Example of Epistasis - Dominant and Recessive Epistasis

It is the phenomenon by which one gene does not allow alleles of another locus to express.

This is a counterpart of dominance. While dominance is intragenic suppression (e.g., allele'A’ dominant over ‘a’), epistasis is intergenic suppression (e.g., A suppresses B/B).

Definition:

• Epistasis can be defined as a gene interaction whereby one gene interferes with the phenotypic expression of another non allelic gene or genes.
• The gene or locus which suppresses or masks the action of a gene at another locus is called epistatic gene. The gene or locus whose expression is suppressed by an epistatic gene is called hypostatic gene.

Types of Epistasis

• Epistasis is of different types namely dominant, recessive and dominant recessive.

1. Dominant epistasis: A dominant epistastic suppresses the expression of a non allelic gene (dominant or recessive).

• F2 phenotypic ratio: 12:3:1
• Example: In summer squash fruit colour may be white, yellow or green. White fruits are produced by a domain epistatic allele ‘W’. At another locus ‘Y’ for yellow fruits is dominant to its allele ‘y’ for green fruits. Dominant white hides the effect of yellow or green. A consequence of this dominant epistasis is that the hybrid ratio is modified into 12 white: 3 yellow: 1 green.

2. Recessive epistasis: A recessive epistastic gene suppresses the expression of a non allelic gene only when the former is in homozygous recessive state.

• F2 phenotypic ratio: 9:3:4
• Example: A black Labrador homozygous for the dominant alleles (BBEE) is crossed with a yellow Labrador homozygous for the recessive alleles (bbee). On intercrossing the F1, the F2 progeny was obtained in the following ratio : 9 black : 3 brown: 4 yellow. 

3. Dominant recessive epistasis: Dominant allele at one locus and recessive allele at another locus give rise to the same effect.

•    F2 phenotypic ratio: 13:3