In genetics, epistasis occurs when two or more different gene loci contribute to the same phenotype, but not additively. Epistasis is often described as occurring when one gene locus masks or modifies the phenotype of a second gene locus. The term epistatic describes the relationship between the genes in epistasis. Epistasis can occur in scenarios other than a step-wise progression, but this is the most common example. Another way to look at this is that a product, like coat color in some animals, is controlled by a pigment (P). Different genes contribute to the steps needed to make P from a precursor molecule. In order to get to P, all these steps have to be fully functional. If there is a mutation in one of these genes, the reaction cannot take place and the phenotype, or in this case, the production of pigment resulting from coat color, is affected.
Epistasis is a phenomenon that consists of the effect of one gene being dependent on the presence of one or more 'modifier genes' (genetic background). Similarly, epistatic mutations have different effects in combination than individually. It was originally a concept from genetics but is now used in biochemistry, population genetics, computational biology and evolutionary biology. It arises due to interactions, either between genes, or within them leading to non-additive effects. Epistasis has a large influence on the shape of evolutionary landscapes which leads to profound consequences for evolution and evolvability of traits.