Epistasis

Epistasis is a form of gene interaction in which the phenotype produced by alleles at one genetic locus is affected by alleles at a different locus. In other words, the effect of a gene on a trait is not independent but is modified or masked by another gene. Epistasis can produce phenotype patterns that deviate from the simple Mendelian dihybrid ratio of 9:3:3:1 and is a major mechanism by which genetic variation is organized in organisms.

Two commonly discussed classes are recessive epistasis, in which recessive alleles at one locus disable the expression of alleles at another locus, and dominant epistasis, in which a dominant allele at one locus masks the second locus. The concept highlights that genetic control over a trait often involves networks of interacting genes rather than a single gene acting alone.

A classic demonstration is coat color in Labrador retrievers. The B locus determines pigment color (black or brown) and the E locus controls pigment deposition in fur. If an individual carries two recessive e alleles (ee), pigment is not deposited, producing yellow coats regardless of B genotype. Crosses of BbEe x BbEe yield phenotypic ratios approximating 9 black: 3 brown: 4 yellow, illustrating recessive epistasis.

Other examples include plants with complementary gene action that produce a 9:7 ratio when both genes are required for pigment production. In practice, epistasis has implications for breeding, genetic mapping, and understanding pathways; it can complicate the interpretation of QTL analyses and the study of gene networks. Detecting epistasis often requires statistical models that include interaction terms and large-scale datasets.