Well, in many cases, a gene creates a protein that goes on to a second gene, and that protein affects the expression of that second gene. And this process can continue until some type of trait is actually expressed, and this is what we call epistasis. So in many cases, a gene at one locus produces a protein that goes on to a second locus and affects the expression of that gene at that second locus. And this is known as epistasis. And an epistasis, two or more pairs of alleles at different loci basically affect a single trait. So to see what we mean, let's take a look at the following example.