3.3 A breeding experiment: stage two

We now turn to the second stage of the breeding experiment, but this time we will follow the phenotypes and genotypes simultaneously. The purple (Gg) grains of the F₁ generation are planted and when these have developed into mature F₁ plants they produce male and female flowers. These F₁ plants are crossed with each other, as shown in Figure 8. The fertilised ovules develop into grains borne on cobs, and these grains are the beginning of the second filial generation (F₂).

The mating diagram and the outcome of the fertilisations of this cross are given in Figure 8. First consider the gametes produced by the F₁ generation.

Now consider the possible fertilisations between the gametes produced by the F₁ generation, as shown in Figure 8, and the genotypes and phenotypes of the F₂ generation that arises from these fertilisations.

Looking at Figure 8, you can see that the allele G of plant 1 might combine with either the G or the g allele of plant 2 at fertilisation. Over a large number of fertilisations a G-bearing ovule of plant 1 would combine with a G-bearing pollen grain of plant 2 in half of the fertilisations and combine with a g-bearing pollen grain of plant 2 in the other half. Similarly, the allele g of plant 1 might combine with either the G or g allele of plant 2. Hence, in the F₂ generation three different genotypes are produced: GG, Gg and gg.

Figure 8 Crossing F₁ plants with F₁ plants gives F₂ cobs, bearing F₂ grains. Drawings of the F₂ offspring grains are included, to show the phenotypes. The recessive phenotype reappears in the F₂ generation. Unlike Figure 7, the chromosomes bearing the grain-colour alleles are not shown; it is conventional to simplify mating diagrams in this way.

This ratio follows for two reasons. First, the two different gametes, G and g, are produced in equal numbers. Second, which ovule is fertilised by which pollen grain occurs at random; that is, a G ovule is equally likely to be fertilised by either a G pollen grain or g pollen grain.

The phenotype of each of these genotypes in the F₂ generation can be determined since we know that the allele G (purple grain) is dominant to g.

The phenotypic ratio of 3 : 1 (three of the dominant phenotype: one of the recessive phenotype) and the genotypic ratio of 1 : 2 : 1 (one homozygous dominant: two heterozygous : one homozygous recessive) are of fundamental importance in genetics.

Figure 9 presents the same information about the breeding experiment as shown in Figure 8, but the information is laid out in a different way. Either way of presenting the details of a cross can be used, and when producing your own diagrams you can use whichever is easier for you (although you are not expected to draw the phenotypes).

Figure 9 An alternative way of laying out the cross shown in Figure 8.

In Figure 9 the fertilisations between the various combinations of gametes from the F₁ generation are shown in boxes at the bottom. Along the top you can see the two types of gamete produced by plant 1 of the F₁ generation; down the left-hand side you can see the two types of gamete produced by plant 2 of the F₁ generation. Inside the other boxes are the genotypes of the products of fertilisation between the various kinds of gamete, that is, the F₂ generation. Thus, for example, the top left-hand box records the outcome of fertilisation between a G ovule and a G pollen grain. An examination of the contents of the four boxes should convince you that the expected ratio of genotypes of the F₂ generation is 1 GG:2Gg: 1 gg (one homozygous dominant: two heterozygous : one homozygous recessive) and that the expected phenotypic ratio is therefore three purple : one white (three of the dominant phenotype : one of the recessive phenotype).

This breeding experiment in maize, which we just have described, is similar in essence to a famous series of experiments carried out by Gregor Mendel (Figure 10) on the garden pea, Pisum sativum. Mendel introduced the scientific approach to breeding experiments by:

• following a single pair of contrasting characters in each investigation;

• breeding beyond one generation;

• counting the individuals showing each of the phenotypes;

• working with large numbers of offspring.

Figure 10 A monk, Gregor Mendel, laid the foundation of the modern science of genetics. The results of his experiments, carried out in the monastery gardens in Brunn, Moravia (subsequently incorporated in Czechoslovakia), were published in 1865. His work was largely ignored or unnoticed until 1900 when it was 'rediscovered' by other workers after they had come to similar conclusions from their own work. Mendel had died 16 years before his work was recognised.

Although the existence of genes and chromosomes was not known in Mendel's time, he predicted the separation or segregation of units (now called genes and alleles) during gamete formation. We now know that meiosis ensures that each gamete contains only one copy of a gene, because the pairs of chromosomes on which the genes are located separate into different gametes (Figure 7). This segregation of genes (units) has been given formal recognition as Mendel's law of segregation.

The breeding experiment in maize shows that characters due to dominant or recessive alleles, such as grain colour, show a particular pattern of inheritance from generation to generation: the F₁ offspring all resemble one of the parents (Figure 7), and the F₂ offspring have the phenotypic ratio of 3 :1 and the genotypic ratio of 1: 2 :1 (Figures 8 and 9). The action of dominant alleles explains why the recessive character 'disappears' in the F₁ generation and why this character reappears in the F₂ generation. Most importantly, this pattern of inheritance is a consequence of two events.

The conclusions we have drawn from the breeding experiment in maize are the same as those drawn by Mendel and apply to all sexually reproducing eukaryotes, including humans. The main features of the experiment that we have described so far, and that can be explained by the theory of inheritance, are summarised in Table 1.