The interaction of two or more genes or gene-gene influences to control a single phenotype, such that one gene is suppressed or masked by an unrelated gene that is not its allele (non-allelic gene). Epistatic genes are sometimes referred to as inhibiting genes because of their effect on other genes that are described as hypostatic. Epistasis/hypostasis is the same kind of relationship between gene pairs as dominance/recessiveness is within a gene pair. Epistasis remains one of the most difficult biological properties to measure or quantify despite its importance in genetics.
See Additive genetic effect, Allele, Behavior genetics, Gene, Polygenic mode of inheritance, Polygenes