mendel's pea plant experiment results

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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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.

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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’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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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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By Elizabeth Quill

February 7, 2022 at 11:00 am

The year was 1900. Three European botanists — one Dutch, one German and one Austrian — all reported results from breeding experiments in plants. Each claimed that they had independently discovered some remarkable patterns in inheritance that had been noticed by Gregor Mendel decades earlier and reported in “Versuche über Pflanzen-Hybriden,” or “Experiments in Plant Hybridization.” All three relied on or built upon the work of the Austrian monk, whose experiments in pea plants are famous today as the foundation of genetics.

Yet at the time, “there was no such discipline as genetics, nor was there a concept of the gene,” says Yafeng Shan, a philosopher of science at the University of Kent in England. Instead, there were many theories of how traits were inherited, including Charles Darwin’s theory of pangenesis, which described particles of inheritance called “gemmules” thought to be given off by all cells in the body and to collect in the reproductive organs.

From the muddle of ideas, Shan says, those three reports at the dawn of the 20th century helped introduce Mendel’s work to other scientists in the fledgling field of heredity. That set the stage for the development of Mendelian genetics as we know it today, and no doubt played into a century’s worth of developments in molecular biology, from the discovery of the structure of DNA to the sequencing of the human genome and the rise of genetic engineering.

To celebrate our 100th anniversary, we’re highlighting some of the biggest advances in science over the last century. To see more from the series, visit Century of Science .

But the path to our current understanding of the inheritance and variation at the heart of modern biology has been far more winding than most biology textbooks reveal. In the conversation that follows, Elizabeth Quill, special projects editor at Science News , talks with Shan about the origins of genetics and what progress over the past century tells us about the nature of science.

Quill: Our understanding of genetics has emerged nearly entirely in the last century. Can you take us back? What did scientists know at the beginning of the century?

Shan: The term genetics was coined to describe the study of heredity in 1905 by the English biologist William Bateson in a letter to his friend. The term gene was introduced later, in 1909, by the Danish biologist Wilhelm Johannsen to refer to the unit of hereditary material.

That said, there were at least 30 different theories of heredity or inheritance at the beginning of the 20th century. So to borrow Charles Dickens’ phrase: It was the best of times, and it was the worst of times for the study of heredity. There were many different theories, methods and lines of inquiry available, but there was no consensus on the mechanism and patterns of inheritance, nor was there any consensus on a reliable way to study them.

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 from each parent, and that there are dominant and recessive forms of genes. But if the concept of gene wasn’t fully developed in Mendel’s day, what did his work actually reveal?

Shan: If you walk into any university library and pick up a copy of a genetics textbook today, you may find the following narrative: Mendel developed a theory of inheritance, but unfortunately, the theory was neglected or overlooked for over three decades, and only rediscovered in 1900.

Actually, there are mistakes in that: Mendel’s theory was not a theory of inheritance. He never used the German word for heredity — Vererbung . His concern was instead about the development of hybrids. In other words, Mendel did propose a theory for patterns of characteristics in plant hybrids, but it is not a theory of inheritance. And Mendel’s theory was not neglected or overlooked. There were more than a dozen citations to his paper before 1900. That’s not a lot, but definitely not overlooked.

Some fascinating things did happen in 1900, though. Mendel’s work was introduced to the study of heredity by Hugo de Vries, Carl Correns and Erich von Tschermak. All of them renewed Mendel’s work for different purposes. That being said, none of these three became a pioneer of Mendelism as we know it today.

Quill: Who was that pioneer?

Shan: After the introduction of Mendel’s work to the study of heredity, one important pioneer was William Bateson, an English biologist. Originally, he was not interested in the problem of heredity. So, to some extent, he was an outsider. He was studying evolution, but he found Mendel’s work useful. Based on Mendel’s findings, he said, we can develop a new theory that is the correct way to study heredity and will further shed light on the nature of evolution. He was one of the most prominent figures in the movement, which at first was resisted by many people.

To cut the story short, Mendelism won the victory — though in the early days, it was quite different from the Mendelian genetics of today, which was mainly established and developed by T.H. Morgan and his students and team at Columbia.

Quill: Thomas H. Morgan isn’t as widely known as Mendel or Darwin, for example. Why was his work so important and what made it different from what came before?

Shan: He may not have become a household name, but Morgan is considered one of the most influential geneticists ever. He actually began his career as a zoologist and had diverse interests in morphology, regeneration, embryology, et cetera. He was using fruit flies as experimental organisms to test the Darwinian theory of evolution. Darwin believed evolution happened through a series of minor and gradual changes. Others, including de Vries, believed species evolved through mutations: radical, sudden change. Morgan bought that argument.

Initially, his work was not very successful, in his own words. He started his experiment in 1908 and found nothing at all until 1910. He mentioned to an office friend that it was two years’ time, just wasted. But sometimes magical things just happen. After two years, he was surprised to find a mutation.

But he was puzzled. This mutation that he observed could not be explained by de Vries’ theory of mutation. Rather, it could be better accounted for by the Mendelian approach. So here is where Morgan and his team began developing a Mendelian approach.

What Morgan did differently from early Mendelians, say Bateson, was that he and his team incorporated Mendelism with another important line of inquiry in the field, the chromosome theory of inheritance, which was developed primarily by American geneticist Walter Sutton and German zoologist Theodor Boveri. They came up with the idea that hereditary material must be somewhere within the chromosomes. That provided a physical basis for hereditary material.

Quill: And that must have proved successful?

Shan: Combining Mendelism and the chromosome theory of inheritance leads to one of the most remarkable achievements of Morgan and his colleagues: They produced the chromosome map for the fruit fly. They located different genes at different locations on the chromosome. With that map, you can calculate the frequency of recombination of genes in the following generations. With that single map, you can identify not only the position of the genes on the chromosomes, but also predict the phenomenon of inheritance.

Quill: We haven’t yet talked about DNA. Were geneticists interested in DNA at that time?

Shan: The study of DNA was part of the job of biochemists. DNA was first identified in the mid-19th century, roughly the same time as when Mendel was working on his peas. Swiss chemist Friedrich Miescher was looking for the most fundamental constituents of life. He identified some substance coming from the nucleus of the cell and named it “nuclein.” That is what we now call DNA.

After his great discovery, the importance of and implications of nuclein, or DNA, were debated for decades. By the turn of the 20th century, nuclein was identified as a nucleic acid, and the five bases of nucleic acids — G, A, C, T and U — were also identified. In the 1920s and ’30s, biochemists came to know that the nucleic acid present in chromosomes is DNA.

But the makeup of DNA was only being pursued by biochemists. Those who studied the problem of heredity did not pay serious attention to DNA until the 1940s.

Quill: How did DNA get incorporated into the study of heredity?

Shan: That is the process of merging of the two lines of inquiry — the line of inquiry in genetics and the line of inquiry in biochemistry. For geneticists, their main concern was about a pattern and mechanism of inheritance and how a particular trait is transmitted from generation to generation. And on the other hand, biochemists were looking for the physical foundations of life.

With the success of T.H. Morgan and his colleagues, geneticists had a better capacity to predict and explain the patterns of inheritance. Then an immediate question arose: So, what are genes?

According to the Morgan school of classical genetics, a gene is just a segment of the chromosome. That’s very easy. There was very popular analogy in which they described genes as beads on the string. But it was still quite unclear what the physical basis was.  

Oswald Avery and his colleagues reported evidence in 1944 that DNA, rather than protein, carries hereditary information. Even though Avery’s experiment was not actually the first — it was confirming work done by others in 1939 — his work was better received and better known within the community. People often refer to Avery’s great experiment, though at the time some skepticism remained.

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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Chapter 8: Introduction to Patterns of Inheritance

8.1 Mendel’s Experiments

Learning objectives.

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.

Watch the interactive video

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 (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 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 . 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 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.

Concept 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.

Section Summary

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

continuous variation: a variation in a characteristic in which individuals show a range of traits with small differences between them

discontinuous variation: a variation in a characteristic in which individuals show two, or a few, traits with large differences between them

dominant: describes a trait that masks the expression of another trait when both versions of the gene are present in an individual

F 1: the first filial generation in a cross; the offspring of the parental generation

F 2: the second filial generation produced when F 1 individuals are self-crossed or fertilized with each other

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

model system: a species or biological system used to study a specific biological phenomenon to gain understanding that will be applied to other species

P: the parental generation in a cross

recessive: describes a trait whose expression is masked by another trait when the alleles for both traits are present in an individual

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

trait: a variation in an inherited 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]

Concepts of Biology - 1st Canadian Edition Copyright © 2015 by Charles Molnar and Jane Gair is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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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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Last updated: April 22, 2013

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Mendel’s Pea Plant Experiment — The Root of All in Genetics

Mendel’s pea plant experiment which lasted for over a decade was a huge scientific breakthrough.

The pea plant has Seven different variable traits . Scientists say that it was due to his luck and the ever important selection of the plant that Mendel succeeded. They are:

• Flower color is purple or white;
• Seed color is yellow or green;
• Flower position is axial or terminal;
• Pod shape is inflated or constricted;
• Stem length is long or short;
• Pod color is yellow or green; and
• Seed shape is round or wrinkled.

So, if we cross bred any pea plants, there can be hundreds of permutations and combinations possible. So, what Mendel did was select one trait at a time. For example: pea albumin color, which can either be yellow or green.

He just wanted to find out whether crossbreeding a pea plant with all other traits except the seed colour the same, would yield a half green and half yellow pea, which would be amazing as well as weird.

However, he found out that the seed color of daughter plants or offspring plants was yellow, which confused him greatly. So, he tried the same thing again and again, this time selecting and crossbreeding plants while focusing on just one trait and found out that either one of the trait was expressed on the combination of two variable features of a same trait.

On basis of this, he proposed the Law of Dominance.

The next two laws were based on further study of this exquisite phenomenon. They are the Law of Segregation and Law of Inheritance .

Definition of Terms

Now before we start understanding the ever-so-important laws, let’s deal with a little more vocabulary first!

Genotype  

These are genes present in an organism . Example: for tall trait, they are TT ; for short, they are tt.

TT and tt is homozygous which means pure breed. If you breed TT with a TT , you will get a pure breed TT .The same goes for tt .

Tt is heterozygous which means hybrid .

Phenotype is how the trait physically shows up in the plant or a living being in general. The simplest way to determine an organism’s phenotype ? Look at it. Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow flowers, small flowers,etc.

Allele is a singular t from tt . Even in Tt , t singularly is known as an allele and T is also another allele. Alleles are little codes , for example ACTGC in DNA means red or say, yellow fruit.

The two of them together are needed to successfully express a trait in an organism. However, they are separated during  meiosis . 

How their separation affects the phenotypic character of an organism, we shall see in detail in the next post, as well as about the Law of Dominance , and phenotypic and genotypic ratios .

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i love for biology and i like for jone mendal but thise expriment was very vell in pea plant risarch

Srushti Excellent! Especially how accurately you explained basic vocabulary without the usual ramblings. Looking forward to future posts. Learned far too late in life my interest in Biology especially genetics. Thank You, Donna

Hey Srushtik Thanks a lot for posting this content 🙂

you’ve really helped me a lot in understanding the Concept of Dominance and helped me out with the question that WHY DID MENDEL EXPERIMENTED WITH PEA PLANTS

Well, if you have some more knowledge about this questions please do share it with me and others who follow your posts.

Looking forward for your future posts.

Thank You, Deipika 😉

Get in touch

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Mendel’s Experiments: Teacher's Manual

In this web lab, students experiment with garden pea plants (Pisum sativum) as did Austrian monk Gregor Mendel (1822-1884). Mendel chose to experiment with peas because they possessed four important qualities:

The traits that Mendel studied are listed below:

The Web Lab

This web lab has five sections that are accessible through the “Sections” button in the lower left-hand corner of the screen. Students can explore the entire web lab by clicking through or can jump to specific sections by using the menu. Each section is described below.

Introduction

Mendel is the guide for students throughout the web lab. When he first appears, he says, “Hello. My name is Gregor Mendel. I lived in Austria in the 1800s long before anyone knew about genes and genetics. I experimented with plants to study how traits are passed from parents to offspring ad discovered the basic rules of inheritance that are still used in your textbooks today. Come and try some of my experiments to see what you can discover about inheritance. Click Next to continue.”

The next text reads, “I used pea plants because they grow quickly and easily, and it is easy to see and recognize their different traits.”

Back To Mendel's Experiment Directory

Plant & Cross

This section of the web lab allows students to explore the traits on which Mendel experimented, then cross pea plants to see what offspring they produce.

Mendel urges students to, “Plant five pea plants and observe what they look like.” When students click the “Plant” button, the animated Mendel plants and waters five pea plants. Each of the pea plants quickly sprouts. By rolling over the plants with the cursor, the student can see the color of the pea pod, the shape of the pod, and the color and form of the ripe seed.

All of the different variations of pea plant can be seen in these growing peas, although the plants are randomly chosen each time the application is run. After they have planted and grown five plants, Mendel asks students how many distinguishing traits they see in the plants. On the next screen, he reveals that there are seven different traits:

These traits are all pictured in the plants below:

Students are then asked to experiment with plant crosses. Using the five plants that they grew, they can cross any plant with itself or with another plant. Students may begin to notice some patterns in the ways in which traits are inherited. For example, they may recognize that a plant with white flowers crossed with itself or another plant with white flowers will produce only white flowered plants, while a purple-flowered plant crossed with itself or another purple-flowered plant sometimes produces white-flowered offspring. By encouraging students to look at individual traits during their experimentation, you may find that they begin to recognize these patterns on their own.

After they have made five crosses, the Next button is enabled and students can move on to the following section.

Predict Results

In this section of the web lab, students explore plant crosses and predict what the offspring of these crosses will look like.

A plant with round peas and a random assortment of other traits appears on the screen. Mendel says “Cross this plant with itself. What pea shapes do the offspring have?”

When the student drags the plant into one of the Parent boxes, the Cross button appears. When the student clicks the Cross button, five offspring grow. Some of the offspring from the plant with round peas have wrinkled peas. Mendel then asks, “Were you surprised that a plant with round peas produced some offspring with wrinkled peas?”

A plant with wrinkled peas appears on the screen and students are asked to cross this plant with itself. As before, when the student drags the plant into one of the Parent boxes, the Cross button appears. When the student clicks the Cross button, five offspring grow.

Mendel appears and says, “What did you learn about your peas?” Students will probably recognize that, while a plant with round peas produced some offspring with wrinkled peas, the plant with wrinkled peas produced only offspring with wrinkled peas. This is one key to Mendel’s experimentation—a trait that was not apparent in a parent generation appeared in the F1 generation.

When the student click Next, two plants appear on the screen, both with wrinkled peas. The student is asked to predict the offsprings’ pea shapes (both round and wrinkled; all round; all wrinkled; or can’t predict). Because the allele that produces wrinkled peas is recessive, the offspring of this cross will all have wrinkled peas.

Mendel then explains the concept of dominant and recessive alleles by saying, “By performing my experiments with peas, I learned a lot about genetics and how traits are passed on. I noticed that sometimes offspring seem to have traits that their parents did not show. I called the traits that appeared to mask (or hide) other traits dominant. I called traits that seemed to be hidden recessive.”

In this section of the web lab, students experiment with pea plants to try to discover which alleles are dominant and which are recessive. Using four different pea plants, students can cross plants with themselves or with each other to determine dominance. One strategy that students might employ is to cross plants with themselves—offspring that show a different trait than the parent of such a cross possess the recessive allele (which was hidden by the dominant allele in the parent generation).

Mendel says, “Using these plants, figure out how the trait for flower color is passed on. Which color is dominant, white or purple? This is a pedigree. You can cross plants with themselves or with each other.”

When a student clicks on one of the plant symbols (a white or a black box), the cross button appears. If the student selects two plants, then the two plants are crossed and the offspring appear below. If a student selects only one plant and clicks the Cross button, then the plant self-fertilizes and the offspring appear below. Students can cross plants as many times as they want before deciding which allele is dominant.

Students can explore all seven of the pea traits that Mendel explored in this section. Four pea plants appear in the pedigree and students can select which trait they are looking at with the pulldown menu in the upper left corner of the screen.

When students have determined which alleles are dominant, they can record their choices in their notepads by clicking on the View Notepad button. The Check button allows students to check the answers they have input into their notepads. The following table shows each of the traits and which traits are dominant and which recessive.

Flowers located near the middle of the plant.

Traits that appear to mask (or hide) other traits.

A diagram of a family history used for tracing a trait through several generations.

Traits that can be hidden in one generation and then appear in the next.

Flowers located at the ends of the stems.

A distinguishing characteristic.

12.1 Mendel’s Experiments and the Laws of Probability

Learning objectives.

In this section, you will explore the following questions:

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

Connection for AP ® Courses

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

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

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

Teacher Support

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

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

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

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

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

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

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

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

Mendel’s Model System

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

Mendelian Crosses

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

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

Garden Pea Characteristics Revealed the Basics of Heredity

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

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

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

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

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

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

Science Practice Connection for AP® Courses

Think about it.

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

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

Possible answer:

Probability basics.

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

The Product Rule and Sum Rule

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

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

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

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

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

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

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• Authors: Julianne Zedalis, John Eggebrecht
• Publisher/website: OpenStax
• Book title: Biology for AP® Courses
• Publication date: Mar 8, 2018
• Location: Houston, Texas
• Book URL: https://openstax.org/books/biology-ap-courses/pages/1-introduction
• Section URL: https://openstax.org/books/biology-ap-courses/pages/12-1-mendels-experiments-and-the-laws-of-probability

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