Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 252

Based upon Mendel's laws of inheritance, statistical probability analysis can be applied to determine the outcome of a cross, given the genotype of the parents and gene action (dominance/recessivity). A genetic grid called a Punnett square (Figure 5.13) facilitates the analysis. For example, a monohybrid cross in which the genotypes of interest are AA × aa, where A is dominant over a, will produce a hybrid genotype Aa in the F₁ (first filial generation) with a AA phenotype. However, in the F₂ (F₁ × F₁), the Punnett square shows a genotypic ratio of 1AA: 2Aa: 1aa, and a phenotypic ratio of 3 : 1, because of dominance. A dihybrid cross (involving simultaneous analysis of two different genes) is more complex but conceptually like a monohybrid cross (only one gene of interest) analysis. An analysis of a dihybrid cross PPRR × pprr, using the Punnett square is illustrated in Figure 5.12. An alternative method of genetic analysis of a cross is by the branch diagram or forked line method (Figure 5.14).

Figure 5.14 The branch diagram method may also be used to predict the phenotypic and genotypic ratios in the F₂ population.

Predicting the outcome of a cross is important to plant breeders. One of the critical steps in a hybrid program is to authenticate the F₁ product. The breeder must be certain that the F₁ truly is a successful cross and not a product of selfing. If a selfed product is advanced, the breeding program will be a total waste of resources. To facilitate the process, breeders may include a genetic marker in their program. If two plants are crossed, for example, one with purple flowers and the other white flowers, we expect the F₁ plant to have purple flowers because of dominance of purple over white flowers. If the F₁ plant has white flowers, it is proof that the cross was unsuccessful (i.e. the product of the “cross” is actually from selfing).