Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 250

Because plant breeders transfer genes from one source to another, an understanding of transmission genetics is crucial to a successful breeding effort. The method of breeding used depends upon the heredity of the trait being manipulated, among other factors. According to Mendel's results from his hybridization studies in pea, traits are controlled by heritable factors that are passed from parents to offspring, through the reproductive cells. Each of these unit factors occurs in pairs in each cell (except reproductive cells or gametes).

In his experiments, Mendel discovered that in a cross between parents displaying two contrasting traits, the hybrid (F₁) expressed one of the traits to the exclusion of the other. He called the expressed trait dominant and the suppressed trait recessive. This is the phenomenon of dominance and recessivity. When the hybrid seed was planted and self‐pollinated, he observed that both traits appeared in the second generation (F₂) (i.e. the recessive trait reappeared), in a ratio of 3 : 1 dominant: recessive individuals (Figures 5.12 and 5.13). Mendel concluded that the two factors that control each trait do not blend but remain distant throughout the life of the individual and segregate in the formation of gametes. This is called the law of segregation. In further studies in which he considered two characters simultaneously, he observed that the genes for different characters are inherited independently of each other. This is called the law of independent assortment. In summary, the two key laws are as follows:

1 Law I: Law of segregation: Paired factors segregate during the formation of gametes in a random fashion such that each gamete receives one form or the other.

2 Law II: Law of independent assortment: When two or more pairs of traits are considered simultaneously, the factors for each pair of traits assort independently to the gametes.

Figure 5.12 Mendel's postulates: (a) dominance, (b) segregation, and (c) independent assortment.

Figure 5.13 The Punnett square procedure may be used to demonstrate the events that occur during hybridization and selfing: (a) a monohybrid cross, and (b) a dihybrid cross shows the proportions of genotypes in the F₂ population and the corresponding Mendelian phenotypic and genotypic ratios.

Mendel's pair of factors is now known as genes, while each factor of a pair (e.g. HH or hh) is called an allele (i.e. the alternative form of a gene; H or h). The specific location on the chromosome where a gene resides is called a gene locus or simply locus (loci for plural).