. Consider the characteristics of seed color and seed texture for two pea plants, one that has green, wrinkled seeds (yyrr) and another that has yellow, round seeds (YYRR). Because each parent is homozygous, the law of segregation indicates that the gametes for the green/wrinkled plant all are yr, and the gametes for the yellow/round plant are all YR. Therefore, the F1 generation of offspring all are YyRr (Figure 12.16).

Figure 8.16

For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele. Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed, as follows: YR, Yr, yR, and yr. Arranging these gametes along the top and left of a 4 by 4 Punnett square (Figure 12.16) gives us 16 equally likely genotypic combinations. From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green (Figure 12.16). These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size. Because of independent assortment and dominance, the 9:3:3:1 dihybrid phenotypic ratio can be collapsed into two 3:1 ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern. Ignoring seed color and considering only seed texture in the above dihybrid cross, we would expect that three quarters of the F2 generation offspring would be round, and one quarter would be wrinkled. Similarly, isolating only seed color, we would assume that three quarters of the F2 offspring would be yellow and one quarter would be green. The sorting of alleles for texture and color are independent events, so we can apply the product rule. Therefore, the proportion of round and yellow F2 offspring is expected to be (3/4) Å~ (3/4) = 9/16, and the proportion of wrinkled and green offspring is expected to be (1/4) Å~ (1/4) = 1/16.

These proportions are identical to those obtained using a Punnett square. Round, green and wrinkled, yellow offspring can also be calculated using the product rule, as each of these genotypes includes one dominant and one recessive phenotype. Therefore, the proportion of each is calculated as (3/4) Å~ (1/4) = 3/16. The law of independent assortment also indicates that a cross between yellow, wrinkled (YYrr) and green, round (yyRR) parents would yield the same F1 and F2 offspring as in the YYRR x yyrr cross.

Linked Genes Violate the Law of Independent Assortment

Although all of Mendel’s pea characteristics behaved according to the law of independent assortment, we now know that some allele combinations are not inherited independently of each other. Genes that are located on separate nonhomologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string. The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair. However, because of the process of recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. To understand this, let’s consider the biological basis of gene linkage and recombination.

Homologous chromosomes possess the same genes in the same linear order. The alleles may differ on homologous chromosome pairs, but the genes to which they correspond do not. In preparation for the first division of meiosis, homologous chromosomes replicate and synapse. Like genes on the homologs align with each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material (Figure 12.18). This process is called recombination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.

Figure 8.18 The process of crossover, or recombination, occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic material. Here, the alleles for gene C were exchanged. The result is two recombinant and two non-recombinant chromosomes.

When two genes are located in close proximity on the same chromosome, they are considered linked, and their alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will go together into a gamete and the short and yellow alleles will go into other gametes. These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles. If you create the Punnett square with these gametes, you will see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply. As the distance between two genes increases, the probability of one or more crossovers between them increases, and the genes behave more like they are on separate chromosomes. Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a measure of how far apart genes are on a chromosome. Using this information, they have constructed elaborate maps of genes on chromosomes for well-studied organisms, including humans. Mendel’s seminal publication makes no mention of linkage, and many researchers have questioned whether he encountered linkage but chose not to publish those crosses out of concern that they would invalidate his independent assortment postulate. The garden pea has seven chromosomes, and some have suggested that his choice of seven characteristics was not a coincidence. However, even if the genes he examined were not located on separate chromosomes, it is possible that he simply did not observe linkage because of the extensive shuffling effects of recombination.

Figure 8.20 In mice, the mottled agouti coat color (A) is dominant to a solid coloration, such as black or gray. A gene at a separate locus (C) is responsible for pigment production. The recessive c allele does not produce pigment, and a mouse with the homozygous recessive cc genotype is albino regardless of the allele present at the A locus. Thus, the C gene is epistatic to the A gene. Epistasis can also occur when a dominant allele masks expression at a separate gene.

Fruit color in summer squash is expressed in this way. Homozygous recessive expression of the W gene (ww) coupled with homozygous dominant or heterozygous expression of the Y gene (YY or Yy) generates yellow fruit, and the wwyy genotype produces green fruit. However, if a dominant copy of the W gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the Y alleles. A cross between white heterozygotes for both genes (WwYy Å~ WwYy) would produce offspring with a phenotypic ratio of 12 white:3 yellow:1 green.

Finally, epistasis can be reciprocal such that either gene, when present in the dominant (or recessive) form, expresses the same phenotype. In the shepherd’s purse plant ( Capsella bursa-pastoris ), the characteristic of seed shape is controlled by two genes in a dominant epistatic relationship. When the genes A and B are both homozygous recessive (aabb), the seeds are ovoid. If the dominant allele for either of these genes is present, the result is triangular seeds. That is, every possible genotype other than aabb results in triangular seeds, and a cross between heterozygotes for both genes (AaBb x AaBb) would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.

For an excellent review of Mendel’s experiments and to perform your own crosses and identify patterns of inheritance, visit the Mendel’s Peas ( http://openstaxcollege.org/l/mendels_peas ) web lab.

Figure 8.21  Chromosomes are threadlike nuclear structures consisting of DNA and proteins that serve as the repositories for genetic information. The chromosomes depicted here were isolated from a fruit fly’s salivary gland, stained with dye, and visualized under a microscope. Akin to miniature bar codes, chromosomes absorb different dyes to produce characteristic banding patterns, which allows for their routine identification. (credit: modification of work by“LPLT”/Wikimedia Commons; scale-bar data from Matt Russell)

8.4 | Chromosomal Theory and Genetic Linkage

Long before chromosomes were visualized under a microscope, the father of modern genetics, Gregor Mendel, began studying heredity in 1843.With the improvement of microscopic techniques during the late 1800s, cell biologists could stain and visualize subcellular structures with dyes and observe their actions during cell division and meiosis. With each mitotic division, chromosomes replicated, condensed from an amorphous (no constant shape) nuclear mass into distinct X-shaped bodies (pairs of identical sister chromatids), and migrated to separate cellular poles.

The speculation that chromosomes might be the key to understanding heredity led several scientists to examine Mendel’s publications and re-evaluate his model in terms of the behavior of chromosomes during mitosis and meiosis. In 1902, Theodor Boveri observed that proper embryonic development of sea urchins does not occur unless chromosomes are present. That same year, Walter Sutton observed the separation of chromosomes into daughter cells during meiosis (Figure13.2). Together, these observations led to the development of the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance.

Figure 8.22  (a) Walter Sutton and (b) Theodor Boveri are credited with developing the Chromosomal Theory ofInheritance, which states that chromosomes carry the unit of heredity (genes).

• During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
• The sorting of chromosomes from each homologous pair into pre-gametes appears to be random.
• Each parent synthesizes gametes that contain only half of their chromosomal complement.
• Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.
• The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.

Despite compelling correlations between the behavior of chromosomes during meiosis and Mendel’s abstract laws, the Chromosomal Theory of Inheritance was proposed long before there was any direct evidence that traits were carried on chromosomes. Critics pointed out that individuals had far more independently segregating traits than they had chromosomes. It was only after several years of carrying out crosses with the fruit fly, Drosophila melanogaster , that Thomas Hunt Morgan provided experimental evidence to support the Chromosomal Theory of Inheritance.

Homologous Recombination

In 1909, Frans Janssen observed chiasmata—the point at which chromatids are in contact with each other and may exchange segments—prior to the first division of meiosis. He suggested that alleles become unlinked and chromosomes physically exchange segments. As chromosomes condensed and paired with their homologs, they appeared to interact at distinct points. Janssen suggested that these points corresponded to regions in which chromosome segments were exchanged. It is now known that the pairing and interaction between homologous chromosomes, known as synapsis, does more than simply organize the homologs for migration to separate daughter cells. When synapsed, homologous chromosomes undergo reciprocal physical exchanges at their arms in a process called homologous recombination, or more simply, “crossing over.”

To better understand the type of experimental results that researchers were obtaining at this time, consider a heterozygous individual that inherited dominant maternal alleles for two genes on the same chromosome (such as AB) and two recessive paternal alleles for those same genes (such as ab). If the genes are linked, one would expect this individual to produce gametes that are either AB or ab with a 1:1 ratio. If the genes are unlinked, the individual should produce AB, Ab, aB, and ab gametes with equal frequencies, according to the Mendelian concept of independent assortment. Because they correspond to new allele combinations, the genotypes Ab and aB are nonparental types that result from homologous recombination during meiosis. Parental types are progeny that exhibit the same allelic combination as their parents. Morgan and his colleagues, however, found that when such heterozygous individuals were test crossed to a homozygous recessive parent (AaBb ~ aabb), both parental and nonparental cases occurred. For example, 950 offspring might be recovered that were either AaBb or aabb, but 50 offspring would also be obtained that were either Aabb or aaBb. These results suggested that linkage occurred most often, but a significant minority of offspring were the products of recombination.

Figure 8.23  Inheritance patterns of unlinked and linked genes are shown.

Figure 8.24  This genetic map orders Drosophila genes on the basis of recombination frequency.

In 1931, Barbara McClintock and Harriet Creighton demonstrated the crossover of homologous chromosomes in corn plants. Weeks later, homologous recombination in Drosophila was demonstrated microscopically by Curt Stern. Stern observed several X-linked phenotypes that were associated with a structurally unusual and dissimilar X chromosome pair in which one X was missing a small terminal segment, and the other X was fused to a piece of the Y chromosome. By crossing flies, observing their offspring, and then visualizing the offspring’s chromosomes, Stern demonstrated that every time the offspring allele combination deviated from either of the parental combinations, there was a corresponding exchange of an X chromosome segment. Using mutant flies with structurally distinct X chromosomes was the key to observing the products of recombination because DNA sequencing and other molecular tools were not yet available. It is now known that homologous chromosomes regularly exchange segments in meiosis by reciprocally breaking and rejoining their DNA at precise locations.

Review Sturtevant’s process to create a genetic map on the basis of recombination frequencies here ( http://openstaxcollege.org/l/gene_crossover ) .

Mendel’s Mapped Traits

Homologous recombination is a common genetic process, yet Mendel never observed it. Had he investigated both linked and unlinked genes, it would have been much more difficult for him to create a unified model of his data on the basis of probabilistic calculations. Researchers who have since mapped the seven traits investigated by Mendel onto the seven chromosomes of the pea plant genome have confirmed that all of the genes he examined are either on separate chromosomes or are sufficiently far apart as to be statistically unlinked. Some have suggested that Mendel was enormously lucky to select only unlinked genes, whereas others question whether Mendel discarded any data suggesting linkage. In any case, Mendel consistently observed independent assortment because he examined genes that were effectively unlinked.

8.5 | Chromosomal Basis of Inherited Disorders

Inherited disorders can arise when chromosomes behave abnormally during meiosis. Chromosome disorders can be divided into two categories: abnormalities in chromosome number and chromosomal structural rearrangements. Because even small segments of chromosomes can span many genes, chromosomal disorders are characteristically dramatic and often fatal.

Identification of Chromosomes

The isolation and microscopic observation of chromosomes forms the basis of cytogenetics and is the primary method by which clinicians detect chromosomal abnormalities in humans. A karyotype is the number and appearance of chromosomes, and includes their length, banding pattern, and centromere position. To obtain a view of an individual’s karyotype, cytologists photograph the chromosomes and then cut and paste each chromosome into a chart, or karyogram, also known as an ideogram (Figure 13.5).

Figure 8.25 This karyotype is of a female human. Notice that homologous chromosomes are the same size, and have the same centromere positions and banding patterns. A human male would have an XY chromosome pair instead of the XX pair shown. (credit: Andreas Blozer et al)

In a given species, chromosomes can be identified by their number, size, centromere position, and banding pattern. In a human karyotype, autosomes or “body chromosomes” (all of the non–sex chromosomes) are generally organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22). The X and Y chromosomes are not autosomes. However, chromosome 21 is actually shorter than chromosome 22. This was discovered after the naming of Down syndrome as trisomy 21, reflecting how this disease results from possessing one extra chromosome 21 (three total). Not wanting to change the name of this important disease, chromosome 21 retained its numbering, despite describing the shortest set of chromosomes. The chromosome “arms” projecting from either end of the centromere may be designated as short or long, depending on their relative lengths. The short arm is abbreviated p (for “petite”), whereas the long arm is abbreviated q (because it follows “p” alphabetically). Each arm is further subdivided and denoted by a number. Using this naming system, locations on chromosomes can be described consistently in the scientific literature.

Geneticists Use Karyograms to Identify Chromosomal Aberrations

Although Mendel is referred to as the “father of modern genetics,” he performed his experiments with none of the tools that the geneticists of today routinely employ. One such powerful cytological technique is karyotyping, a method in which traits characterized by chromosomal abnormalities can be identified from a single cell. To observe an individual’s karyotype, a person’s cells (like white blood cells) are first collected from a blood sample or other tissue. In the laboratory, the isolated cells are stimulated to begin actively dividing. A chemical called colchicine is then applied to cells to arrest condensed chromosomes in metaphase. Cells are then made to swell using a hypotonic solution so the chromosomes spread apart. Finally, the sample is preserved in a fixative and applied to a slide.

The geneticist then stains chromosomes with one of several dyes to better visualize the distinct and reproducible banding patterns of each chromosome pair. Following staining, the chromosomes are viewed using bright-field microscopy. A common stain choice is the Giemsa stain. Giemsa staining results in approximately 400–800 bands (of tightly coiled DNA and condensed proteins) arranged along all of the 23 chromosome pairs; an experienced geneticist can identify each band. In addition to the banding patterns, chromosomes are further identified on the basis of size and centromere location. To obtain the classic depiction of the karyotype in which homologous pairs of chromosomes are aligned in numerical order from longest to shortest, the geneticist obtains a digital image, identifies each chromosome, and manually arranges the chromosomes into this pattern (Figure 12.25).

At its most basic, the karyogram may reveal genetic abnormalities in which an individual has too many or too few chromosomes per cell. Examples of this are Down Syndrome, which is identified by a third copy of chromosome 21, and Turner Syndrome, which is characterized by the presence of only one X chromosome in women instead of the normal two. Geneticists can also identify large deletions or insertions of DNA. For instance, Jacobsen Syndrome—which involves distinctive facial features as well as heart and bleeding defects—is identified by a deletion on chromosome 11. Finally, the karyotype can pinpoint translocations, which occur when a segment of genetic material breaks from one chromosome and reattaches to another chromosome or to a different part of the same chromosome. Translocations are implicated in certain cancers, including chronic myelogenous leukemia.

During Mendel’s lifetime, inheritance was an abstract concept that could only be inferred by performing crosses and observing the traits expressed by offspring. By observing a karyogram, today’s geneticists can actually visualize the chromosomal composition of an individual to confirm or predict genetic abnormalities in offspring, even before birth.

Disorders in Chromosome Number

Of all of the chromosomal disorders, abnormalities in chromosome number are the most obviously identifiable from a karyogram. Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes. They are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis. Misaligned or incomplete synapsis, or a dysfunction of the spindle apparatus that facilitates chromosome migration, can cause nondisjunction. The risk of nondisjunction occurring increases with the age of the parents.

Nondisjunction can occur during either meiosis I or II, with differing results (Figure 13.6). If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome and two gametes with two copies of the chromosome. If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.

An individual with the appropriate number of chromosomes for their species is called euploid; in humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes. An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome). Monosomic human zygotes missing any one copy of an autosome invariably fail to develop to birth because they lack essential genes. This underscores the importance of “gene dosage” in humans. Most autosomal trisomies also fail to develop to birth; however, duplications of some of the smaller chromosomes (13, 15, 18, 21, or 22) can result in offspring that survive for several weeks to many years. Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose. Individuals with an extra chromosome may synthesize an abundance of the gene products encoded by that chromosome. This extra dose (150 percent) of specific genes can lead to a number of functional challenges and often precludes development. The most common trisomy among viable births is that of chromosome 21, which corresponds to Down Syndrome. Individuals with this inherited disorder are characterized by short stature and stunted digits, facial distinctions that include a broad skull and large tongue, and significant developmental delays. The incidence of Down syndrome is correlated with maternal age; older women are more likely to become pregnant with fetuses carrying the trisomy 21 genotype ( Figure 8.27 ).

Figure 8.27 The incidence of having a fetus with trisomy 21 increases dramatically with maternal age. Visualize the addition of a chromosome that leads to Down syndrome in this video simulation (http://openstaxcollege.org/l/down_syndrome) .

An individual with more than the correct number of chromosome sets (two for diploid species) is called polyploid. For instance, fertilization of an abnormal diploid egg with a normal haploid sperm would yield a triploid zygote. Polyploid animals are extremely rare, with only a few examples among the flatworms, crustaceans, amphibians, fish, and lizards.

Polyploid animals are sterile because meiosis cannot proceed normally and instead produces mostly aneuploid daughter cells that cannot yield viable zygotes. Rarely, polyploid animals can reproduce asexually by haplodiploidy, in which an unfertilized egg divides mitotically to produce offspring. In contrast, polyploidy is very common in the plant kingdom, and polyploid plants tend to be larger and more robust than euploids of their species ( Figure 8.28 ).

Figure 8.28 As with many polyploid plants, this triploid orange daylily (Hemerocallis fulva) is particularly large and robust, and grows flowers with triple the number of petals of its diploid counterparts. (credit: Steve Karg)

Sex Chromosome Nondisjunction in Humans

Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome. Rather than a gain or loss of autosomes, variations in the number of sex chromosomes are associated with relatively mild effects. In part, this occurs because of a molecular process called X inactivation. Early in development, when female mammalian embryos consist of just a few thousand cells (relative to trillions in the newborn), one X chromosome in each cell inactivates by tightly condensing into a quiescent (dormant) structure called a Barr body. The chance that an X chromosome (maternally or paternally derived) is inactivated in each cell is random, but once the inactivation occurs, all cells derived from that one will have the same inactive X chromosome or Barr body. By this process, females compensate for their double genetic dose of X chromosome. In so-called “tortoiseshell” cats, embryonic X inactivation is observed as color variegation (Figure 13.9). Females that are heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region.

Figure 8.29 In cats, the gene for coat color is located on the X chromosome. In the embryonic development of female cats, one of the two X chromosomes is randomly inactivated in each cell, resulting in a tortoiseshell pattern if the cat has two different alleles for coat color. Male cats, having only one X chromosome, never exhibit a tortoiseshell coat color. (credit: Michael Bodega)

An individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each of her cells. However, even inactivated X chromosomes continue to express a few genes, and X chromosomes must reactivate for the proper maturation of female ovaries. As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the individual will not develop in utero.

Several errors in sex chromosome number have been characterized. Individuals with three X chromosomes, called triplo- X, are phenotypically female but express developmental delays and reduced fertility. The XXY genotype, corresponding to one type of Klinefelter syndrome, corresponds to phenotypically male individuals with small testes, enlarged breasts, and reduced body hair. More complex types of Klinefelter syndrome exist in which the individual has as many as five X chromosomes. In all types, every X chromosome except one undergoes inactivation to compensate for the excess genetic dosage. This can be seen as several Barr bodies in each cell nucleus. Turner syndrome, characterized as an X0 genotype (i.e., only a single sex chromosome), corresponds to a phenotypically female individual with short stature, webbed skin in the neck region, hearing and cardiac impairments, and sterility.

Duplications and Deletions

In addition to the loss or gain of an entire chromosome, a chromosomal segment may be duplicated or lost. Duplications and deletions often produce offspring that survive but exhibit physical and mental abnormalities. Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus. Cri-du-chat (from the French for “cry of the cat”) is a syndrome associated with nervous system abnormalities and identifiable physical features that result from a deletion of most of 5p (the small arm of chromosome 5) ( Figure 8.30 ). Infants with this genotype emit a characteristic high-pitched cry on which the disorder’s name is based.

Figure 8.30 This individual with cri-du-chat syndrome is shown at two, four, nine, and 12 years of age. (credit: Paola Cerruti Mainardi)

Chromosomal Structural Rearrangements

Cytologists have characterized numerous structural rearrangements in chromosomes, but chromosome inversions and translocations are the most common. Both are identified during meiosis by the adaptive pairing of rearranged chromosomes with their former homologs to maintain appropriate gene alignment. If the genes carried on two homologs are not oriented correctly, a recombination event could result in the loss of genes from one chromosome and the gain of genes on the other. This would produce aneuploid gametes.

Chromosome Inversions

A chromosome inversion is the detachment, 180Åã rotation, and reinsertion of part of a chromosome. Inversions may occur

in nature as a result of mechanical shear, or from the action of transposable elements (special DNA sequences capable of facilitating the rearrangement of chromosome segments with the help of enzymes that cut and paste DNA sequences). Unless they disrupt a gene sequence, inversions only change the orientation of genes and are likely to have more mild effects than aneuploid errors. However, altered gene orientation can result in functional changes because regulators of gene expression could be moved out of position with respect to their targets, causing aberrant levels of gene products. An inversion can be pericentric and include the centromere, or paracentric and occur outside of the centromere ( Figure 8.31 ). A pericentric inversion that is asymmetric about the centromere can change the relative lengths of the chromosome arms, making these inversions easily identifiable.

Figure 8.31 Pericentric inversions include the centromere, and paracentric inversions do not. A pericentric inversion can change the relative lengths of the chromosome arms; a paracentric inversion cannot. When one homologous chromosome undergoes an inversion but the other does not, the individual is described as an inversion heterozygote. To maintain point-for-point synapsis during meiosis, one homolog must form a loop, and the other homolog must mold around it. Although this topology can ensure that the genes are correctly aligned, it also forces the homologs to stretch and can be associated with regions of imprecise synapsis (Figure 12.32).

Figure 8.32 When one chromosome undergoes an inversion but the other does not, one chromosome must form an inverted loop to retain point-for-point interaction during synapsis. This inversion pairing is essential to maintaining gene alignment during meiosis and to allow for recombination.

The Chromosome 18 Inversion

Not all structural rearrangements of chromosomes produce nonviable, impaired, or infertile individuals. In rare instances, such a change can result in the evolution of a new species. In fact, a pericentric inversion in chromosome 18 appears to have contributed to the evolution of humans. This inversion is not present in our closest genetic relatives, the chimpanzees. Humans and chimpanzees differ cytogenetically by pericentric inversions on several chromosomes and by the fusion of two separate chromosomes in chimpanzees that correspond to chromosome two in humans.

The pericentric chromosome 18 inversion is believed to have occurred in early humans following their divergence from a common ancestor with chimpanzees approximately five million years ago. Researchers characterizing this inversion have suggested that approximately 19,000 nucleotide bases were duplicated on 18p, and the duplicated region inverted and reinserted on chromosome 18 of an ancestral human. A comparison of human and chimpanzee genes in the region of this inversion indicates that two genes—ROCK1 and USP14—that are adjacent on chimpanzee chromosome 17 (which corresponds to human chromosome 18) are more distantly positioned on human chromosome 18. This suggests that one of the inversion breakpoints occurred between these two genes. Interestingly, humans and chimpanzees express USP14 at distinct levels in specific cell types, including cortical cells and fibroblasts. Perhaps the chromosome 18 inversion in an ancestral human repositioned specific genes and reset their expression levels in a useful way. Because both ROCK1 and USP14 encode cellular enzymes, a change in their expression could alter cellular function. It is not known how this inversion contributed to hominid evolution, but it appears to be a significant factor in the divergence of humans from other primates.

Translocations

A translocation occurs when a segment of a chromosome dissociates and reattaches to a different, non-homologous chromosome. Translocations can be benign or have devastating effects depending on how the positions of genes are altered with respect to regulatory sequences. Notably, specific translocations have been associated with several cancers and with schizophrenia. Reciprocal translocations result from the exchange of chromosome segments between two non-homologous chromosomes such that there is no gain or loss of genetic information ( Figure 8.33 ).

Figure 8.33 A reciprocal translocation occurs when a segment of DNA is transferred from one chromosome to another, nonhomologous chromosome. (credit: modification of work by National Human Genome Research/USA)

• Violaine Goidts et al., “Segmental duplication associated with the human-specific inversion of chromosome 18: a further example of the impact of segmental duplications on karyotype and genome evolution in primates,” Human Genetics. 115 (2004):116-122

allele gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes

autosomes any of the non-sex chromosomes

codominance in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic

continuous variation inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them

dihybrid result of a cross between two true-breeding parents that express different traits for two characteristics

dominant trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait

dominant lethal inheritance pattern in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age

epistasis antagonistic interaction between genes such that one gene masks or interferes with the expression of another

genotype underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism

hemizygous presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant

heterozygous having two different alleles for a given gene on the homologous chromosome

homozygous having two identical alleles for a given gene on the homologous chromosome

hybridization process of mating two individuals that differ with the goal of achieving a certain characteristic in their offspring

incomplete dominance in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype

law of dominance in a heterozygote, one trait will conceal the presence of another trait for the same characteristic

law of independent assortment genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur

law of segregation paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors

linkage phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together

monohybrid result of a cross between two true-breeding parents that express different traits for only one characteristic parental generation in a cross

phenotype observable traits expressed by an organism

recessive trait that appears “latent” or non-expressed when the individual also carries a dominant trait for that same

recessive lethal characteristic; when present as two identical copies, the recessive trait is expressed

sex-linked any gene on a sex chromosome

trait variation in the physical appearance of a heritable characteristic

X-linked gene present on the X, but not the Y chromosome

CHAPTER SUMMARY

8.1 mendel’s experiments and the laws of probability.

Working with garden pea plants, Mendel found that crosses between parents that differed by one trait produced F1 offspring that all expressed the traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the F2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P0 parent. Reciprocal crosses generated identical F1 and F2 offspring ratios. By examining sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that the traits were inherited as independent events.

When true-breeding or homozygous individuals that differ for a certain trait are crossed, all of the offspring will be heterozygotes for that trait. If the traits are inherited as dominant and recessive, the F1 offspring will all exhibit the same phenotype as the parent homozygous for the dominant trait. If these heterozygous offspring are self-crossed, the resulting F2 offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive. Because homozygous dominant and heterozygous individuals are phenotypically identical, the observed traits in the F2 offspring will exhibit a ratio of three dominant to one recessive. Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles of a gene to exist in a population. In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two. Finally, some alleles can be lethal. Recessive lethal alleles are only lethal in homozygotes, but dominant lethal alleles are fatal in heterozygotes as well.

Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, alleles are generally not more likely to segregate into a gamete with a particular allele of another gene. A dihybrid cross demonstrates independent assortment when the genes in question are on different chromosomes or distant from each other on the same chromosome. For crosses involving more than two genes, use the forked line or probability methods to predict offspring genotypes and phenotypes rather than a Punnett square.

Although chromosomes sort independently into gametes during meiosis, Mendel’s law of independent assortment refers to genes, not chromosomes, and a single chromosome may carry more than 1,000 genes. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel’s law of independent assortment. However, recombination serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on a given chromosome are not always inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chromosomal space.

Whether or not they are sorting independently, genes may interact at the level of gene products such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis.

REVIEW QUESTIONS

Mendel performed hybridizations by transferring pollen

from the _______ of the male plant to the female ova.

Which is one of the seven characteristics that Mendel

observed in pea plants?

a. flower size

b. seed texture

c. leaf shape

d. stem color

 Imagine you are performing a cross involving seed

color in garden pea plants. What F1 offspring would you

expect if you cross true-breeding parents with green seeds

and yellow seeds? Yellow seed color is dominant over

a. 100 percent yellow-green seeds

b. 100 percent yellow seeds

c. 50 percent yellow, 50 percent green seeds

d. 25 percent green, 75 percent yellow seeds

Consider a cross to investigate the pea pod texture trait,

involving constricted or inflated pods. Mendel found that

the traits behave according to a dominant/recessive pattern

in which inflated pods were dominant. If you performed

this cross and obtained 650 inflated-pod plants in the F2

generation, approximately how many constricted-pod

plants would you expect to have?

The observable traits expressed by an organism are

described as its ________.

a. phenotype

b. genotype

A recessive trait will be observed in individuals that

are ________ for that trait.

a. heterozygous

b. homozygous or heterozygous

c. homozygous

If black and white true-breeding mice are mated and

the result is all gray offspring, what inheritance pattern

would this be indicative of?

a. dominance

b. codominance

c. multiple alleles

d. incomplete dominance

The ABO blood groups in humans are expressed as

the IA, IB, and i alleles. The IA allele encodes the A blood

group antigen, IB encodes B, and i encodes O. Both A and

B are dominant to O. If a heterozygous blood type A

parent (IAi) and a heterozygous blood type B parent (IBi)

mate, one quarter of their offspring will have AB blood

type (IAIB) in which both antigens are expressed equally.

Therefore, ABO blood groups are an example of:

a. multiple alleles and incomplete dominance

b. codominance and incomplete dominance

c. incomplete dominance only

d. multiple alleles and codominance

In a mating between two individuals that are

heterozygous for a recessive lethal allele that is expressed

in utero, what genotypic ratio (homozygous

dominant:heterozygous:homozygous recessive) would you

expect to observe in the offspring?

X-linked recessive traits in humans (or in Drosophila)

are observed ________.

a. in more males than females

b. in more females than males

c. in males and females equally

d. in different distributions depending on the trait

The first suggestion that chromosomes may physically

exchange segments came from the microscopic

identification of ________.

a. synapsis

b. sister chromatids

c. chiasmata

Which recombination frequency corresponds to

independent assortment and the absence of linkage?

Which recombination frequency corresponds to perfect

linkage and violates the law of independent assortment?

Which of the following codes describes position 12 on

the long arm of chromosome 13?

In agriculture, polyploid crops (like coffee,

strawberries, or bananas) tend to produce ________.

a. more uniformity

b. more variety

c. larger yields

d. smaller yields

Assume a pericentric inversion occurred in one of two

homologs prior to meiosis. The other homolog remains

normal. During meiosis, what structure—if any—would

these homologs assume in order to pair accurately along

their lengths?

a. V formation

b. cruciform

d. pairing would not be possible

8. The genotype XXY corresponds to

a. Klinefelter syndrome

b. Turner syndrome

c. Triplo-X

d. Jacob syndrome

Abnormalities in the number of X chromosomes tends to have milder phenotypic effects than the same abnormalities in autosomes because of ________.

a. deletions

b. nonhomologous recombination

c. synapsis

d. X inactivation

By definition, a pericentric inversion includes the ________.

a. centromere

c. telomere

CRITICAL THINKING QUESTIONS

Describe one of the reasons why the garden pea was an excellent choice of model system for studying inheritance.

How would you perform a reciprocal cross for the characteristic of stem height in the garden pea?

The gene for flower position in pea plants exists as axial or terminal alleles. Given that axial is dominant to terminal, list all of the possible F1 and F2 genotypes and phenotypes from a cross involving parents that are homozygous for each trait. Express genotypes with conventional genetic abbreviations.

Use a Punnett square to predict the offspring in a cross between a dwarf pea plant (homozygous recessive) and a tall pea plant (heterozygous). What is the phenotypic ratio of the offspring?

Can a human male be a carrier of red-green color blindness?

Explain epistatis in terms of its Greek-language roots “standing upon.”

Adapted from:

OpenStax, Biology . OpenStax. May 20, 2013. < http://cnx.org/content/col11448/latest/ >

“Download for free at http://cnx.org/content/col11448/latest/ .”

Human Biology Copyright © by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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12.1 Mendel’s Experiments and the Laws of Probability

Learning objectives.

In this section, you will explore the following questions:

• Why was Mendel’s experimental work so successful?
• How do the sum and product rules of probability predict the outcomes of monohybrid crosses involving dominant and recessive alleles?

Connection for AP ® Courses

Genetics is the science of heredity. Austrian monk Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Working with garden peas, Mendel found that crosses between true-breeding parents (P) that differed in one trait (e.g., color: green peas versus yellow peas) produced first generation (F1) offspring that all expressed the trait of one parent (e.g., all green or all yellow). Mendel used the term dominant to refer to the trait that was observed, and recessive to denote that non-expressed trait, or the trait that had “disappeared” in this first generation. When the F1 offspring were crossed with each other, the F2 offspring exhibited both traits in a 3:1 ratio. Other crosses (e.g., height: tall plants versus short plants) generated the same 3:1 ratio (in this example, tall to short) in the F2 offspring. By mathematically examining sample sizes, Mendel showed that genetic crosses behaved according to the laws of probability, and that the traits were inherited as independent events. In other words, Mendel used statistical methods to build his model of inheritance.

As you have likely noticed, the AP Biology course emphasizes the application of mathematics. Two rules of probability can be used to find the expected proportions of different traits in offspring from different crosses. To find the probability of two or more independent events (events where the outcome of one event has no influence on the outcome of the other event) occurring together, apply the product rule and multiply the probabilities of the individual events. To find the probability that one of two or more events occur, apply the sum rule and add their probabilities together.

The content presented in this section supports the learning objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The AP ® learning objectives merge essential knowledge content with one or more of the seven science practices. These objectives provide a transparent foundation for the AP ® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP ® exam questions.

Teacher Support

Two rules of probability are used in solving genetics problems: the rule of multiplication and the rule of addition. The probability that independent events will occur simultaneously is the product of their individual probabilities. If two dices are tossed, what is the probability of landing two ones? A die has 6 faces, and assuming the die is not loaded, each face has the same probability of outcome. The probability of obtaining the number 1 is equal to the number on the die divided by the total number of sides: 1 6 1 6 . The probability of rolling two ones is equal to 1 6   ×   1 6   =   1 36 1 6   ×   1 6   =   1 36 .

The probability that any one of a set of mutually exclusive events will occur is the sum of their individual probabilities. The probability of rolling a 1 or a 2 is equal to 1 6   +   1 6   =   1 3 1 6   +   1 6   =   1 3 because the two outcomes are mutually exclusive. If we roll a 1, it cannot be a 2.

Tell students that Gregor Mendel was a monk who had received a solid scientific education and had excelled at mathematics. He brought this knowledge of science into his experiments with peas.

Engage students in describing what makes a good organism to study genetics. One approach is to ask the class if they would use elephants to study genetics. The disadvantages of using elephants actually highlight the advantages of using peas, corn, fruit flies, or mice for genetics studies: short life cycle, easy to maintain and handle, large number of offspring for statistical analysis, etc.

The concepts of statistics are not intuitive. Practice with dice and coins. Explain that the probability ratios are achieved with large numbers of trials.

Dominant traits are the ones expressed in a dominant/recessive situation. They do not usually repress the recessive trait. A dominant trait is not necessarily the most common trait in a population. For example, type O blood is a recessive trait, but it is the most frequent blood group in many ethnic groups. A dominant trait can be lethal. A dominant allele is not better than the recessive allele. Whether a trait is beneficial depends on the environment. Give the example of wing color in moths. Dark pigmentation is beneficial in a polluted environment where predators would not pick up the moths on dark tree barks. For example, the population peppered moths in 19th century London shifted so that their wing colors were darker to blend in with the soot of the Industrial Revolution. After pollution levels dropped, light pigmentation became more prevalent because it helped the moths to escape notice.

Johann Gregor Mendel (1822–1884) ( Figure 12.2 ) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization, 1 in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community that believed, incorrectly, that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring; this hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation results from the action of many genes to determine a characteristic like human height. Offspring appear to be a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. The blending theory of inheritance asserted that the original parental traits were lost or absorbed by the blending in the offspring, but we now know that this is not the case. Mendel was the first researcher to see it. Instead of continuous characteristics, Mendel worked with traits that were inherited in distinct classes (specifically, violet versus white flowers); this is referred to as discontinuous variation . Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring, nor were they absorbed, but rather that they kept their distinctness and could be passed on. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel’s Model System

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum , to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendelian Crosses

Mendel performed hybridizations , which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. In plants, pollen carries the male gametes (sperm) to the stigma, a sticky organ that traps pollen and allows the sperm to move down the pistil to the female gametes (ova) below. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant’s flowers before they had a chance to mature.

Plants used in first-generation crosses were called P 0 , or parental generation one, plants ( Figure 12.3 ). Mendel collected the seeds belonging to the P 0 plants that resulted from each cross and grew them the following season. These offspring were called the F 1 , or the first filial ( filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendel’s experiments extended beyond the F 2 generation to the F 3 and F 4 generations, and so on, but it was the ratio of characteristics in the P 0 −F 1 −F 2 generations that were the most intriguing and became the basis for Mendel’s postulates.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants, reporting results from 19,959 F 2 plants alone. His findings were consistent.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait in the F 1 generation had completely disappeared.

Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that, of F 2 -generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers per one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of which parent, male or female, contributed which trait. This is called a reciprocal cross —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics Mendel examined, the F 1 and F 2 generations behaved in the same way as they had for flower color. One of the two traits would disappear completely from the F 1 generation only to reappear in the F 2 generation at a ratio of approximately 3:1 ( Table 12.1 ).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these, respectively, dominant and recessive traits. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear, in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate (not blended) in the plants of the F 1 generation. Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

So why did Mendel repeatedly obtain 3:1 ratios in his crosses? To understand how Mendel deduced the basic mechanisms of inheritance that lead to such ratios, we must first review the laws of probability.

Science Practice Connection for AP® Courses

Think about it.

Students are performing a cross involving seed color in garden pea plants. Yellow seed color is dominant to green seed color. What F1 offspring would be expected when cross true-breeding plants with green seeds with true-breading plants with yellow seeds? Express the answer(s) as percentage.

This question is an application of Learning Objectives 3.14 and Science Practice 2.2 because students are applying a mathematical routine (probability) to determine a Mendelian pattern of inheritance.

Possible answer:

Probability basics.

Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur. It is also possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur by the number of times that it could occur. Empirical probabilities come from observations, like those of Mendel. Theoretical probabilities come from knowing how the events are produced and assuming that the probabilities of individual outcomes are equal. A probability of one for some event indicates that it is guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur. An example of a genetic event is a round seed produced by a pea plant. In his experiment, Mendel demonstrated that the probability of the event “round seed” occurring was one in the F 1 offspring of true-breeding parents, one of which has round seeds and one of which has wrinkled seeds. When the F 1 plants were subsequently self-crossed, the probability of any given F 2 offspring having round seeds was now three out of four. In other words, in a large population of F 2 offspring chosen at random, 75 percent were expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds. Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses.

The Product Rule and Sum Rule

Mendel demonstrated that the pea-plant characteristics he studied were transmitted as discrete units from parent to offspring. As will be discussed, Mendel also determined that different characteristics, like seed color and seed texture, were transmitted independently of one another and could be considered in separate probability analyses. For instance, performing a cross between a plant with green, wrinkled seeds and a plant with yellow, round seeds still produced offspring that had a 3:1 ratio of green:yellow seeds (ignoring seed texture) and a 3:1 ratio of round:wrinkled seeds (ignoring seed color). The characteristics of color and texture did not influence each other.

The product rule of probability can be applied to this phenomenon of the independent transmission of characteristics. The product rule states that the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone. To demonstrate the product rule, imagine that you are rolling a six-sided die (D) and flipping a penny (P) at the same time. The die may roll any number from 1–6 (D # ), whereas the penny may turn up heads (P H ) or tails (P T ). The outcome of rolling the die has no effect on the outcome of flipping the penny and vice versa. There are 12 possible outcomes of this action ( Table 12.2 ), and each event is expected to occur with equal probability.

Of the 12 possible outcomes, the die has a 2/12 (or 1/6) probability of rolling a two, and the penny has a 6/12 (or 1/2) probability of coming up heads. By the product rule, the probability that you will obtain the combined outcome 2 and heads is: (D 2 ) x (P H ) = (1/6) x (1/2) or 1/12 ( Table 12.3 ). Notice the word “and” in the description of the probability. The “and” is a signal to apply the product rule. For example, consider how the product rule is applied to the dihybrid cross: the probability of having both dominant traits (for example, yellow and round) in the F 2 progeny is the product of the probabilities of having the dominant trait for each characteristic, as shown here:

On the other hand, the sum rule of probability is applied when considering two mutually exclusive outcomes that can come about by more than one pathway. The sum rule states that the probability of the occurrence of one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities. Notice the word “or” in the description of the probability. The “or” indicates that you should apply the sum rule. In this case, let’s imagine you are flipping a penny (P) and a quarter (Q). What is the probability of one coin coming up heads and one coin coming up tails? This outcome can be achieved by two cases: the penny may be heads (P H ) and the quarter may be tails (Q T ), or the quarter may be heads (Q H ) and the penny may be tails (P T ). Either case fulfills the outcome. By the sum rule, we calculate the probability of obtaining one head and one tail as [(P H ) × (Q T )] + [(Q H ) × (P T )] = [(1/2) × (1/2)] + [(1/2) × (1/2)] = 1/2 ( Table 12.3 ). You should also notice that we used the product rule to calculate the probability of P H and Q T , and also the probability of P T and Q H , before we summed them. Again, the sum rule can be applied to show the probability of having exactly one dominant trait in the F 2 generation of a dihybrid cross:

To use probability laws in practice, it is necessary to work with large sample sizes because small sample sizes are prone to deviations caused by chance. The large quantities of pea plants that Mendel examined allowed him to calculate the probabilities of the traits appearing in his F 2 generation. As you will learn, this discovery meant that when parental traits were known, the offspring’s traits could be predicted accurately even before fertilization.

• 1 Johann Gregor Mendel, Versuche über Pflanzenhybriden Verhandlungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr , 1865 Abhandlungen, 3–47. [go here for the English translation here ]

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Who was Gregor Mendel?

Gregor Mendel was an Austrian scientist, teacher, and Augustinian prelate who lived in the 1800s. He experimented on garden pea hybrids while living at a monastery and is known as the father of modern genetics .

Through his careful breeding of garden peas, Gregor Mendel discovered the basic principles of heredity and laid the mathematical foundation of the science of genetics . He formulated several basic genetic laws, including the law of segregation, the law of dominance, and the law of independent assortment, in what became known as Mendelian inheritance .  

Gregor Mendel (born July 20, 1822, Heinzendorf, Silesia, Austrian Empire [now Hynčice, Czech Republic]—died January 6, 1884, Brünn , Austria-Hungary [now Brno, Czech Republic]) was a botanist, teacher, and Augustinian prelate, the first person to lay the mathematical foundation of the science of genetics , in what came to be called Mendelism .

Born to a family with limited means in German-speaking Silesia , Mendel was raised in a rural setting. His academic abilities were recognized by the local priest , who persuaded his parents to send him away to school at the age of 11. His Gymnasium (grammar school) studies completed in 1840, Mendel entered a two-year program in philosophy at the Philosophical Institute of the University of Olmütz (Olomouc, Czech Republic), where he excelled in physics and mathematics , completing his studies in 1843. His initial years away from home were hard, because his family could not sufficiently support him. He tutored other students to make ends meet, and twice he suffered serious depression and had to return home to recover. As his father’s only son, Mendel was expected to take over the small family farm, but he preferred a different solution to his predicament, choosing to enter the Altbrünn monastery as a novitiate of the Augustinian order, where he was given the name Gregor.

The move to the monastery took him to Brünn, the capital of Moravia , where for the first time he was freed from the harsh struggle of former years. He was also introduced to a diverse and intellectual community . As a priest, Mendel found his parish duty to visit the sick and dying so distressing that he again became ill. Abbot Cyril Napp found him a substitute-teaching position at Znaim ( Znojmo , Czech Republic), where he proved very successful. However, in 1850 Mendel failed an exam—introduced through new legislation for teacher certification—and was sent to the University of Vienna for two years to benefit from a new program of scientific instruction. As at Olmütz, Mendel devoted his time at Vienna to physics and mathematics, working under Austrian physicist Christian Doppler and mathematical physicist Andreas von Ettinghausen. He also studied the anatomy and physiology of plants and the use of the microscope under botanist Franz Unger, an enthusiast for the cell theory and a supporter of the developmentalist (pre-Darwinian) view of the evolution of life. Unger’s writings on the latter made him a target for attack by the Roman Catholic press of Vienna shortly before and during Mendel’s time there.

In the summer of 1853, Mendel returned to the monastery in Brünn, and in the following year he was again given a teaching position, this time at the Brünn Realschule (secondary school), where he remained until elected abbot 14 years later. He attempted the teacher exam again in 1856, although the event caused a nervous breakdown and a second failure. However, these years were his greatest in terms of success both as teacher and as consummate experimentalist. Once abbot, his administrative duties came to occupy the majority of his time. Moreover, Mendel’s refusal to permit the monastery to pay the state’s new taxes for a religious fund led to his involvement in a long and bitter dispute with the authorities. Convinced that this tax was unconstitutional, he continued his opposition, refusing to comply even when the state took over the administration of some of the monastery’s estates and directed the profits to the religious fund.

In 1854 Abbot Cyril Napp permitted Mendel to plan a major experimental program in hybridization at the monastery. The aim of this program was to trace the transmission of hereditary characters in successive generations of hybrid progeny. Previous authorities had observed that progeny of fertile hybrids tended to revert to the originating species , and they had therefore concluded that hybridization could not be a mechanism used by nature to multiply species—though in exceptional cases some fertile hybrids did appear not to revert (the so-called “constant hybrids”). On the other hand, plant and animal breeders had long shown that crossbreeding could indeed produce a multitude of new forms. The latter point was of particular interest to landowners, including the abbot of the monastery, who was concerned about the monastery’s future profits from the wool of its Merino sheep, owing to competing wool being supplied from Australia.

Mendel chose to conduct his studies with the edible pea ( Pisum sativum ) because of the numerous distinct varieties, the ease of culture and control of pollination , and the high proportion of successful seed germinations . From 1854 to 1856 he tested 34 varieties for constancy of their traits. In order to trace the transmission of characters, he chose seven traits that were expressed in a distinctive manner, such as plant height (short or tall) and seed colour (green or yellow). He referred to these alternatives as contrasted characters, or character-pairs. He crossed varieties that differed in one trait—for instance, tall crossed with short. The first generation of hybrids (F 1 ) displayed the character of one variety but not that of the other. In Mendel’s terms, one character was dominant and the other recessive . In the numerous progeny that he raised from these hybrids (the second generation, F 2 ), however, the recessive character reappeared, and the proportion of offspring bearing the dominant to offspring bearing the recessive was very close to a 3 to 1 ratio. Study of the descendants (F 3 ) of the dominant group showed that one-third of them were true-breeding and two-thirds were of hybrid constitution. The 3:1 ratio could hence be rewritten as 1:2:1, meaning that 50 percent of the F 2 generation were true-breeding and 50 percent were still hybrid. This was Mendel’s major discovery, and it was unlikely to have been made by his predecessors, since they did not grow statistically significant populations, nor did they follow the individual characters separately to establish their statistical relations.

Mendel’s approach to experimentation came from his training in physics and mathematics , especially combinatorial mathematics . The latter served him ideally to represent his result. If A represents the dominant characteristic and a the recessive, then the 1:2:1 ratio recalls the terms in the expansion of the binomial equation: ( A + a ) 2 = A 2 + 2 A a + a 2 Mendel realized further that he could test his expectation that the seven traits are transmitted independently of one another. Crosses involving first two and then three of his seven traits yielded categories of offspring in proportions following the terms produced from combining two binomial equations, indicating that their transmission was independent of one another. Mendel’s successors have called this conclusion the law of independent assortment .