Gene action

Gene action
  • There are four types of gene action: additive, dominance, epistatic, and overdominance.
    1. Additive gene action
  • The effect of a gene is said to be additive when each additional gene enhances the expression of the trait byEqual increments. Consequently, if one gene adds one unit to a trait, the effect of aabb = 0, Aabb = 1, AABb = 3,and AABB = 4. For a single locus (A, a) the heterozygote would be exactly intermediate between the parents(i.e., AA = 2, Aa =1, aa = 0). That is, the performance of an allele is the same irrespective of other alleles atthe same locus. This means that the phenotype reflects the genotype in additive action, assuming the absenceof environmental effect. Additive effects apply to the allelic relationship at the same locus. Furthermore, asuperior phenotype will breed true in the next generation, making selection for the trait more effective toconduct. Selection is most effective for additive variance;it can be fixed in plant breeding (i.e., develop acultivar that is homozygous).
    2. Dominance gene action
  • Dominance effects are deviations from additivity that make the heterozygote resemble one parent morethan the other. When dominance is complete, the heterozygote is equal to the homozygote in effects (i.e.Aa = AA). 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. Epistatic gene action:In quantitative inheritance, epistasis is described as non-allelic gene interaction. When two genes interact, an effect can be produced where there was none (e.g., Aabb = 0, aaBB = 0, but A–B– = 4). The estimation of gene action or genetic variance requires the use of large populations and a mating design. The effect of the environment on polygenes makes estimations more challenging.
    3. Overdominance gene action
  • exists when each allele at a locus produces a separate effect on the phenotype, and their combined effect exceeds the independent effect of the alleles (i.e., aa =1, AA =1, and Aa= 2) From the breeding standpoint; the breeder can fix overdominance effects only in the first generation (i.e., F1 hybrid cultivars) through apomixis, or through chromosome doubling of the product of a wide cross.
Last modified: Tuesday, 13 March 2012, 6:39 AM