Recall that the first step in genetic dissection is to obtain variants that differ in the property under scrutiny. With the assumption that we have acquired a collection of relevant mutants, the next question is whether each of the mutations is inherited as a single gene.

The first-ever analysis of single-gene inheritance as a pathway to gene discovery was carried out by Gregor Mendel. His is the analysis that we shall follow as an example. Mendel chose the garden pea, Pisum sativum, as his research organism. The choice of organism for any biological research is crucial, and Mendel’s choice proved to be a good one because peas are easy to grow and breed. Note, however, that Mendel did not embark on a hunt for mutants of peas; instead, he made use of mutants that had been found by others and had been used in horticulture. Moreover, Mendel’s work differs from most genetics research undertaken today in that it was not a genetic dissection; he was not interested in the properties of peas themselves, but rather in the way in which the hereditary units that influenced those properties were inherited from generation to generation. Nevertheless, the laws of inheritance deduced by Mendel are exactly those that we use today in modern genetics in identifying single-gene inheritance patterns.

Mendel chose to investigate the inheritance of seven properties of his chosen pea species: pea color, pea shape, pod color, pod shape, flower color, plant height, and position of the flowering shoot. In genetics, the terms character and trait are used more or less synonymously; they roughly mean “property.” For each of these seven characters, he obtained from his horticultural supplier two lines that showed distinct and contrasting phenotypes. These contrasting phenotypes are illustrated in Figure 2-2. His results were substantially the same for each character, and so we can use one character, pea seed color, as an illustration. All of the lines used by Mendel were pure lines, meaning that, for the phenotype in question, all offspring produced by matings within the members of that line were identical. For example, within the yellow-seeded line, all the progeny of any mating were yellow seeded.

35

Mendel’s analysis of pea heredity made extensive use of crosses. To make a cross in plants such as the pea, pollen is simply transferred from the anthers of one plant to the stigmata of another. A special type of mating is a self (self-pollination), which is carried out by allowing pollen from a flower to fall on its own stigma. Crossing and selfing are illustrated in Figure 2-3. The first cross made by Mendel mated plants of the yellow-seeded lines with plants of the green-seeded lines. In his overall breeding program, these lines constituted the parental generation, abbreviated P. In Pisum sativum, the color of the seed (the pea) is determined by the seed’s own genetic makeup; hence, the peas resulting from a cross are effectively progeny and can be conveniently classified for phenotype without the need to grow them into plants. The progeny peas from the cross between the different pure lines were found to be all yellow, no matter which parent (yellow or green) was used as male or female. This progeny generation is called the first filial generation, or F₁. The word filial comes from the Latin words filia (daughter) and filius (son). Hence, the results of these two reciprocal crosses were as follows, where × represents a cross:

female from yellow line × male from green line → F₁ peas all yellow

female from green line × male from yellow line → F₁ peas all yellow

The results observed in the descendants of both reciprocal crosses were the same, and so we will treat them as one cross. Mendel grew F₁ peas into plants, and he selfed these plants to obtain the second filial generation, or F₂. The F₂ was composed of 6022 yellow peas and 2001 green peas. In summary,

Mendel noted that this outcome was very close to a mathematical ratio of three-fourths (75%) yellow and one-fourth (25%) green. A simple calculation shows us that 6022/8023 = 0.751 or 75.1%, and 2001/8023 = 0.249 or 24.9%. Hence, there was a 3:1 ratio of yellow to green. Interestingly, the green phenotype, which had disappeared in the F₁, had reappeared in one-fourth of the F₂ individuals, showing that the genetic determinants for green must have been present in the yellow F₁, although unexpressed.

To further investigate the nature of the F₂ plants, Mendel selfed plants grown from the F₂ seeds. He found three different types of results. The plants grown from the F₂ green seeds, when selfed, were found to bear only green peas.

36

However, plants grown from the F₂ yellow seeds, when selfed, were found to be of two types: one-third of them were pure breeding for yellow seeds, but two-thirds of them gave mixed progeny: three-fourths yellow seeds and one-fourth green seeds, just as the F₁ plants had. In summary,

Hence, looked at another way, the F₂ was comprised of

Thus, the 3:1 ratio at a more fundamental level is a 1:2:1 ratio.

Mendel made another informative cross between the F₁ yellow-seeded plants and any green-seeded plant. In this cross, the progeny showed the proportions of one-half yellow and one-half green. In summary:

These two types of matings, the F₁ self and the cross of the F₁ with any green-seeded plant, both gave yellow and green progeny, but in different ratios. These two ratios are represented in Figure 2-4. Notice that the ratios are seen only when the peas in several pods are combined.

The 3:1 and 1:1 ratios found for pea color were also found for comparable crosses for the other six characters that Mendel studied. The actual numbers for the 3:1 ratios for those characters are shown in Table 2-1.

Initially, the meaning of these precise and repeatable mathematical ratios must have been unclear to Mendel, but he was able to devise a brilliant model that not only accounted for all the results, but also represented the historical birth of the science of genetics. Mendel’s model for the pea-color example, translated into modern terms, was as follows:

37

Here, we introduce some terminology. A fertilized egg, the first cell that develops into a progeny individual, is called a zygote. A plant with a pair of identical alleles is called a homozygote (adjective homozygous), and a plant in which the alleles of the pair differ is called a heterozygote (adjective heterozygous). Sometimes a heterozygote for one gene is called a monohybrid. An individual can be classified as either homozygous dominant (such as Y/Y), heterozygous (Y/y), or homozygous recessive (y/y). In genetics generally, allelic combinations underlying phenotypes are called genotypes. Hence, Y/Y, Y/y, and y/y are all genotypes.

38

Figure 2-5 shows how Mendel’s postulates explain the progeny ratios illustrated in Figure 2-4. The pure-breeding lines are homozygous, either Y/Y or y/y. Hence, each line produces only Y gametes or only y gametes and thus can only breed true. When crossed with each other, the Y/Y and the y/y lines produce an F₁ generation composed of all heterozygous individuals (Y/y). Because Y is dominant, all F₁ individuals are yellow in phenotype. Selfing the F₁ individuals can be thought of as a cross of the type Y/y × Y/y, which is sometimes called a monohybrid cross. Equal segregation of the Y and y alleles in the heterozygous F₁ results in gametes, both male and female, half of which are Y and half of which are y. Male and female gametes fuse randomly at fertilization, with the results shown in the grid in Figure 2-5. The composition of the F₂ is three-fourths yellow seeds and one-fourth green, a 3:1 ratio. The one-fourth of the F₂ seeds that are green breed true as expected of the genotype y/y. However, the yellow F₂ seeds (totaling three-fourths) are of two genotypes: two-thirds of them are clearly heterozygotes Y/y, and one-third are homozygous dominant Y/Y. Hence, we see that underlying the 3:1 phenotypic ratio in the F₂ is a 1:2:1 genotypic ratio:

The general depiction of an individual expressing the dominant allele is Y/–; the dash represents a slot that can be filled by either another Y or a y. Note that equal segregation is detectable only in the meiosis of a heterozygote. Hence, Y/y produces one-half Y gametes and one-half y gametes. Although equal segregation is taking place in homozygotes too, neither segregation Y : Y nor segregation y : y is meaningful or detectable at the genetic level.

We can now also explain results of the cross between the plants grown from F₁ yellow seeds (Y/y) and the plants grown from green seeds (y/y). In this case, equal segregation in the yellow heterozygous F₁ gives gametes with a Y : y ratio. The y/y parent can make only y gametes, however; so the phenotype of the progeny depends only on which allele they inherit from the Y/y parent. Thus, the Y : y gametic ratio from the heterozygote is converted into a Y/y : y/y genotypic ratio, which corresponds to a 1:1 phenotypic ratio of yellow-seeded to green-seeded plants. This is illustrated in the right-hand panel of Figure 2-5.

39

Note that, in defining the allele pairs that underlay his phenotypes, Mendel had identified a gene that radically affects pea color. This identification was not his prime interest, but we can see how finding single-gene inheritance patterns is a process of gene discovery, identifying individual genes that influence a biological property.

Mendel’s research in the mid-nineteenth century was not noticed by the international scientific community until similar observations were independently published by several other researchers in 1900. Soon research in many species of plants, animals, fungi, and algae showed that Mendel’s law of equal segregation was applicable to all sexual eukaryotes and, in all cases, was based on the chromosomal segregations taking place in meiosis, a topic that we turn to in the next section.