Epistasis

Interactions of genes at different loci is called epistasis.

When the expression of one gene affects the expression of another gene, this interaction is called epistasis. (Epistasis is the interaction between the genes at two or more loci, so that the phenotype differs from what would be expected if the loci were expressed independently.)

There are different kinds of epistasis. It can be synergistic (enhancing) or antagonistic (inhibiting), it can be also complete or partial. Genes with epistatic effects are very often called modifier genes.


 * Complete antagonistic (disabling) epistasis - When a gene disables the expression of another gene. The perfect example of this is recessive white in pigeons (and indeed many other animals). When recessive white is expressed (that it when it is homozygous) all other colour genes are inhibited. We cannot see whether the bird is ash-red, blue or brown; whether it is T-pattern or bar. We cannot even see expression of genes like pearl eye, smokey or anything else. This means that recessive white is epistatic to genes at the base colour locus, pattern locus, etc.
 * Incomplete antagonistic (inhibiting) epistasis - Here we see a partial inhibition of the expression of one gene because of the expression of another. Spread could be said to show incomplete antagonistic epistasis to grizzle.
 * Incomplete synergistic (enhancing) epistasis - Here we see a partial enhancing of the expression of one gene because of the expression of another gene. An example of this is the expression of dirty enhancing the expression of spread.
 * Complete synergistic (enabling) epistasis - When a the expression of one gene enables the expression of another gene. The examples we saw in the earlier thread was the whiteside interaction. The whiteside gene can only express when the recessive red gene is expressed. We could say that the recessive red enables the whiteside gene.