Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 166

Gene action is affected by several factors, the key one being the type of genetic material, mode of pollination, mode of inheritance, presence of linkage, as well as biometrical parameters (e.g. simple size, sampling method, and method of calculation). Alleles with a dominant, additive, or deleterious phenotypic effect affect heritability differently depending on whether they are in homozygous or heterozygous condition. Knowledge of the way genes act and interact will determine which breeding system optimizes gene action more efficiently and will elucidate the role of breeding systems in the evolution of crop plants.

Self‐pollinated materials (e.g. mass selected cultivar, multiline, varietal blends) express additive and additive epistasis. A pure line cultivar will have additive gene action but without genetic variation. On the other hand, products derived from cross‐pollinated species (e.g. composite variety, synthetic variety) will display additive, dominance, and epistatic gene action. F₁ hybrid material will have no additive gene action and no genetic variation.

In terms of pollination, self‐pollinated species exhibit additive gene action since this gene action is associated with homozygosity. On the contrary, non‐additive gene action is associated with heterozygosity and hence is more prevalent in cross‐pollinated species than self‐pollinated ones. Simply inherited (qualitative, oligogenic) traits predominantly exhibit non‐additive and epistatic gene action, while polygenic traits are governed predominantly by additive gene action.

Gene action estimates are affected by the presence of genetic linkage. Estimates of additive gene action and dominance gene action can be biased up when the genes of interest are in the coupling phase (AB/ab). In the repulsion phase (Ab/aB), genes can cause estimates of dominance gene action to be biased upwards and additive action downwards.