When a whole bunch of genes affect a particular trait, these genes can interact in different ways. The effect of the different genes can be additive, though some genes might be more important than others in determining the final phenotype. Imagine a lot of genes for milk production where, at each locus (location on the DNA strand), some alleles result in more milk and some result in less. The heritable part of milk production will be determined by the sum of the effects at all the involved loci.
Different loci can also interact in non-additive ways. In some cases, the nonadditive interaction of the different genes is important. Imagine that a particular locus may influence milk production, for example, but only when a specific allele occurs at some particular other locus. Without that allele at the second locus, the first locus doesn't have anything to do with how much milk a cow makes. This process is called epistasis.
Epistatic interactions are those in which the fitness of a particular allele at one locus depends on alleles at another locus.
Imagine epistatic interactions that affect the A locus, in which the relative fitness of the aa and AA individuals depends on the alleles at a second locus, which I'll call B. If that second locus has one combination of alleles, AA individuals will have higher fitness than aa individuals. But if the B locus has a different combination of alleles, aa individuals will have higher fitness than AA individuals.
I made up the milk example for illustrative purposes, but one actual case of epistasis that you might be familiar with concerns those colored squash that you see around Halloween and Thanksgiving. In one genetic system, the colors white, green, and yellow are controlled by two genes, each of which have two alleles (of the dominant/recessive type; refer to Chapter 3).
When at least one of the genes at the white locus is the dominant form, the squash is white and the other locus has no effect on color. But when both alleles at the white locus are of the recessive form, then the color of the squash (green or yellow) is determined by the alleles at the green/yellow locus. You can see that it starts to get complicated even with only two loci involved — there can be many more.
These examples have consequences for the way selection operates. In both cases, the extent to which selection increases the frequency of the alleles at the one locus depends on the frequency of the alleles at other locus.
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