Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 157

Dominance action describes the relationship of alleles at the same locus. Dominance variance has two components – variance due to homozygous alleles (which is additive) and variance due to heterozygous genotypic values. Dominance effects are deviations from additivity that make the heterozygote resemble one parent more than the other. When dominance is complete, the heterozygote is equal to the homozygote in effects (i.e. Aa = AA). The breeding implication is that the breeder cannot distinguish between the heterozygous and homozygous phenotypes. Consequently, both kinds of plants will be selected, the homozygotes breeding true while the heterozygotes will not breed true in the next generation (i.e. fixing superior genes will be less effective with dominance gene action).

Using the previous figure for additive effect, the extent of dominance (d_A) is calculated as the deviation of the heterozygote, Aa, from the mean of the two homozygotes (AA, aa). Also, d_A = 0 when there is no dominance while d is positive if A is dominant, and negative if a_A is dominant. Further, if dominance is complete d_A = a_A, whereas d_A < a_A for incomplete (partial) dominance and d_A > a_A for overdominance. For a single locus, m = ½ (AA + aa), a_A = ½ (AA − aa), while d_A = Aa − ½ (AA + aa).