Epistasis's the interaction between genes. Epistasis takes place when the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype's expressed's said to be epistatic, while the phenotype altered or suppressed's said to be hypostatic. In general, the fitness increment of any one allele depends in a complicated way on many other alleles; but, because of the way that the science of population genetics was developed, evolutio… (More on Epistasis)