The genes that Nilsson-Ehle studied, which affected kernel color in wheat, were additive in this way.
Second, there is dominance genetic variance (VD) when some genes have a dominance component. In this case, the alleles at a locus are not additive; rather, the effect of an allele depends on the identity of the other allele at that locus. Here, we cannot simply add the effects of the alleles together. Instead, we must add a component (VD) to the genetic variance to account for the way that alleles interact.
Third, genes at different loci may interact in the same way that alleles at the same locus interact. When this genic interaction occurs, the effects of genes are not additive, and we must include a third component, called genic interaction variance (Vj), to the genetic variance:
Summary equation We can now integrate these components into one equation to represent all the potential contributions to the phenotypic variance:
This equation provides us with a model that describes the potential causes of differences that we observe among individual phenotypes. It's important to note that this model deals strictly with the observable differences (variance) in phenotypes among individual members of a population; it says nothing about the absolute value of the characteristic or about the underlying genotypes that produce these differences.
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