that is not explained by the additive genetic variance due to differences in alleles among individuals. The magnitudes of VD and Vj in the statistical sense are a function of both the genotype frequencies in the population as well as the relationship between genotype and phenotype. These non-additive parts of the phenotypic variance are sometimes called dominance and epistasis deviations since they are measured as differences from the variance that would be expected in a population if all genetic effects were additive.

The distinction between additive and dominance genetic variance in quantitative traits can be seen with a modification of the rules that specify the relationship between genotypes and phenotypes. Under additivity, each genotype's phenotypic value is determined by the sum of the phenotypic effects of the alleles that compose it. An alternative relationship between the genotype and phenotype is seen with complete dominance, where heterozygotes have the same phenotype as one of the homozygotes.

With dominance, the phenotypic value of the heterozygote is no longer determined by adding together the phenotypic effects of the two alleles that compose it. When gene action shows dominance, the pheno-typic contribution of an allele depends on pairing of alleles in the genotype where it resides. Dominance causes the phenotype to be defined by the genotype context rather than being independent of the pairing of alleles in a genotype as it is under additivity.

The additivity rule that applies in Fig. 9.2 is changed to complete dominance in Fig. 9.5a. Dominance markedly changes the phenotypic frequency distribution, even though allele frequencies remain constant in the two panels. In Fig. 9.2 the pheno-typic distribution is symmetric and exhibits every possible value of the phenotype. In contrast, Fig. 9.5a shows that about 56% of the individuals in the population have phenotypic values of 9. There are no longer any individuals with phenotypic values of 3 or 7 units of pigment. In this example, dominance


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