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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 and the Principles of Inheritance

Traits are passed down in families in different patterns. Pedigrees can illustrate these patterns by following the history of specific characteristics, or phenotypes, as they appear in a family. For example, the pedigree in Figure 1 shows a family in which a grandmother (generation I) has passed down a characteristic (shown in solid red) through the family tree. The inheritance pattern of this characteristic is considered dominant , because it is observable in every generation. Thus, every individual who carries the genetic code for this characteristic will show evidence of the characteristic. In contrast, Figure 2 shows a different pattern of inheritance, in which a characteristic disappears in one generation, only to reappear in a subsequent one. This pattern of inheritance, in which the parents do not show the phenotype but some of the children do, is considered recessive . But where did our knowledge of dominance and recessivity first come from?

Gregor Mendel’s Courage and Persistence

Mendel was curious about how traits were transferred from one generation to the next, so he set out to understand the principles of heredity in the mid-1860s. Peas were a good model system, because he could easily control their fertilization by transferring pollen with a small paintbrush. This pollen could come from the same flower (self-fertilization), or it could come from another plant's flowers (cross-fertilization). First, Mendel observed plant forms and their offspring for two years as they self-fertilized, or "selfed," and ensured that their outward, measurable characteristics remained constant in each generation. During this time, Mendel observed seven different characteristics in the pea plants, and each of these characteristics had two forms (Figure 3). The characteristics included height (tall or short), pod shape (inflated or constricted), seed shape (smooth or winkled), pea color (green or yellow), and so on. In the years Mendel spent letting the plants self, he verified the purity of his plants by confirming, for example, that tall plants had only tall children and grandchildren and so forth. Because the seven pea plant characteristics tracked by Mendel were consistent in generation after generation of self-fertilization, these parental lines of peas could be considered pure-breeders (or, in modern terminology, homozygous for the traits of interest). Mendel and his assistants eventually developed 22 varieties of pea plants with combinations of these consistent characteristics.

Mendel not only crossed pure-breeding parents, but he also crossed hybrid generations and crossed the hybrid progeny back to both parental lines. These crosses (which, in modern terminology, are referred to as F 1 , F 1 reciprocal, F 2 , B 1 , and B 2 ) are the classic crosses to generate genetically hybrid generations.

Understanding Dominant Traits

Understanding recessive traits.

When conducting his experiments, Mendel designated the two pure-breeding parental generations involved in a particular cross as P 1 and P 2 , and he then denoted the progeny resulting from the crossing as the filial, or F 1 , generation. Although the plants of the F 1 generation looked like one parent of the P generation, they were actually hybrids of two different parent plants. Upon observing the uniformity of the F 1 generation, Mendel wondered whether the F 1 generation could still possess the nondominant traits of the other parent in some hidden way.

To understand whether traits were hidden in the F 1 generation, Mendel returned to the method of self-fertilization. Here, he created an F 2 generation by letting an F 1 pea plant self-fertilize (F 1 x F 1 ). This way, he knew he was crossing two plants of the exact same genotype . This technique, which involves looking at a single trait, is today called a monohybrid cross . The resulting F 2 generation had seeds that were either round or wrinkled. Figure 4 shows an example of Mendel's data.

When looking at the figure, notice that for each F 1 plant, the self-fertilization resulted in more round than wrinkled seeds among the F 2 progeny. These results illustrate several important aspects of scientific data:

Multiple trials are necessary to see patterns in experimental data.
There is a lot of variation in the measurements of one experiment.
A large sample size, or "N," is required to make any quantitative comparisons or conclusions.

In Figure 4, the result of Experiment 1 shows that the single characteristic of seed shape was expressed in two different forms in the F 2 generation: either round or wrinkled. Also, when Mendel averaged the relative proportion of round and wrinkled seeds across all F 2 progeny sets, he found that round was consistently three times more frequent than wrinkled. This 3:1 proportion resulting from F 1 x F 1 crosses suggested there was a hidden recessive form of the trait. Mendel recognized that this recessive trait was carried down to the F 2 generation from the earlier P generation .

Mendel and Alleles

As mentioned, Mendel's data did not support the ideas about trait blending that were popular among the biologists of his time. As there were never any semi-wrinkled seeds or greenish-yellow seeds, for example, in the F 2 generation, Mendel concluded that blending should not be the expected outcome of parental trait combinations. Mendel instead hypothesized that each parent contributes some particulate matter to the offspring. He called this heritable substance "elementen." (Remember, in 1865, Mendel did not know about DNA or genes.) Indeed, for each of the traits he examined, Mendel focused on how the elementen that determined that trait was distributed among progeny. We now know that a single gene controls seed form, while another controls color, and so on, and that elementen is actually the assembly of physical genes located on chromosomes. Multiple forms of those genes, known as alleles , represent the different traits. For example, one allele results in round seeds, and another allele specifies wrinkled seeds.

One of the most impressive things about Mendel's thinking lies in the notation that he used to represent his data. Mendel's notation of a capital and a lowercase letter ( Aa ) for the hybrid genotype actually represented what we now know as the two alleles of one gene : A and a . Moreover, as previously mentioned, in all cases, Mendel saw approximately a 3:1 ratio of one phenotype to another. When one parent carried all the dominant traits ( AA ), the F 1 hybrids were "indistinguishable" from that parent. However, even though these F 1 plants had the same phenotype as the dominant P 1 parents, they possessed a hybrid genotype ( Aa ) that carried the potential to look like the recessive P 1 parent ( aa ). After observing this potential to express a trait without showing the phenotype, Mendel put forth his second principle of inheritance: the principle of segregation . According to this principle, the "particles" (or alleles as we now know them) that determine traits are separated into gametes during meiosis , and meiosis produces equal numbers of egg or sperm cells that contain each allele (Figure 5).

Dihybrid Crosses

Mendel had thus determined what happens when two plants that are hybrid for one trait are crossed with each other, but he also wanted to determine what happens when two plants that are each hybrid for two traits are crossed. Mendel therefore decided to examine the inheritance of two characteristics at once. Based on the concept of segregation , he predicted that traits must sort into gametes separately. By extrapolating from his earlier data, Mendel also predicted that the inheritance of one characteristic did not affect the inheritance of a different characteristic.

Mendel tested this idea of trait independence with more complex crosses. First, he generated plants that were purebred for two characteristics, such as seed color (yellow and green) and seed shape (round and wrinkled). These plants would serve as the P 1 generation for the experiment. In this case, Mendel crossed the plants with wrinkled and yellow seeds ( rrYY ) with plants with round, green seeds ( RRyy ). From his earlier monohybrid crosses, Mendel knew which traits were dominant: round and yellow. So, in the F 1 generation, he expected all round, yellow seeds from crossing these purebred varieties, and that is exactly what he observed. Mendel knew that each of the F 1 progeny were dihybrids; in other words, they contained both alleles for each characteristic ( RrYy ). He then crossed individual F 1 plants (with genotypes RrYy ) with one another. This is called a dihybrid cross . Mendel's results from this cross were as follows:

315 plants with round, yellow seeds
108 plants with round, green seeds
101 plants with wrinkled, yellow seeds
32 plants with wrinkled, green seeds

Thus, the various phenotypes were present in a 9:3:3:1 ratio (Figure 6).

Next, Mendel went through his data and examined each characteristic separately. He compared the total numbers of round versus wrinkled and yellow versus green peas, as shown in Tables 1 and 2.

Table 1: Data Regarding Seed Shape

Table 2: Data Regarding Pea Color

The proportion of each trait was still approximately 3:1 for both seed shape and seed color. In other words, the resulting seed shape and seed color looked as if they had come from two parallel monohybrid crosses; even though two characteristics were involved in one cross, these traits behaved as though they had segregated independently. From these data, Mendel developed the third principle of inheritance: the principle of independent assortment . According to this principle, alleles at one locus segregate into gametes independently of alleles at other loci. Such gametes are formed in equal frequencies.

Mendel’s Legacy

More lasting than the pea data Mendel presented in 1862 has been his methodical hypothesis testing and careful application of mathematical models to the study of biological inheritance. From his first experiments with monohybrid crosses, Mendel formed statistical predictions about trait inheritance that he could test with more complex experiments of dihybrid and even trihybrid crosses. This method of developing statistical expectations about inheritance data is one of the most significant contributions Mendel made to biology.

But do all organisms pass their on genes in the same way as the garden pea plant? The answer to that question is no, but many organisms do indeed show inheritance patterns similar to the seminal ones described by Mendel in the pea. In fact, the three principles of inheritance that Mendel laid out have had far greater impact than his original data from pea plant manipulations. To this day, scientists use Mendel's principles to explain the most basic phenomena of inheritance.

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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

Image is a sketch of Johann Gregor Mendel.

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.

$The diagram shows a cross between pea plants that are true-breeding for purple flower color and plants that are true-breeding for white flower color. This cross-fertilization of the P generation resulted in an F_{1} generation with all violet flowers. Self-fertilization of the F_{1} generation resulted in an F_{2} generation that consisted of 705 plants with violet flowers, and 224 plants with white flowers.$

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 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).

Seven characteristics of Mendel’s pea plants are illustrated. The flowers can be purple or white. The peas can be yellow or green, or smooth or wrinkled. The pea pods can be inflated or constricted, or yellow or green. The flower position can be axial or terminal. The stem length can be tall or dwarf.

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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8.1 Mendel’s Experiments

Learning objectives.

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 8.2 ) 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 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. As stated earlier, in genetics, "parent" is often used to describe the individual organism(s) that contribute genetic material to an offspring, usually in the form of gamete cells.

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. Because every pea plant has both male reproductive organs and female reproductive organs, each plant produces both types of gametes required for reproduction—both pollen and ova. In plants, just as in animals, reproductive organs are classified by the size of the gametes produced. The organs producing the smaller pollen are called male reproductive organs, while the organs producing the larger ova are called female reproductive organs.

In garden peas, 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.

Plants used in first-generation crosses were called P, or parental generation, plants ( Figure 8.3 ). 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

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. Mendel performed an additional experiment to ascertain differences in inheritance of traits carried in the pollen versus the ovum. When Mendel transferred pollen from a plant with violet flowers to fertilize the ova of a plant with white flowers and vice versa, he obtained approximately the same ratio irrespective of which gamete 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 8.4 ).

1 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]

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Microbe Notes

Mendelian Inheritance: Mendelism or Mendelian Genetics

Mendelian inheritance, also known as Mendelism or Mendelian genetics, is a set of principles that explain how hereditary traits are passed from parents to their offspring.

These principles were initially developed by Gregor Johann Mendel, an Austrian monk, and botanist, who is regarded as the father of genetics. Mendel conducted pioneering experiments with garden peas ( Pisum sativum ) in the 19th century and established the fundamental laws of inheritance.

Mendelian Inheritance- Mendelism or Mendelian Genetics

Mendel’s contributions to the field of genetics were initially overlooked but were rediscovered and recognized in the early 20th century. Despite facing initial controversy, Mendel’s work laid the foundation for classical genetics and has since provided a framework for understanding the basic principles of heredity.

Table of Contents

Interesting Science Videos

Mendel’s Experiment

Gregor Mendel conducted breeding experiments on the pea plant between 1856 and 1863 in order to study the patterns of inheritance. He specifically chose pea plants for a number of reasons including their availability in various varieties, self-pollination capabilities, short life cycles, ease of cultivation, and distinct characteristics. Mendel focused on studying seven specific traits in pea plants: seed shape, seed color, flower color, pod shape, pod color, flower position, and stem height.

Mendel conducted two main experiments, monohybrid and dihybrid crosses, to determine the laws of inheritance.

Monohybrid cross

In the monohybrid cross, Mendel studied the inheritance of a single trait. Mendel conducted crosses between pea plants with different traits of the same character, such as tallness (TT) and dwarfness (tt), and observed their inheritance patterns.
The parental generation (P) are the organisms involved in the initial cross, while the first filial generation (F 1 ) represents the offspring of this cross.
In the F 1 generation, all the plants showed the dominant trait (tallness), while the recessive trait (dwarfness) was not present. This pattern of displaying only the dominant trait in the F 1 generation was the same across all the traits Mendel studied.
When the F1 plants were crossed among themselves, resulting in the second filial generation (F 2 ), some offspring showed the recessive trait, which was not observed in the F 1 generation. F 2 generation exhibited a 3:1 ratio of the dominant and recessive traits.
Mendel observed and found that this pattern was consistent in all the traits he studied.

Dihybrid cross

In the dihybrid cross, Mendel studied the inheritance of two different traits. He crossed purebred parental plants with different traits. For example- plants with yellow, round seeds (YYRR) were crossed with plants with green, wrinkled seeds (yyrr).
The resulting F1 generation displayed only the dominant traits of yellow and round seeds. In the F2 generation, both parental traits appeared in four types of combination in a phenotypic ratio of approximately 9:3:3:1, showing the independent assortment of the two traits.

Mendel’s Laws of Inheritance

Mendel proposed three laws explaining the inheritance of traits.

Law of Dominance

According to the law of dominance, when there are two alternative forms (alleles) of a particular trait present in an organism, one allele will be dominant and the other recessive. In the F1 generation, only the dominant allele is expressed, while the recessive allele remains masked or unexpressed. This law explains how the traits of the parents are expressed in the offspring during a monohybrid cross.

Law of Segregation

The Law of Segregation, also known as the Law of Purity of Gametes, explains how the alleles responsible for a specific trait separate during the formation of gametes and how they are passed on to the offspring. According to this law, each individual possesses two alleles for a particular trait, one inherited from each parent. During gamete formation, these alleles separate from each other, so that each gamete carries only one allele for each trait. Since each gamete carries only one allele for a trait, they are considered pure or homozygous for that particular characteristic.

Law of Independent Assortment

According to the Law of Independent Assortment, alleles for different traits separate and are inherited independently during the formation of gametes. This means that the alleles for one trait are not linked or influenced by the alleles for other traits. Mendel’s dihybrid cross provides support for the Law of Independent Assortment.

Modes of Inheritance

Mendelian inheritance patterns can be categorized into three major types: autosomal dominant, autosomal recessive, and X-linked inheritance.

Autosomal Dominant Inheritance is a type of inheritance where the presence of a single dominant allele is sufficient to express a trait or disease, even if the other chromosome carries a normal allele. This means that an affected individual only needs to inherit one copy of the dominant allele from either parent to exhibit the trait or disease. An affected individual has a 50% chance of passing the trait to each independent offspring. Examples of autosomal dominant diseases are Huntington’s disease and Marfan syndrome.

Autosomal Recessive Inheritance is the mode of genetic inheritance where the expression of a trait or disease requires the presence of two copies of an abnormal recessive allele, one inherited from each parent. In this inheritance pattern, both alleles must be abnormal for the trait to be expressed. Carriers of a single copy of the recessive allele do not display the trait but can pass it on to their children. Couples who are carriers have a 25% risk of having an affected child. Examples of autosomal recessive diseases are cystic fibrosis and sickle cell anemia.

X-Linked Inheritance refers to the inheritance of traits or diseases associated with genes located on the X chromosome. Since males have one X and one Y chromosome, they typically exhibit the phenotype of X-linked traits inherited from their mother, as they only inherit one X chromosome. On the other hand, females have two X chromosomes, so they may be carriers of X-linked traits without displaying the phenotype. Examples of X-linked diseases are hemophilia and color blindness.

Deviation from Mendel’s Findings

Mendel’s principles laid the foundation for genetics. However, exceptions and variations in Mendel’s findings have been discovered that have helped us develop a more complete understanding of inheritance patterns. Some of these variations are:

Incomplete Dominance occurs when the offspring’s phenotype is not the same as either of the parents but is intermediate between the phenotypes of the parents. It happens when one allele for a trait is not completely dominant over the other, resulting in a combination of the phenotypes of both alleles. This goes against Mendel’s law of dominance, which states that one allele is dominant and masks the expression of the other. For example, in the snapdragon flower, crossing plants with red and white flowers resulted in pink flowers.

Co-dominance occurs when both alleles in an organism are fully expressed. This means that neither allele dominates over the other, and both traits are simultaneously present in the phenotype. This also differs from Mendel’s law of dominance, where one allele dominates over the other. An example is blood type inheritance, where the A and B alleles are co-dominant, resulting in individuals with both A and B antigens.

Multiple Alleles refer to a gene having more than two variations or alleles within a population. Unlike Mendel’s experiments that involved traits controlled by only two alleles, some traits can have multiple alleles. For example, the ABO blood group system has three alleles: A, B, and O.

Genetic linkage refers to the phenomenon where genes located near each other on the same chromosome have a tendency to be inherited together. This is against the principle of independent assortment, which states that genes segregate and inherit independently.

Epistasis occurs when the expression of one gene masks or affects the expression of another gene. Epistasis deviates from Mendel’s laws as it involves the interaction of multiple genes and shows that the expression of one gene can modify the expression of another gene.\

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About Author

Sanju Tamang

Chapter 18. Mendelian Genetics

Chapter Outline

18.1 Mendel’s Experiments
18.2 Mendel’s Principles of Inheritance
18.3 Exceptions to Mendel’s Principles of Inheritance

Introduction

Genetics is the study of heredity. Johann Gregor Mendel (1822–1884) set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood ( Figure 18.2 ). 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.

18.1 | Mendel’s Experiments

Learning Objectives

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

Describe 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) 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 . 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, Exp eriments 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 when many genes work together to determine a characteristic, such as human height or eye color. Offspring appear to be a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation.

Mendel worked with traits that were inherited in distinct classes, such as violet versus white flowers. These traits display discontinuous variation . Mendel’s choice of these kinds of traits allowed him to see 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.

18.1.1 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.

18.1.2 Mendelian Crosses

Mendel performed hybridizations , which involve mating two true-breeding individuals that have different traits. In the pea, this is done by manually transferring pollen from one pea plant to the stigma of another pea plant. 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 pollen-producing anthers from the plant’s flowers before they had a chance to mature.

Plants used in first-generation crosses were called P , or parental generation, plants ( Figure 18.3 ). Mendel collected the seeds that resulted from each cross and grew them the following season. These offspring were called the F1 , or the first filial ( filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize. He then collected and grew the seeds from the F1 plants to produce the F2 , or second filial, generation. Mendel’s experiments extended beyond the F2 generation to the F3 and F4 generations, and so on, but it was the ratio of characteristics in the P−F1−F2 generations that were the most intriguing and became the basis for Mendel’s principles.

18.1.3 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 include: tall vs. short plant height, wrinkled vs. round seeds, green vs. yellow seeds, violet vs. white flowers, etc. ( Table 18.1 ). To fully examine each characteristic, Mendel generated large numbers of F1 and F2 plants, reporting results from 19,959 F2 plants alone.

As an example, let us look at Mendel’s results for the flower color trait. 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 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 F1 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 18.1 ).

Table 18.1 The Results of Mendel’s Garden Pea Hybridizations

18.2 | Mendel’s Principles of Inheritance

Describe the three principles of inheritance.
Explain the relationship between phenotype and genotype.
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.
Draw and interpret a pedigree.

Mendel generalized the results of his pea-plant experiments into three principles that describe the basis of inheritance in diploid organisms. They are: the principle of segregation, the principle of dominance, and the principle of independent assortment. Together, these principles summarize the basics of classical, or Mendelian, genetics.

18.2.1 The Principle of Segregation

Since the white flower trait reappeared in the F2 generation, Mendel saw that the traits remained separate (not blended) in the plants of the F1 generation. This led to the principle of segregation , which states that individuals have two copies of each trait, and that each parent transmits one of its two copies to its offspring.

We now know that the traits that are passed on are a result of genes that are inherited on chromosomes during meiosis and fertilization. The fact that the genetic factors proposed by Mendel were carried on chromosomes was proposed in 1902 by Walter and Sutton and Theodor Boveri ( Figure 18.4 ) as the Chromosomal Theory of Inheritance .

Different versions of genes are called alleles . Diploid organisms that have two identical alleles of a gene on their two homologous chromosomes are homozygous for that trait. Diploid organisms that have two different alleles of a gene on their two homologous chromosomes are heterozygous for that trait.

The physical basis of the principle 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. Since each gamete receives only one homolog of each chromosome, it follows that they receive only one allele for each trait. At fertilization, the zygote receives one of each homologous chromosome, and one of each allele, from each parent.

18.2.2 The Principle of Dominance

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into dominant and recessive traits. Dominant traits are those that are expressed in a hybridization. Recessive traits become latent, or disappear, in the offspring of a hybridization but reappear in the progeny of the hybrid offspring. Thus, the violet-flower trait is dominant and the white-flower trait is recessive.

The principle of dominance states that in a heterozygote, only the dominant allele will be expressed. 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 18. 5 ). Individuals with a dominant trait could have either two dominant versions of the trait or one dominant and one recessive version of the trait. Individuals with a recessive trait have two recessive alleles.

In Mendel’s experiments, the principle of dominance explains why the F1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical. The recessive allele will only be observed in homozygous recessive individuals. Some examples of human dominant and recessive traits are shown in Table 18.2 .

Table 18.2 Examples of dominant and recessive traits in humans.

The principles of segregation and dominance could be deduced by simple crosses that follow only one genetic trait. These crosses are called monohybrid crosses . Before we discuss the principle of independent assortment, let’s look at some tools and terminology used for monohybrid crosses.

18.2.3 Phenotypes and Genotypes

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, green is the dominant trait for pea pod color, so the pod-color gene would be abbreviated as G (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 green pods as GG , a homozygous recessive pea plant with yellow pods as gg , and a heterozygous pea plant with green pods as Gg .

The two alleles for each given gene in a diploid organism may be 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, which alleles it has, 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 green pods. Although the hybrid offspring had the same phenotype as the true-breeding parent with green pods, we know that the genotype of the parent was homozygous dominant ( GG ), while the genotype of the F1 offspring was heterozygous ( Gg ). We know this since the yellow pod allele reappeared in some of the F2 offspring ( gg ).

18.2.4 Using Punnett Squares for Monohybrid Crosses

Punnett squares , devised by the British geneticist Reginald Punnett, can be used to predict the possible outcomes of a genetic cross or mating and their expected frequencies. 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. 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 18. 6 ).

A self-cross of one of the Yy heterozygous offspring can be represented in a 2 × 2 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 18. 6 ). 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.

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 18. 6 ). Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, 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.

Using a Test Cross to Determine Genotype

Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. Called the test cross , this technique is still used by plant and animal breeders. In a test cross, an organism with the dominant phenotype is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant- expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait. Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes ( Figure 18. 7 ). The test cross further validates Mendel’s postulate that pairs of unit factors segregate equally.

Concept Check

In pea plants, round peas (R) are dominant to wrinkled peas (r). You do a test cross between a pea plant with wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. You end up with three plants, all which have round peas.

From this data, can you tell if the round pea parent plant is homozygous dominant or heterozygous?
If the round pea parent plant is heterozygous, what is the probability that a random sample of 3 progeny peas will all be round?

18.2.5 Using Pedigrees to Study Inheritance Patterns

Many human diseases are inherited genetically. A healthy person in a family in which some members suffer from a recessive genetic disorder may want to know if he or she has the disease-causing gene and what risk exists of passing the disorder on to his or her offspring. Of course, doing a test cross in humans is unethical and impractical. Instead, geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases.

Each row of a pedigree represents one generation of the family. Women are represented by circles; males by squares. People who had children together are connected with a horizontal line and their children are connected to this line with a vertical line. See Figure 18. 8 for an example of a pedigree for a human genetic disease.

People with the recessive genetic disease alkaptonuria cannot properly metabolize two amino acids, phenylalanine and tyrosine. 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, both parents 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?

18.2.6 Principle of Independent Assortment

Mendel’s principle 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 a 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 principle 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 18.9 ).

For the F2 generation, the principle 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 principle 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 × 4 Punnett square 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 18.9 ).

The physical basis for the principle of independent assortment also lies in meiosis I, in which the different homologous pairs line up in random orientations. Each gamete can contain any combination of paternal and maternal chromosomes (and therefore the genes on them) because the orientation of tetrads on the metaphase plane is random.

Testing the Hypothesis of Independent Assortment

To better appreciate the amount of labor and ingenuity that went into Mendel’s experiments, proceed through one of Mendel’s dihybrid crosses.

Question : What will be the offspring of a dihybrid cross?

Background : Consider that you have access to a large garden in which you can cultivate thousands of pea plants. There are several true-breeding plants with the following pairs of traits: tall plants with inflated pods, and dwarf plants with constricted pods. Before the plants have matured, you remove the pollen-producing organs from the tall/inflated plants in your crosses to prevent self-fertilization. When the plants mature, they are manually crossed by transferring pollen from the dwarf/constricted plants to the stigmata of the tall/inflated plants.

Hypothesis : Both trait pairs will sort independently according to Mendelian principles. When the true-breeding parents are crossed, all of the F1 offspring are tall and have inflated pods, which indicates that the tall (T ) and inflated (I) traits are dominant over the dwarf (t) and constricted (i) traits, respectively. A self-cross of the F1 heterozygotes results in 2,000 F2 progeny.

Test the hypothesis : You cross the dwarf and tall plants and then self-cross the offspring. For best results, this is repeated with hundreds or even thousands of pea plants. What special precautions should be taken in the crosses and in growing the plants?

If these traits sort independently, the ratios of tall:dwarf and inflated:constricted will each be 3:1. Each member of the F1 generation therefore has a genotype of TtIi . Figure 18.1 0 shows a cross between two TtIi individuals. There are 16 possible offspring genotypes. The offspring proportions: tall/inflated:tall/constricted:dwarf/inflated:dwarf/constricted show a 9:3:3:1 ratio. Notice from the grid that when considering the tall/dwarf and inflated/constricted trait pairs in isolation, they are each inherited in 3:1 ratios.

Analyze your data: You observe the following plant phenotypes in the F2 generation: 2706 tall/inflated, 930 tall/constricted, 888 dwarf/inflated, and 300 dwarf/constricted. Reduce these findings to a ratio and determine if they are consistent with Mendelian principles.

Form a conclusion: Were the results close to the expected 9:3:3:1 phenotypic ratio? Do the results support the prediction? What might be observed if far fewer plants were used, given that alleles segregate randomly into gametes? Try to imagine growing that many pea plants, and consider the potential for experimental error. For instance, what would happen if it was extremely windy one day?

18.3 | Exceptions to Mendel’s Principles of Inheritance

Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, and sex linkage.
Describe genetic linkage.
Describe how chromosome maps are created.
Explain the phenotypic outcomes of epistatic effects between genes.

Although Mendel’s principles still apply to some situations, many situations exist in which they do not apply. These “exceptions” to Mendelian genetics are discussed below.

18.3.1 Alternatives to Dominance and Recessiveness

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

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 18. 11 ), 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 red flowers is incompletely dominant over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses. In this case, the genotypic ratio would be 1 CRCR :2 CRCW :1 CWCW , and the phenotypic ratio would be 1:2:1 for red:pink:white.

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 and LNLN ) 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 18. 12 ). 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. The Himalayan 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.

An example of multiple allelism in humans pertains to ABO blood type. A person’s blood type (e.g., type A or type O) is caused by different combinations of three alleles: IA, IB, and IO. A person with type A blood could have either IAIA or IAIO genotype. A person with type B blood could have IBIB or IBIO genotype. A person with type O blood must have the IOIO genotype. Note that type AB blood is an example of codominance (IAIB).

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 rabbit fur color, the wild-type allele may supply a given dosage of fur pigment, whereas the mutants supply a lesser dosage or none at all.

Multiple Alleles Confer Drug Resistance in the Malaria Parasite

Malaria is a parasitic disease that is transmitted to humans by infected female Anopheles gambiae mosquitos ( Figure 18.13a ). It is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia. Plasmodium falciparum is the most deadly causative agent of malaria ( Figure 18.13b ). When promptly and correctly treated, P. falciparum malaria has a mortality rate of 0.1%. 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.

In Southeast Asia, Africa, and South America, P. falciparum has developed resistance to the anti-malarial drugs chloroquine, mefloquine, and sulfadoxine-pyrimethamine. P. falciparum , which is haploid during the life stage in which it infects humans, has evolved multiple drug-resistant mutant alleles of the dhps gene. Varying degrees of sulfadoxine resistance are associated with each of these alleles. Being haploid, P. falciparum needs only one drug-resistant allele to express this trait.

Environmental Effects

Interestingly, the Himalayan phenotype in rabbits 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. In this case, the protein product of the gene does not fold correctly at high temperatures. A similar gene gives Siamese cats their distinctive coloration.

Temperature-sensitive proteins are also at work in arctic foxes and rabbits, which are white in the winter and darker colored during the summer. In these cases, the protein product of the gene does not fold correctly at colder temperatures. The mutation that caused this coloration was advantageous to these species, so they persisted in the populations.

18.3.2 X-Linked Traits are an Exception to the Principle of Segregation

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 is present on the X chromosome, it is said to be X-linked .

Eye color in Drosophila was one of the first X-linked traits to be identified. Like humans, Drosophila males are XY and females are XX. In flies, the wild-type eye color is red (X W ) which is dominant to white eye color (X w ) ( Figure 18.1 4) . Females can be X W X W , X W X w or X w X w . However, Drosophila males lack a second allele copy on the Y chromosome, so their genotype can only be X W Y or X w Y. 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.

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 P generation. When the P male expresses the white-eye phenotype and the female is homozygous red-eyed, all members of the F1 generation exhibit red eyes ( Figure 18.1 5 ). The F1 females are heterozygous (X W X w ), and the males are all X W Y, since they received their X chromosome from the homozygous dominant P female and their Y chromosome from the P male. A cross between a X W X w female and an X W Y male would produce only red-eyed females and both red- and white-eyed males. A cross between a homozygous white-eyed female and a male with red eyes would produce only heterozygous red-eyed females and only white-eyed males.

What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color?

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 Disorders

Sex-linkage studies in Morgan’s laboratory provided the fundamentals for understanding X-linked recessive disorders in humans, which included red-green color blindness, Types A and B hemophilia, and muscular dystrophy. 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 recessive X-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 18.1 6 ). Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted to subsequent generations.

18.3.3 Lethal Alleles are Apparent Exceptions to the Principle of Segregation

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 through heterozygous carriers. The wild-type allele functions at a capacity sufficient to sustain life and is therefore considered to be dominant over the nonfunctional allele. If two heterozygous parents mate, one quarter of their offspring will be homozygous recessive. Because the gene is essential, these individuals will die. This will cause the genotypic ratio among surviving offspring to be 2:1 rather than 3:1. This inheritance pattern is referred to as recessive lethal .

The dominant lethal inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote. Dominant lethal alleles are very rare because, as you might expect, the allele only lasts one generation and is not transmitted. However, dominant lethal alleles might not be expressed until adulthood. The allele may be unknowingly passed on, resulting in a delayed death in both generations. An example of this in humans is Huntington disease, in which the nervous system gradually wastes away ( Figure 18.1 7 ). People who are heterozygous for the dominant Huntington allele ( Hh ) will inevitably develop the fatal disease. However, the onset of Huntington 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.

18.3.4 Linked Genes Violate the Principle of Independent Assortment

Although all of Mendel’s pea characteristics behaved according to the principle of independent assortment, we now know that some allele combinations are not inherited independently of each other. Genes that are located on different chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string. Genes that are on the same chromosome are linked and are therefore likely to be inherited together. When homologs separate during meiosis I, entire chromosomes segregate into separate daughter cells, carrying all of their linked genes with them.

However, because of 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 order. However, since each homolog came from a different parent, the alleles may differ on homologous chromosome pairs. Prior to meiosis I, homologous chromosomes replicate and synapse so that genes on the homologs align with each other. At this stage, segments of homologous chromosomes cross over and exchange segments of genetic material ( Figure 18.1 8 ). Because the genes are aligned, 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.

When two genes are located in close proximity on the same chromosome, their alleles are more likely 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 the homologous chromosome from one parent has alleles for tall plants and red flowers, and the homolog from the other parent has alleles 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. Since the genes were close together on the same chromosomes, the chance of a crossover event happening between them is slim. Therefore, 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 crossovers between them increases, and the genes behave more as if they are on separate chromosomes. The further apart two linked genes are on a chromosome, the more progeny with nonparental genotypes will appear.

Genetic Linkage and Distances

Geneticists have used the proportion of nonparental gametes as a measure of how far apart genes are on a chromosome. Using this information, they have constructed elaborate maps of genes on chromosomes. Briefly, the more crossover that occurs between two linked genes, the further apart they are on the chromosome. The frequency of crossover is measured by counting the number of offspring that have nonparental genotypes. By using recombination frequency to predict genetic distance, the relative order of genes on chromosome 2 could be inferred.

18.3.5 Epistasis is an Exception to the Principle of Independent Assortment

Mendel’s studies in pea plants implied that every characteristic was distinctly and completely controlled by a single gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.

Genes may function in complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype. Genes may also oppose each other. In epistasis , the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.

An example of epistasis is pigmentation in mice. The wild-type coat color, agouti ( AA ), is dominant to solid-colored fur ( aa ). However, a separate gene ( C ) is necessary for pigment production. A mouse with a recessive c allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus A . Therefore, the genotypes AAcc , Aacc , and aacc all produce an albino phenotype. A cross between heterozygotes for both genes ( AaCc x AaCc ) would generate offspring with a phenotypic ratio of 9 agouti:3 solid color:4 albino ( Figure 18.19 ). In this case, 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 × 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.

As you work through genetics problems, keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction. Recall the phenotypic inheritance pattern for Mendel’s dihybrid cross, which considered two non-interacting genes—9:3:3:1. Similarly, we would expect interacting gene pairs to also exhibit ratios expressed as 16 parts. Note that we are assuming the interacting genes are not linked; they are still assorting independently into gametes.

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] ↵

Introduction to Molecular and Cell Biology Copyright © 2020 by Katherine R. Mattaini is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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13 Introduction to Mendelian Genetics

Donald Lee; Walter Suza; Amy Kohmetscher; and Marjorie Hanneman

Outline the experimental approach Mendel used to propose the idea that genes exist, control traits, and are inherited in predictable ways.
Compare the methods used by Mendel and Punnett to predict trait inheritance.

Introduction

In plant and animal genetics research, the decisions a scientist will make are based on a high level of confidence in the predictable inheritance of the genes that control the trait being studied. This confidence comes from a past discovery by a biologist named Gregor Mendel, who explained the inheritance of trait variation using the idea of monogenic traits.

Monogenic characters are controlled by the following biological principles:

Living things have genes in their cells that encode the information to control a single trait. These genes are stable and passed on from cell to cell without changing.
The genes are in pairs in somatic cells.  When these cells divide to form gametes, the pair of genes is divided.  One gene from the pair goes into a gamete.
Male gametes (pollen) combine with female gametes (eggs) in the wheat flower pistil and fuse to form the next generation (zygote).  Gamete union is random.
The zygote, again, has two copies of each gene. As the zygote grows into a multicellular seed and the seed grows into a plant, the same two gene copies are found in every cell.

Let’s take a short genetics history lesson to understand their confidence.

black and white photo of Gregor Mendel, a white man wearing glasses.

Mendel’s Peas

In the mid 1800’s, an Austrian monk named Gregor Mendel (Figure 1) decided he should try to understand how inherited traits are controlled.  He needed a model organism he could work with in his research facility, a small garden in the monastery, and a research plan.  His plan was designed to test a hypothesis for the inheritance of trait variation.

Since Mendel could obtain different varieties of peas that differed in easy to observe traits such as flower color, seed color and seed shape, and he could grow these peas in his garden, he chose peas as the model organism for conducting his inheritance control study. A model is easy to work with and often what you learn from the model you can apply to other organisms.

The Hypothesis

While many biologists were interested in trait inheritance, at the time Mendel conducted his experiments none of the biologists had published evidence that inheritance could be predicted.  Mendel made this bold statement.  His hypothesis was that he could observe “mathematical” regularities in the appearance of a trait that was passed on from parents to their offspring.  Mendel had the idea that mathematical regularities could be observed and could be used to explain the biology of inheritance!

Mendel’s experimental plan was designed to test the hypothesis.  He identified true breeding lines of peas by allowing them to self pollinate (which we will refer to as “selfing”) and examining their offspring. Pea plants have flowers that contain both male and female reproductive parts; if a pea flower is left undisturbed, the male and female gametes from the same flower will combine to produce seeds, the next generation.  If the pea always made offspring like itself, Mendel had his true breeding line.  He then made planned crosses between lines that differed by just one trait (monohybrid crosses). The controlled monohybrid cross was the first step in his experiment that allowed him to look for mathematical regularities in the data for three generations.  Table 1 below shows the data from a series of these monohybrid cross experiments.

The Analysis

By summarizing his data in a single table, Mendel could look for those hypothesized math regularities. A regularity is a repeated observation.

*Gray seed coat also had purple flowers; White seed coat had white flowers. 

Table 1 demonstrates that Mendel was serious about the math.  He generated large numbers of offspring that allowed him to observe mathematical ratios.  From his table of data, we can see mathematical patterns appear with every monohybrid cross he made.

F 1 :  All the plants had the same phenotype as one of the parents.
F 2 :  Both phenotypes are present, the phenotype that was not expressed in the F 1 appears again in the F 2 but is always the least frequently produced.  The average ratio is about 3:1 for the two phenotypes.

What was striking to Mendel was that every character in his study exhibited the same kind of mathematical pattern.  This suggested that the same fundamental processes inside the plant’s reproductive cells were at work controlling the inheritance of each trait.

Now Mendel had the task of providing a description of the fundamental biology process controlling each of these traits.  He needed to come up with ideas that no one had yet proposed to explain biology.

New Idea #1:

The traits expressed in the pea plant were controlled by some kind of particle. These hereditary particles are stable and passed on intact from parent to offspring through the sex cells. (NOTE: Sex cells or gametes were not a new idea, Mendel was aware that biologists knew sexually reproducing plants and animals needed to make gametes.)  We now call these particulate factors genes and will use that term in the rest of this reading.

New Idea #2:

Genes are stable, and genes can have alternative versions (alleles).

New Idea #3:

Genes are in pairs in somatic cells and these paired genes separate during gamete formation.  Each gamete will have one gene from the pair of genes. The segregating of the paired genes from the somatic cells of the parent into gametes is random.  Because segregation is random, a parent that has two different alleles for a gene pair will make two kinds of gametes and makes these gametes at equal frequencies.

From Mendel’s ideas, we can see that in a situation in which there was a normal version of a gene (we can call it the R gene) and an alternate version (r), the plant could produce gametes with just the R gene or just the r gene.

New Idea #4:

Plant flowers are designed to allow male gametes (pollen) to combine randomly with the female gametes (egg).  When the gametes randomly come together, they bring the genes they carry to the same zygote. This means plants could have the genotype RR, Rr , or  rr  in families that have both the R and r alleles.

New Idea #5:

Mendel proposed that the genes controlling a trait not only paired in somatic cells, they also interacted in controlling the traits of the plants.  For the traits in his experiment, he proposed that one allele interacted with the other in a dominant fashion.  That means a plant that is the genotype RR would have the same phenotype as an Rr plant.  The R allele is dominant to the r allele.

Ideas and Data advance science

Those were Mendel’s new ideas; he used them to make sense of his experiment data and observations. Let’s think like Mendel and apply those ideas.

All the F1 were the same

Mendel’s new ideas could explain this observation. Since his parents were true breeding, he was always making a cross between homozygous parents.  Homo means the same, so the parents had two copies of the same version of the gene.

Crossing RR X rr plants to produce Rr

Since the R is dominant to r , then the Rr offspring (named the F 1 ) look the same (have the same phenotype) as the RR parent. Therefore, only one phenotype is observed in the F 1 .  But the F 1  genotype is different from either parent.  It is heterozygous (two different alleles).

Somatic cells (with two genes) are made up of two gametes (each with one gene), represented as sets of capital and lowercase letters.

The F 2 :  both traits appear in about a 3:1 ratio

Mendel could explain the reappearance of the recessive trait and the ratio by combining the idea of genes with the idea of random segregation.  Mendel used simple algebra to explain this result.

First, he wrote out a mathematical expression to account for the gametes made in the male part of the F 1 flower or in the female part.

½ R + ½ r = all the gametes made (Figure 2).

Next, he reasoned that if pollen randomly united with the egg to combine the genes in the gametes, then algebra could be used to predict the result by multiplying the gamete expressions.

(½ R + ½ r) X (½ R + ½ r) = all the F 2 offspring made.

If we do the multiplication above, we get …

¼ RR + ¼ Rr + ¼ Rr + ¼ rr = ¼ RR + ½ Rr + ¼ rr = predicted fractions of F2 genotypes.

If this math is causing your brain to lose focus, you might be experiencing what Mendel’s contemporaries experienced when they read his published research paper.  While many biologists were motivated to understand how the variation among animals and plants was controlled and inherited, it took biologists 30 years to recognize that Mendel’s new ideas to explain inheritance of traits in peas could be applied to inheritance of traits in other living organisms.

One possible explanation for this  30-year delay in appreciation is that it was difficult for biologists to understand how math could explain biology. One biologist that did understand what Mendel was describing was Punnett.  Punnett decided to convert Mendel’s algebra into a more graphic representation of the process of gamete segregation and random union.

The Punnett Square

Math: (¼ RR + ½ Rr + ¼ rr).

Punnett designated the gametes made in the male and female parents with single letters (Figure 3). The diagram shows that when the gametes combine, the offspring (inside the squares) again have the genes in pairs in their cells.  Accounting for the random union of gametes is accomplished with the four squares in the diagram.  Two squares give the same Rr result, one the RR genotype and one  rr . Both the algebra and diagram approaches provide the same prediction. Crossing an Rr with an Rr will produce three genotypes, RR, Rr and  rr . They will be produced in a ratio based on the principle of segregation.

Gene inheritance from two plants, each with one uppercase and one lowercase r. The results are one double uppercase, one double lowercase, and two half upper- half lowercase Rs. This implies that there is a 50% chance of similar offspring, and a 25% chance of all-dominant or all-recessive offspring.

The genes controlling the monogenic traits behaved in predictable ways

Punnett’s diagram clarified for many biologists what Mendel was telling them in his published article. This was a challenging idea to understand because he was asking biologists to use something they could not see (genes) and explain something they could see (traits in peas or some other living organism).

Because Mendel recognized he was proposing a very different idea with the segregation principle, he was likely motivated to share the most convincing evidence possible.  Mendel conducted additional experiments.  One experiment was to test the hypothesis that there were two different kinds of F 2 which expressed the dominant trait, and these two types were being made by the F 1 in predictable fractions.  How would Mendel show that F 2 which had the same phenotype did not always have the same genotype?

Mendel tested the breeding behavior of the F 2 .  Mendel harvested all the selfed seed produced by his F 2 and grew progeny rows of F 3 .  His segregation principle predicted that of the dominant F 2 , there should be two that are heterozygous for every one homozygote made (on average).  The results of this experiment are summarized in  Table 2 .  Did Mendel’s data support the hypothesis?

Average ratio heterozygote F 2 to homozygote F 2 was 2.06 to 1.

The data show that, if we select a sample of F 2 with the dominant trait (Round seed or Yellow cotyledon), the principle of segregation predicts that there should be 2 heterozygotes for every 1 homozygotes.

Mendel’s data from rows of F 3 that all came from F 2 with the dominant trait supported his hypothesis. There were always two kinds of rows (true breeding and mixed) and the rows were in a 2:1 ratio.  This fits with the principle of segregation .

By publishing these results in a scientific journal, Mendel allowed other scientists to learn from his work. This story reveals the real power of publishing research in the “permanent” scientific literature. The power of publication does not mean you were right with your science. The real power is that other scientists can find your paper, read it, think about your ideas, and then test them.  In Mendel’s case, he was already dead when his fellow biologists discovered that his new ideas to explain the biology of peas were not only correct, but universal in their application.

Mendel’s Dihybrid Cross Experiments

Proper credit must be given to the idea of independent assortment. Gregor Mendel was the first to put this idea down on paper based on what he observed with his pea experiments. Furthermore, Mendel performed additional experiments to back up his ideas. Let’s examine his experiments with peas from the late 1800’s.

The outline below describes Mendel’s dihybrid cross experiments. The pattern observed in the results should look familiar!

The Experiment

Parents: round seeds, yellow seeds (RRYY) x wrinkled seeds, green seeds (rryy).
F 1 : All round and yellow seeds (RrYy).
Selfing: F 1 (RrYy x RrYy):

Mendel explained his results as follows:

The F 1 plants have the genotype RrYy and can make four kinds of gametes RY, Ry, rY and ry.

Note that with both the Mendel algebra and Punnett square, the RRYY genotype occurs one time and the  RrYy  genotype occurs four times (Table 4). Mendel’s algebra and Punnett’s squares can be summarized to give the same results.

Selfing the F 2 to produce F 3

The easiest experiment to perform was to let the plants self-pollinate and then keep good records. After scoring his 556 F 2 seeds (Table 5) he took the 315 that were round and yellow and planted them in one part of his garden. The plants that grew were allowed to self-pollinate. Of the 315 round and yellow seeds planted, 301 plants matured and produced seed. The seed produced was the F 3 generation. At harvest, Mendel needed to exercise the utmost care. Each F 2 plant was handled separately. The seeds from the plant were harvested and Mendel then scored the F 3 seeds that came from the same F 2 plant. This can be referred to as F 2:3 data and the table below summarizes his complete experiment using all of the F 2 phenotypes.

Mendel’s F 2 data supported his principle of independent assortment. There were four different types of round yellow F 2 based on the kinds of progeny they could produce or their breeding behaviors. Based on the F 3 progeny produced, the F 2 genotype was deduced. For example, if a round, yellow seed gave all round progeny it must have the genotype RR . If it gave both round and wrinkled it was Rr .

Furthermore, the numbers of F 2 plants with each breeding behavior were in agreement with what was expected with independent assortment. There were four times as many round and yellow F 2 that gave all four phenotypes of F 3 seeds (138) compared to the round and yellow F 2 that were true breeding (38). Overall, there were nine types of breeding behaviors demonstrated in the F 2 demonstrating that there were nine F 2 genotypes. In all cases, the fractions observed in the F 2 agreed to the principle of independent assortment. Mendel’s well-planned experiment provided a convincing demonstration that genes behaved in this predictable manner.

The only thing better than performing an experiment that shows you were right about a new hypothesis is performing two experiments that show that you were right. That is what Gregor Mendel did! In his second experiment he crossed dihybrid F 1 plants with homozygous recessive plants in a test cross. This type of cross is named because the geneticist wants to perform a cross that will test or reveal the genotype of an organism. Therefore, a test cross is usually made between an organism with a dominant trait and a partner with a recessive version of this trait. Mendel performed the  RrYy  x  rryy  testcross and the expected progeny are shown in the Punnett square below:

RrYy gametes: RY, Ry, rY, ry

rryy gametes: all ry

Mendel established a rigorous precedent for using carefully planned multi-generation experiments to reveal the principles that governed trait inheritance. The beauty of Mendel’s accomplishments is that both the principles and his experimental approach can be applied to understanding the genetic control and inheritance of traits in many kinds of organisms still today.

Mendel’s principles of segregation and independent assortment are valid explanations for genetic variation observed in many organisms. Alleles of a gene pair may interact in a dominant vs. recessive manner or show a lack of dominance. Even so, these principles can be used to predict the future…at least the potential outcome of specific crosses.

Watch this video about Punnett Squares for more information

Genetics, Agriculture, and Biotechnology Copyright © 2021 by Donald Lee; Walter Suza; Amy Kohmetscher; and Marjorie Hanneman is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Science News

How we got from gregor mendel’s pea plants to modern genetics.

Philosopher Yafeng Shan explains how today's understanding of inheritance emerged from a muddle of ideas

In 1900, Gregor Mendel’s experiments on pea plants were introduced into the study of heredity.

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Quill: That background helps explain why the discovery of the double-helix structure of DNA, from James Watson and Francis Crick, along with Rosalind Franklin and Maurice Wilkins, was so monumental. By knowing the structure of DNA, people could think about how the physical process of inheritance might work. Is that right?

Shan: Today we say, ‘Ah, so the process of inheritance is quite straightforward: Basically, DNA can be transcribed to RNA, and RNA can be translated into protein, and protein is responsible for phenotypic traits.’ Roughly speaking, it is like that.

That double-helix model provided a very reliable and useful framework to study DNA replication, and transcription. That’s crucially important for the later work in molecular genetics. At the time, in 1953, when Watson and Crick proposed that model, their work was not immediately well-received. It was not cited a lot — just like Mendel’s paper — until the end of the 1950s, when other work confirmed that the structure of DNA provides a mechanism of controlling protein synthesis.

There are quite a lot of important discoveries that followed. It’s probably unfair, but from my point of view, the others aren’t as exciting as the discovery of the double helix. If I can borrow a phrase from American philosopher Thomas Kuhn, we are now in the period of “normal science,” or what he calls “mopping up.” It took another 40 or 50 years to get where we are now, but in terms of milestones in the history of genetics, if you ask me if there’s anything as important as the introduction of Mendel’s work and the discovery of the double helix, I would say I’m afraid nothing else is as fascinating.

Quill: Looking back at the history of genetics, are there lessons to take away in how we think about science and scientific progress?

Shan: When we look back, we see that genetics developed through multiple parallel lines from the very beginning. We’ve got Darwin. We’ve got de Vries developing Darwin’s approach. We’ve got Francis Galton and his biometric approach, developed further by Karl Pearson and Raphael Weldon — which we didn’t even get to discuss. We’ve got Bateson borrowing ideas from Mendel. And there is also the important line of inquiry, the chromosome theory, independently developed primarily by Sutton and Boveri.

Across the century, we start from classical genetics, then molecular genetics and now epigenetics (which studies changes in an organism that result from how genes are turned on and off, rather than alterations to the DNA sequence). That’s three historical episodes. One popular interpretation is that these three historical episodes or paradigms can be viewed as three scientific revolutions. But these paradigms are interactive with each other, not destructive or revolutionary. For instance, molecular genetics arises from the need to better understand the physical basis of heredity in classical genetics. Even today, the methods of classical genetics are still used in some problems.

I think there are lessons here about the nature and the aim of science. Science seems to be often characterized as an enterprise in explaining or understanding the phenomena of the world. It’s right to say scientists do make efforts to explain and understand. But there is another essential feature of science, namely exploratory or investigative. From the very beginning, none of the geneticists of the past century probably had a very clear idea of what a good explanation, what a good theory, what a good experiment would look like.

Our understanding of inheritance improved with the development of investigative or exploratory research. Ultimately, some of science’s most important features cannot be simply captured by concepts like truth or knowledge or understanding.

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Mendel's Experiments: The Study Of Pea Plants & Inheritance

** Gregor Mendel ** was a 19th-century pioneer of genetics who today is remembered almost entirely for two things: being a monk and relentlessly studying different traits of pea plants. Born in 1822 in Austria, Mendel was raised on a farm and attended the University of Vienna in Austria's capital city.

There, he studied science and math, a pairing that would prove invaluable to his future endeavors, which he conducted over an eight-year period entirely at the monastery where he lived.

In addition to formally studying the natural sciences in college, Mendel worked as a gardener in his youth and published research papers on the subject of crop damage by insects before taking up his now-famous work with Pisum sativum, the common pea plant. He maintained the monastery greenhouses and was familiar with the artificial fertilization techniques required to create limitless numbers of hybrid offspring.

An interesting historical footnote: While Mendel's experiments and those of the visionary biologist ** Charles Darwin ** both overlapped to a great extent, the latter never learned of Mendel's experiments.

Darwin formulated his ideas about inheritance without knowledge of Mendel's thoroughly detailed propositions about the mechanisms involved. Those propositions continue to inform the field of biological inheritance in the 21st century.

Understanding of Inheritance in the Mid-1800s

From the standpoint of basic qualifications, Mendel was perfectly positioned to make a major breakthrough in the then-all-but-nonexistent field of genetics, and he was blessed with both the environment and the patience to get done what he needed to do. Mendel would end up growing and studying nearly 29,000 pea plants between 1856 and 1863.

When Mendel first began his work with pea plants, the scientific concept of heredity was rooted in the concept of blended inheritance, which held that parental traits were somehow mixed into offspring in the manner of different-colored paints, producing a result that was not quite the mother and not quite the father every time, but that clearly resembled both.

Mendel was intuitively aware from his informal observation of plants that if there was any merit to this idea, it certainly didn't apply to the botanical world.

Mendel was not interested in the appearance of his pea plants per se. He examined them in order to understand which characteristics could be passed on to future generations and exactly how this occurred at a functional level, even if he didn't have the literal tools to see what was occurring at the molecular level.

Pea Plant Characteristics Studied

Mendel focused on the different traits, or characters, that he noticed pea plants exhibiting in a binary manner. That is, an individual plant could show either version A of a given trait or version B of that trait, but nothing in between. For example, some plants had "inflated" pea pods, whereas others looked "pinched," with no ambiguity as to which category a given plant's pods belonged in.

The seven traits Mendel identified as being useful to his aims and their different manifestations were:

• Flower color: Purple or white. • Flower position: Axial (along the side of the stem) or terminal (at the end of the stem). • Stem length: Long or short. • Pod shape: Inflated or pinched. • Pod color: Green or yellow. • Seed shape: Round or wrinkled. • Seed color: Green or yellow.

Pea Plant Pollination

Pea plants can self-pollinate with no help from people. As useful as this is to plants, it introduced a complication into Mendel's work. He needed to prevent this from happening and allow only cross-pollination (pollination between different plants), since self-pollination in a plant that does not vary for a given trait does not provide helpful information.

In other words, he needed to control what characteristics could show up in the plants he bred, even if he didn't know in advance precisely which ones would manifest themselves and in what proportions.

Mendel's First Experiment

When Mendel began to formulate specific ideas about what he hoped to test and identify, he asked himself a number of basic questions. For example, what would happen when plants that were true-breeding for different versions of the same trait were cross-pollinated?

"True-breeding" means capable of producing one and only one type of offspring, such as when all daughter plants are round-seeded or axial-flowered. A true line shows no variation for the trait in question throughout a theoretically infinite number of generations, and also when any two selected plants in the scheme are bred with each other.

• To be certain his plant lines were true, Mendel spent two years creating them.

If the idea of blended inheritance were valid, blending a line of, say, tall-stemmed plants with a line of short-stemmed plants should result in some tall plants, some short plants and plants along the height spectrum in between, rather like humans. Mendel learned, however, that this did not happen at all. This was both confounding and exciting.

Mendel's Generational Assessment: P, F1, F2

Once Mendel had two sets of plants that differed only at a single trait, he performed a multigenerational assessment in an effort to try to follow the transmission of traits through multiple generations. First, some terminology:

• The parent generation was the P generation , and it included a P1 plant whose members all displayed one version of a trait and a P2 plant whose members all displayed the other version. • The hybrid offspring of the P generation was the F1 (filial) generation . • The offspring of the F1 generation was the F2 generation (the "grandchildren" of the P generation).

This is called a _ monohybrid cross _: "mono" because only one trait varied, and "hybrid" because offspring represented a mixture, or hybridization, of plants, as one parent has one version of the trait while one had the other version.

For the present example, this trait will be seed shape (round vs. wrinkled). One could also use flower color (white vs. purpl) or seed color (green or yellow).

Mendel's Results (First Experiment)

Mendel assessed genetic crosses from the three generations to assess the heritability of characteristics across generations. When he looked at each generation, he discovered that for all seven of his chosen traits, a predictable pattern emerged.

For example, when he bred true-breeding round-seeded plants (P1) with true-breeding wrinkled-seeded plants (P2):

• All of the plants in the F1 generation had round seeds . This seemed to suggest that the wrinkled trait had been obliterated by the round trait. • However, he also found that, while about three-fourths of the plants in the F2 generation has round seeds, about one-fourth of these plants had wrinkled seeds . Clearly, the wrinkled trait had somehow "hidden" in the F1 generation and re-emerged in the F2 generation.

This led to the concept of dominant traits (here, round seeds) and recessive traits (in this case, wrinkled seeds).

This implied that the plants' _ phenotype (what the plants actually looked like) was not a strict reflection of their genotype _ (the information that was actually somehow coded into the plants and passed along to subsequent generations).

Mendel then produced some formal ideas to explain this phenomenon, both the mechanism of heritability and the mathematical ratio of a dominant trait to a recessive trait in any circumstance where the composition of allele pairs is known.

Mendel's Theory of Heredity

Mendel crafted a theory of heredity that consisted of four hypotheses:

1. Genes (a gene being the chemical code for a given trait) can come in different types. 2. For each characteristic, an organism inherits one allele (version of a gene) from each parent. 3. When two different alleles are inherited, one may be expressed while the other is not. 4. When gametes (sex cells, which in humans are sperm cells and egg cells) are formed, the two alleles of each gene are separated.

The last of these represents the ** law of segregation **, stipulating that the alleles for each trait separate randomly into the gametes.

Today, scientists recognize that the P plants that Mendel had "bred true" were homozygous for the trait he was studying: They had two copies of the same allele at the gene in question.

Since round was clearly dominant over wrinkled, this can be represented by RR and rr, as capital letters signify dominance and lowercase letters indicate recessive traits. When both alleles are present, the trait of the dominant allele was manifested in its phenotype.

The Monohybrid Cross Results Explained

Based on the foregoing, a plant with a genotype RR at the seed-shape gene can only have round seeds, and the same is true of the Rr genotype, as the "r" allele is masked. Only plants with an rr genotype can have wrinkled seeds.

And sure enough, the four possible combinations of genotypes (RR, rR, Rr and rr) yield a 3:1 phenotypic ratio, with about three plants with round seeds for every one plant with wrinkled seeds.

Because all of the P plants were homozygous, RR for the round-seed plants and rr for the wrinkled-seed plants, all of the F1 plants could only have the genotype Rr. This meant that while all of them had round seeds, they were all carriers of the recessive allele, which could therefore appear in subsequent generations thanks to the law of segregation.

This is precisely what happened. Given F1 plants that all had an Rr genotype, their offspring (the F2 plants) could have any of the four genotypes listed above. The ratios were not exactly 3:1 owing to the randomness of the gamete pairings in fertilization, but the more offspring that were produced, the closer the ratio came to being exactly 3:1.

Mendel's Second Experiment

Next, Mendel created _ dihybrid crosses _, wherein he looked at two traits at once rather than just one. The parents were still true-breeding for both traits, for example, round seeds with green pods and wrinkled seeds with yellow pods, with green dominant over yellow. The corresponding genotypes were therefore RRGG and rrgg.

As before, the F1 plants all looked like the parent with both dominant traits. The ratios of the four possible phenotypes in the F2 generation (round-green, round-yellow, wrinkled-green, wrinkled-yellow) turned out to be 9:3:3:1

This bore out Mendel's suspicion that different traits were inherited independently of one another, leading him to posit the ** law of independent assortment **. This principle explains why you might have the same eye color as one of your siblings, but a different hair color; each trait is fed into the system in a manner that is blind to all of the others.

Linked Genes on Chromosomes

Today, we know the real picture is a little more complicated, because in fact, genes that happen to be physically close to each other on chromosomes can be inherited together thanks to chromosome exchange during gamete formation.

In the real world, if you looked at limited geographical areas of the U.S., you would expect to find more New York Yankees and Boston Red Sox fans in close proximity than either Yankees-Los Angeles Dodgers fans or Red Sox-Dodgers fans in the same area, because Boston and New York are close together and both are close to 3,000 miles from Los Angeles.

Mendelian Inheritance

As it happens, not all traits obey this pattern of inheritance. But those that do are called Mendelian traits . Returning to the dihybrid cross mentioned above, there are sixteen possible genotypes:

RRGG, RRgG, RRGg, RRgg, RrGG, RrgG, RrGg, Rrgg, rRGG, rRgG, rRGg, rRgg, rrGG, rrGg, rrgG, rrgg

When you work out the phenotypes, you see that the probability ratio of

round green, round yellow, wrinkled green, wrinkled yellow

turns out to be 9:3:3:1. Mendel's painstaking counting of his different plant types revealed that the ratios were close enough to this prediction for him to conclude that his hypotheses were correct.

• Note: A genotype of rR is functionally equivalent to Rr. The only difference is which parent contributes which allele to the mix.

Scitable by Nature Education: Gregor Mendel and the Principles of Inheritance
Biology LibreTexts: Mendel's Pea Plants
OpenText BC: Concepts of Biology: Laws of Inheritance
Forbes Magazine: How Mendel Channeled Darwin

Cite This Article

Beck, Kevin. "Mendel's Experiments: The Study Of Pea Plants & Inheritance" sciencing.com , https://www.sciencing.com/mendels-experiments-the-study-of-pea-plants-inheritance-13718433/. 8 May 2019.

Beck, Kevin. (2019, May 8). Mendel's Experiments: The Study Of Pea Plants & Inheritance. sciencing.com . Retrieved from https://www.sciencing.com/mendels-experiments-the-study-of-pea-plants-inheritance-13718433/

Beck, Kevin. Mendel's Experiments: The Study Of Pea Plants & Inheritance last modified August 30, 2022. https://www.sciencing.com/mendels-experiments-the-study-of-pea-plants-inheritance-13718433/

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.

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.

Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!

Select the correct answer and click on the “Finish” button Check your score and answers at the end of the quiz

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Mendelian Inheritance

Mendelian inheritance refers to certain patterns of how traits are passed from parents to offspring. These general patterns were established by the Austrian monk Gregor Mendel, who performed thousands of experiments with pea plants in the 19th century. Mendel’s discoveries of how traits (such as color and shape) are passed down from one generation to the next introduced the concept of dominant and recessive modes of inheritance.

Mendelian Inheritance. This is one of those classic textbook terms that any student in a basic genetics class will learn. In textbooks, you often see pictures of plants or mice with certain Mendelian traits. This became much more real to me when I worked in a fruit fly lab in college and we were searching for mutations that correlated with smooth or rough surfaces of their eyes. We literally spent hours counting flies under the microscope to carefully track the numbers of flies in each category. Of course, it's important to note that not every trait is easily observable. Also, it's interesting that some Mendelian traits occur so rarely that new variants are being discovered all the time. It's a fascinating area of current research.

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COMMENTS

Mendel's experiments
Mendel is known as the father of genetics because of his ground-breaking work on inheritance in pea plants 150 years ago. ... 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. ...
Gregor Mendel and the Principles of Inheritance
By experimenting with pea plant breeding, Mendel developed three principles of inheritance that described the transmission of genetic traits, before anyone knew genes existed. Mendel's insight ...
Mendel's Experiments
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. Figure 2: Mendel's process for performing crosses included examining flower color.
8.1 Mendel's Experiments
As stated earlier, in genetics, "parent" is often used to describe the individual organism(s) that contribute genetic material to an offspring, usually in the form of gamete cells. ... 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
Mendelian inheritance
Mendel also developed the law of dominance, in which one allele exerts greater influence than the other on the same inherited character.Mendel developed the concept of dominance from his experiments with plants, based on the supposition that each plant carried two trait units, one of which dominated the other. For example, if a pea plant with the alleles T and t (T = tallness, t = shortness ...
Mendelian inheritance
Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularized by William Bateson. [1] These principles were initially controversial. When Mendel's theories were integrated with the Boveri-Sutton chromosome theory ...
Gregor Mendel
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.
Mendelian Genetics: lab
Mendelian Genetics: lab — The Biology Primer. Gregor Mendel was an Austrian monk that devoted nearly as much of his life to understanding the nature of heredity as he did in his fraternal duties. From his experiments with peas, he was able to determine several basic principles of how traits were passed from parents to offspring.
Mendelian Inheritance: Mendelism or Mendelian Genetics
Mendel conducted pioneering experiments with garden peas (Pisum sativum) in the 19th century and established the fundamental laws of inheritance. Mendelian Inheritance- Mendelism or Mendelian Genetics. Mendel's contributions to the field of genetics were initially overlooked but were rediscovered and recognized in the early 20th century.
Chapter 18. Mendelian Genetics
Chapter 18. Mendelian Genetics. Figure 18.1 Mendel experimented with garden peas to uncover the fundamentals of genetics. (Credit: modification of work by Jerry Kirkhart) Chapter Outline. 18.1 Mendel's Experiments. 18.2 Mendel's Principles of Inheritance. 18.3 Exceptions to Mendel's Principles of Inheritance.
5.10 Mendel's Experiments and Laws of Inheritance
Figure 5.10.5 Mendel's first experiment with pea plants. Figure 5.10.5 shows Mendel's first experiment with pea plants. The F1 generation results from the cross-pollination of two parent (P) plants, and it contains all purple flowers. The F2 generation results from the self-pollination of F1 plants, and contains 75% purple flowers and 25% ...
Introduction to Mendelian Genetics
Introduction. In plant and animal genetics research, the decisions a scientist will make are based on a high level of confidence in the predictable inheritance of the genes that control the trait being studied. This confidence comes from a past discovery by a biologist named Gregor Mendel, who explained the inheritance of trait variation using ...
How we got from Gregor Mendel's pea plants to modern genetics
Quill: In biology classes, we learn that Gregor Mendel's experiments breeding pea plants in the mid-19th century taught us that inherited traits are delivered to offspring on pairs of genes, one ...
Mendel's Experiments: The Study of Pea Plants & Inheritance
Mendel's Experiments: The Study of Pea Plants & Inheritance. Gregor Mendel was a 19th-century pioneer of genetics who today is remembered almost entirely for two things: being a monk and relentlessly studying different traits of pea plants. Born in 1822 in Austria, Mendel was raised on a farm and attended the University of Vienna in Austria's ...
Mendel's Laws of Inheritance
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.
Mendelian Inheritance
Definition. 00:00. …. Mendelian inheritance refers to certain patterns of how traits are passed from parents to offspring. These general patterns were established by the Austrian monk Gregor Mendel, who performed thousands of experiments with pea plants in the 19th century. Mendel's discoveries of how traits (such as color and shape) are ...

NOTIFICATIONS

Studying traits in peas

Traits in pea plants

Dominant and recessive traits

Traits are inherited independently

The next generations

Mendel’s findings were ignored

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Gregor Mendel’s Courage and Persistence

Understanding Dominant Traits

Mendel and Alleles

Dihybrid Crosses

Mendel’s Legacy

References and Recommended Reading

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21 Mendel’s Experiments

Mendel’s Crosses

Garden Pea Characteristics Revealed the Basics of Heredity

CONCEPTS IN ACTION

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8.1 Mendel’s Experiments

Mendel’s Crosses

Garden Pea Characteristics Revealed the Basics of Heredity

Mendelian Inheritance: Mendelism or Mendelian Genetics

Mendel’s Experiment

Monohybrid cross

Dihybrid cross

Mendel’s Laws of Inheritance

Law of Dominance

Law of Segregation

Law of Independent Assortment

Modes of Inheritance

Deviation from Mendel’s Findings

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Chapter 18. Mendelian Genetics

Introduction

18.1 | Mendel’s Experiments

18.1.1 Mendel’s Model System

18.1.2 Mendelian Crosses

18.1.3 Garden Pea Characteristics Revealed the Basics of Heredity

18.2 | Mendel’s Principles of Inheritance

18.2.1 The Principle of Segregation

18.2.2 The Principle of Dominance

18.2.3 Phenotypes and Genotypes

18.2.4 Using Punnett Squares for Monohybrid Crosses

Using a Test Cross to Determine Genotype

18.2.5 Using Pedigrees to Study Inheritance Patterns

18.2.6 Principle of Independent Assortment

Testing the Hypothesis of Independent Assortment

18.3 | Exceptions to Mendel’s Principles of Inheritance

18.3.1 Alternatives to Dominance and Recessiveness

Incomplete Dominance

Codominance

Multiple Alleles

Environmental Effects

18.3.2 X-Linked Traits are an Exception to the Principle of Segregation

Human Sex-linked Disorders

18.3.3 Lethal Alleles are Apparent Exceptions to the Principle of Segregation

18.3.4 Linked Genes Violate the Principle of Independent Assortment

Genetic Linkage and Distances

18.3.5 Epistasis is an Exception to the Principle of Independent Assortment

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13 Introduction to Mendelian Genetics

Introduction

Monogenic characters are controlled by the following biological principles:

Mendel’s Peas

The Hypothesis

The Analysis

New Idea #1:

New Idea #2:

New Idea #3:

New Idea #4:

New Idea #5:

Ideas and Data advance science

All the F1 were the same

Crossing RR X rr plants to produce Rr

The F 2 : both traits appear in about a 3:1 ratio

Crossing RR X rr plants to produce Rr

The F 2 :  both traits appear in about a 3:1 ratio

Selfing the F 2 to produce F 3

Mendelian Inheritance