Epistasis

Epistasis is an interaction in which the genotype at once locus will mask/suppress the phenotypic expression of the genotype at another locus.

An example of epistasis occurs in Labrador Retriever coat colour. Coat colour of Labrador Retreivers is determined by two genes: the B gene and the E gene and the phenotypes can be either black coats, chocolate brown coats or yellow coats. A dominant B and a dominant E gene present in the genoytype (e.g. BBEE, BBEe, BbEE, BbEe) will give a Labrador with a black coat, a dominant E gene present with recessive B genes (e.g. bbEe, bbEE) will give Labradors with a chocolate brown coats. However, the genotype of recessive E (e) is recessive but also epistatic in homozygous form (ee) so if the genotype 'ee' is present the Labrador will have a yellow coat, regardless of the B gene genotype. This is an example of epistasis as the genotype at the E gene determines the expression of the B gene.

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