122.16 Genetic-environmental interaction variance occurs when the effect of a gene depends on the specific environment in which it is found. In this example, the genotype affects plant weight, but the environmental conditions determine which genotype produces the heavier plant.

mances of the genotypes depend on whether the plants are grown in a wet or dry environment. In this case, the influences on phenotype cannot be neatly allocated into genetic and environmental components, because the expression of the genotype depends on the environment in which the plant grows. The phenotypic variance must therefore include a component that accounts for the way in which genetic and environmental factors interact.

In summary, the total phenotypic variance can be apportioned into three components:

Components of genetic variance Genetic variance can be further subdivided into components consisting of different types of genetic effects. First, additive genetic variance (VA) comprises the additive effects of genes on the pheno-type, which can be summed to determine the overall effect on the phenotype. For example, suppose that, in a plant, allele A1 contributes 2 g in weight and allele A2 contributes 4 g. If the alleles are strictly additive, then the genotypes would have the following weights:

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