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

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

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

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

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

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

Understanding of Inheritance in the Mid-1800s

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

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

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

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

Pea Plant Characteristics Studied

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

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

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

Pea Plant Pollination

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

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

Mendel's First Experiment

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

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

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

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

Mendel's Generational Assessment: P, F1, F2

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

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

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

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

Mendel's Results (First Experiment)

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

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

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

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

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

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

Mendel's Theory of Heredity

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

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

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

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

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

The Monohybrid Cross Results Explained

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

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

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

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

Mendel's Second Experiment

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

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

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

Linked Genes on Chromosomes

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

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

Mendelian Inheritance

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

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

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

round green, round yellow, wrinkled green, wrinkled yellow

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

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

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