15.1: How Mendel did what he did

To make genetic behaviors intelligible, Mendel purposefully selected (and bred) plants whose mating partners he controlled (↑); plants that produced high numbers of offspring, and that displayed easily characterized and uniform (from plant to plant) traits. The traits he chose were independent of one another and were not dramatically influenced by environmental effects (growth conditions). His most famous work involved the garden pea Pisum sativum, which displays all of these features. Mating in peas involves male pollen (the plant equivalent of animal sperm). During fertilization a pollen cell fuses with an ovule cell, the plant equivalent of an animal egg. Pea plants can self fertilize, but this can be prevented and the experimenter can control the source of the pollen.^[506]

Mendel identified or developed lines of peas that displayed one or the other of various pairs of traits (↓). In many cases, this involved "breeding out" natural variation. A case in point is pea color. The type of pea plants that Mendel worked with normally display a continuous range of seed colors, from green to yellow. Over a number of generations, Mendel selected the greenest and yellowest plants for in-breeding, leading to strains that produced seeds of uniform green or yellow color, with no intermediates. The plants "bred true" for seed color.

An individual plant is derived from a single pollen grain fertilizing an ovule. To say that a plant line breeds true means that when a plant is allowed to self-fertilize all of the offspring produced display the same form of the trait. These offspring will, if allowed to self-fertilize or to fertilize each other, again produce offspring that display the same form of the trait as the parent(s). The are in-bred, that is genetically homogeneous (similar), with little plant to plant variation - a situation quite different from most natural populations.

Next he crossed (fertilized) one plant with gametes from another. For example he fertilized a plant with white flowers with pollen from a plant with purple flowers, and examined the traits expressed in the offspring, known as the F₁ generation. On analyzing the traits of a large number of F₁ offspring he found that among this set of traits, only one of the pair of traits was displayed or expressed. When the parents (the F₀ generation) had purple or white petals, all of the offspring (F₁) individuals had purple flowers. It did not matter if the purple plant was the maternal or the paternal parent. In such a cross the parental trait displayed in the F₁ generation was said to be "dominant" to the "recessive" parental trait, that is the trait that was not displayed. The traits Mendel worked with all behaved in this way (that is one reason he studied them). Moreover, when two or three of these traits were displayed in the same individual, they did not influence each other - they behaved independently. The result was not surprising in that Mendel did not start with random traits, he selected traits that followed these rules, they were "well behaved", a fact we will consider further.

Mendel continued his experiments; he crossed true breeding F₀ individuals expressing one or the other trait, to produce F₁ individuals. He then crossed F₁ individuals to themselves or to F₁ individuals from other crosses (↓). Here came the surprise, from F₁ x F₁ crosses there emerged F₂ individuals that displayed the recessive form of the trait. As he collected large enough numbers of F₂ individuals, he discerned a pattern - approximately 25% of the F₂ individuals displayed the recessive form of the trait: a clear 1 to 3 ratio of individuals expressing the recessive traits compared to those that expressed the dominant trait. When the F₂ individuals that display the recessive trait were crossed to one another (or themselves), the resulting F₃ individuals all (100%) expressed the recessive trait (→). The F₂'s that expressed the recessive trait were like the original in-bred, recessive F₀ parent. However, the F₂ individuals that displayed the dominant trait were not all the same. When Mendel crossed the F₂ individuals that expressed the dominant trait to recessive F₀ individuals and then examined large numbers of offspring, the results fell into two classes. In one third (~33%) of cases, all the offspring displayed the dominant trait, while in two thirds (~66%) of the cases approximately half of the offspring expressed the dominant trait and half expressed the recessive trait.

Mendel used his data to come up with a model for trait behavior. He assumed that each trait was controlled by two factors (alleles) of what we now refer to as a gene. In each of the parental lines these two factors were the same, they were "homozygous" for either a dominant or a recessive allele. All of the gametes produced by an F₀ individual therefore carry the same allele of the gene associated with the trait. In a cross between parents homozygous for different alleles of a particular locus, the model predicts that all F₁ individuals are "heterozygous" and display the same "dominant" phenotype.

An F₁ individual heterozygous (for a particular gene) will (normally) produce equal numbers of two different types of gametes; 50% of the gametes will carry the dominant allele while 50% of the gametes will carry the recessive allele. When two F₁ individuals mate, there are four possibilities for the F₂ offspring. An F₁ gamete carrying the dominant allele can fuse with an F₁ gamete carrying either the dominant or the recessive allele. Similarly, an F₁ gamete carrying the recessive allele can fuse with a F₁ gamete carrying either the dominant or the recessive allele. If we assume that these events are equally probable. If the number of offspring is large enough we expect to find dominant to recessive phenotypes in the F₂ generation in a 3:1 ratio, All of the recessive phenotype individuals should be homozygous for the recessive allele. The individuals displaying the dominant phenotype can be either heterozygous or homozygous for the dominant allele. We can distinguish these two classes of individuals using what is known as a backcross; this involves crossing them to a homozygous recessive individual. If the individual to be tested is homozygous for the dominant allele, all of the offspring from the backcross will display a dominant phenotype. If the individual tested is heterozygous, if we collect a large enough number of offspring 50% should display the dominant phenotype and 50% should display the recessive phenotype. Given large enough numbers we will find that the F₂ generation consists of 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive, a 1 to 2 to 1 ratio. Mendel's observations were consistent with these ratios.

Key to Mendel's model was the assumption that the variants of a specific trait he examined were unambiguously distinguishable and determined by the alleles of a single gene. These are what are known as monoallelic traits. In addition, the alleles, and the traits they influenced displayed clear dominant-recessive behaviors with respect to one another; few alleles/traits behave this way. In many cases individuals heterozygous for a particular gene display a phenotype distinct from either homozygous form of the trait. Finally, while Mendel examined a number of traits, they all behaved independently of one another. That is because he selected them for their independence. As we know now, that means that these behaved (segregated) independently during meiosis. But remember, Mendel knew nothing about chromosomes and the molecular mechanisms involved, it was just that his choices of traits made his data intelligible and enabled him to build a relatively simple predictive model.

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