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21 Mendel’s Experiments

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

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

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

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

Mendel’s Crosses

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

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

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

Garden Pea Characteristics Revealed the Basics of Heredity

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

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

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

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

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

CONCEPTS IN ACTION

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

Also, check out the following video as review

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

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

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How evolution works? - Class 12

Course: how evolution works - class 12   >   unit 4.

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Why Mendel chose peas

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show/hide words to know

Anther: the part of a flower that creates and stores the male reproductive cells (pollen) of a plant.

Cultural generation: all of the individuals born at about the same time.

Inheritance: genetic information passed down from a parent.

Mated: putting together male and female reproductive cells to create offspring.

Ovary: creates and stores the female reproductive cells in plants and animals.

Phenotype: the appearance of an individual that results from the interaction between their genetic makeup and the environment. Phenotypic trait...  more

Trait: a characteristic of an organism that can be the result of genes and/or influenced by the environment. Traits can be physical like hair color or the shape and size of a plant leaf. Traits can also be behaviors such as nest building behavior in birds.

Mendel's Pea Garden

When looking for something to experiment with, Mendel turned to what was already available in his own backyard: the common pea plant.

The pea plant was perfect for Mendel's experiments for a number of reasons. First, pea plants were easy to grow and could be grown quickly in large numbers.

With the help of a small brush, Mendel was able to move pollen from one plant to another to control which plants were being mated.

Second, the shape of the flowers made it easy to control which plants were being mated.

In flowering plants, male reproductive cells called pollen are created and stored on the anther. The female reproductive cells are created and stored in the ovary. When pollen touches the stigma, it falls through a tube and into the ovary. Here, it combines with female reproductive cells which begin to grow into seeds.

Mendel controlled breeding by separating the male and female parts of the flowers so they couldn't reproduce on their own. Next, he used a small brush to move pollen between plants.

Lastly, pea plants had a number of visible traits, called phenotypes, that were easy to identify. The inner pea color, for example, could be either green or yellow.

At first glance, pea plants might seem to have very little in common with animals or human beings. A closer look into the inside of the cell, however, will show you that the way that genes and chromosomes work is extremely similar in all living things. The same rules that determine how traits like pea color are passed down from parent to offspring also determine how traits like freckles or dimples are passed down in humans.

Parent Generation

Mendel began his experiments with true breeding strains, meaning groups of plants that pass down only one phenotype to their offspring. These true breeding strains were created by mating plants with the same traits for many generations.

First Generation

Mendel mated two different true breeding strains together, a green pea strain and a yellow pea strain, to see what phenotype the first generation of offspring would have. When Mendel looked at the offspring, called the F1 (or first) generation, he saw that every single one of the plants had yellow seeds.

Second Generation

Next, Mendel took the first generation plants and mated them with each other. What color seeds would you expect the next generation to have? To Mendel’s surprise, 25% of the offspring, called the F2 (or second) generation, actually had green seeds, even though all of the F1 parent plants had yellow seeds!

This result led Mendel to believe that it was possible for a trait to be present, but not visible, in an individual. Something from the original green parent plants was skipping a generation and being passed to the grandchildren. Mendel repeated this experiment with many different characteristics. He tested inner pea color, outer pea color, pea shape, flower position, stem length, unripe pod color, and pod shape. He had similar results every single time.

How is this possible? Let’s take a closer look at what’s happening on a genetic level with the help of a Punnett Square .

Additional images via Wikimedia Commons. Pea plant flower via AnRo0002. Purple sweet pea flower taken by  Giligone .

Read more about: Solving a Genetic Mystery

View citation, bibliographic details:.

• Article: Mendel’s Garden
• Author(s): Sabine Deviche
• Publisher: Arizona State University School of Life Sciences Ask A Biologist
• Site name: ASU - Ask A Biologist
• Date published: July 20, 2010
• Date accessed: June 12, 2024
• Link: https://askabiologist.asu.edu/mendel-garden

Sabine Deviche. (2010, July 20). Mendel’s Garden. ASU - Ask A Biologist. Retrieved June 12, 2024 from https://askabiologist.asu.edu/mendel-garden

Chicago Manual of Style

Sabine Deviche. "Mendel’s Garden". ASU - Ask A Biologist. 20 July, 2010. https://askabiologist.asu.edu/mendel-garden

MLA 2017 Style

Sabine Deviche. "Mendel’s Garden". ASU - Ask A Biologist. 20 Jul 2010. ASU - Ask A Biologist, Web. 12 Jun 2024. https://askabiologist.asu.edu/mendel-garden

The shape of pea flowers helped Mendel control breeding of the plants for his experiments.

Solving a Genetic Mystery

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1865: Mendel's Peas

1865: mendel's peas.

From earliest time, people noticed the resemblance between parents and offspring, among animals and plants as well as in human families. Gregor Johann Mendel turned the study of heredity into a science.

Mendel was a monk in the Augustinian order, long interested in botany. He studied mathematics and science at the University of Vienna to become a science teacher. For eight years, starting in 1857, he studied the peas he grew in the garden of his monastery. He carefully pollinated the plants, saved seeds to plant separately, and analyzed the succeeding generations.

He self-pollinated plants until they bred true - giving rise to similar characteristics generation after generation. He studied easily distinguishable characteristics like the color and texture of the peas, the color of the pea pods and flowers, and the height of the plants.

When he crossed true-breeding lines with each other, he noticed that the characteristics of the offspring consistently showed a three to one ratio in the second generation. For example, for approximately every three tall plants, one would be short; for about every three plants with yellow peas, one would have green peas. Further breeding showed that some traits are dominant (like tall or yellow) and others recessive (like short or green). In other words, some traits can mask others. But the traits don't blend: they are inherited from the parents as discrete units and remain distinct. Furthermore, different traits - like height and seed color - are inherited independently of each other.

More Information

References:.

Mendel read his paper, "Experiments in Plant Hybridization" at meetings on February 8 and March 8, 1865. He published papers in 1865 and 1869 in the Transactions of the Brunn Natural History Society .

Some Biographies of Mendel:

Iltis, Hugo, Life of Mendel . Eden and Cedar Paul, trans. London: George Allen & Unwin Ltd. 1932. From the German publication, "Gregor Johann Mendel, Leben, Werk, und Wirkung", Berlin: Julius Springer, 1924.

Orel, Vitezslav, Gregor Mendel: The First Geneticist . Oxford & London: Oxford University Press, 1996.

In the following paper, scientists explained, in molecular detail, the cause of the wrinkled seed trait that Mendel had observed in his peas:

Bhattacharyya M.K., Smith A.M., Ellis T.H., Hedley C., and Martin C.. The wrinkled-seed character of pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branding enzyme. Cell , 60: 115-122, 1990.

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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:

• Genes  (a gene being the chemical code for a given trait) can come in different types.
• For each characteristic, an organism inherits one  allele  (version of a gene) from each parent.
• When two different alleles are inherited, one may be expressed while the other is not.
• 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.

Related Articles

What is the main function of the punnett square, what are the three steps of the monohybrid cross, what is it when an allele of a gene masks a recessive..., what is an allele, what is the genotypic ratio in the f2 generation if..., how do genotype and phenotype affect how you look, how to determine an unknown genotype using a test cross, 2 examples of heterozygous traits, genotype & phenotype definition, what is the study of heredity, where do the physical traits we inherit come from, what is the dominant phenotype, what are examples of homozygous dominants, what makes an allele dominant, recessive or co-dominant.

• 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

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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Mendel Selected the Garden Pea Plants for His Experiments. Explain - Biology

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Mendel selected the garden pea plants for his experiments. Explain

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Mendel selected the garden pea Pisum sativum for his experiments on inheritance for the following reasons:-

• They are easy to cultivate.
• Their life cycle is quite short.
• The flowers of the pea plant are bisexual. Besides performing self-pollination, these plants can undergo cross-pollination if it is performed on them.
• The true-breeding or pure plants can be easily obtained through self-pollination.
• Their characteristics can be distinguished properly without any difficulty. This means that the traits in this plant can be easily observed (for example; tall versus dwarf plant).
• These plants possess large flowers. Therefore, anthers (male) can be easily removed to make this flower cross with the pollen from another flower.

RELATED QUESTIONS

Organisms produced by ______ show minor variations. 

Organisms produced through ______ show major variations.

Multiple choice question.

If the centromere is situated near the end of the chromosome, the chromosome is called ______.

Chromosomal theory of inheritance was proposed by _______.

If the genes are located in a chromosome as p-q-r-s-t, which of the following gene pairs will have the least probability of being inherited together?

Find the mismatch pair:

A colour-blind man marries a woman, who is homozygous for normal colour vision, the probability of their son being colour blind is –

Short Answer Question.

Enlist seven traits of pea plant selected/ studied by Mendel.

What are the reasons for Mendel’s success?

Human skin colour is an example of ______

The differences between parents and offspring, among offsprings of the same parents and individuals of the same species are called ______ 

How many different phenotypes are produced by the mulattoes in the F 2 generation?

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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 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. 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.

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 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

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. 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 ).

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.

• 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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Mendel selected pea plant for his experiments because of the following reasons, except

The correct option is d cross pollination could not be done artificially mendel selected pea plants for his experiments due to their easily detectable, contrasting characters. these are bisexual plants and grow to maturity in a single season only. in these plants, cross-pollination can also be done artificially..

Select the incorrect option:

Mendel used the garden pea plant for his hybridization experiment because:

Why did Mendel choose pea plant for his experiments?

Correct option is d. all of the above gregor mendel chose the pea plants for his experiments because the garden pea is an ideal subject in the study of genetics for the following reasons: presence of observable traits with contrasting forms. it produces many offspring in one cross. short life cycle. ease in manipulating pollination (cross pollination)..

Why did mendel choose pea plants for his experiments?

why did mendel choose pea plant 4 experiment?

Why did Gregor Mendel choose pea plants for his experiment?