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 (dA) is calculated as the deviation of the heterozygote, Aa, from the mean of the two homozygotes (AA, aa). Also, dA = 0 when there is no dominance while d is positive if A is dominant, and negative if aA is dominant. Further, if dominance is complete dA = aA, whereas dA < aA for incomplete (partial) dominance and dA > aA for overdominance. For a single locus, m = ½ (AA + aa), aA = ½ (AA − aa), while dA = Aa − ½ (AA + aa).
Overdominance gene action
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. From the breeding standpoint, the breeder can fix overdominance effects only in the first generation (i.e. F1 hybrid cultivars) through apomixes.
Epistasis gene action
Epistasis is the interaction of alleles at different loci. It complicates gene action in that the value of a genotype or allele at one locus depends on the genotype at other epistatically interacting loci. In other words, the allelic effects at one locus depend on the genotype at a second locus. An effect of epistasis is that an allele may be deemed “favorable” at one locus and then deemed “unfavorable” under a different genetic background. In the absence of epistasis, the total genetic value of an individual is simply the sum total of the individual genotype values because the loci are independent. Epistasis is sometimes described as the masking effect of the expression of one gene by another at a different locus.
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. As N.W. Simmonds observed, at the end of the day, what qualitative genetic analysis allows the breeder to conclude from partitioning variance in an experiment is to say that a portion of the variance behaves as though it could be attributed to additive gene action or dominance effect, and so forth.
4.2.6 Gene action and plant breeding
Understanding gene actions is critical to the success of plant breeding. It is used by breeders several ways:
In the selection of parents used in crosses to create segregating populations in which selection is practiced.
In the choice of the method of breeding used in crop improvement.
In research applications to gain understanding of the breeding material by estimating genetic parameters.
4.2.7 Gene action and methods of breeding
Breeding methods are discussed in detail in Chapters 17–20. The methods are grouped according to modes of pollination: self‐pollinated or cross‐pollinated.
Self‐pollinated species
When additive gene action predominates in a self‐pollinated species, breeders should consider using selections methods such as pure line selection, mass selection, progeny selection, and hybridization. However, when non‐additive gene action predominates, effective methods of breeding are the exploitation of heterosis in breeding hybrid cultivars.
Cross‐pollinated species
When additive gene action predominates in a cross pollinated species, recurrent selection may be used to achieve general combining ability (GCA). Specific breeding products to pursue include synthetic varieties and composites. In case of non‐additive gene action, heterosis breeding, just like in self‐pollinated species, is recommended for breeding hybrid cultivars. Alternatively, breeders may consider recurrent selection for specific combining ability (SCA) for population improvement. Where both additive and non‐additive gene action occurs together, reciprocal recurrent selection may be used for population improvement.
Impact of breeding method on genetic variance
Additive genetic variance is known to decrease proportionally to the improvement following selection. In pure line selection, genetic variance is completely depleted with time, until further improvement is impossible. However, mutational events as well as genetic recombination can replenish some of the lost additive genetic variance. On the contrary, additive genetic variance cannot be depleted in intermating populations because auto conversion (self conversion) of non‐additive genetic variance to additive genetic variance occurs. This conversion occurs because heterozygotes become fixed into homozygotes.
Estimating gene action
Gene action may be estimated by creating various crosses (e.g. diallelee, partial diallelee, line × tester cross, biparental cross, etc.) and applying various biometrical analyses to estimate components of genetic variance. Additive genetic variance is very important to breeders because it is the only genetic variance that responds to selection. In addition to the components of genetic variance, combining ability variances may also be used to measure gene action.
Factors affecting gene action
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).
