Medical Definition of Epistasy
1. 1. The formation of a pellicle or scum on the surface of a liquid, especially as on standing urine. 2. Phenotypic interaction of non-allelic genes. 3. A form of gene interaction whereby one gene masks or interferes with the phenotypic expression of one or more genes at other loci; the gene whose phenotype is expressed is said to be "epistatic," while the phenotype altered or suppressed is then said to be "hypostatic". Synonym: epistasy. Origin: G. Scum; epi-+ G. Stasis, a standing (05 Mar 2000)