b) Recessive epistasis (9:3:4):

Recessive alleles at one locus (aa) mask the phenotypic expression of other gene locus (BB, Bb or bb) such epistasis is called recessive epistasis. The alleles of ‘B’ locus express themselves only when epistatic locus ‘A’ has dominant allele like AA or Aa. The phenotypic ratio is 9: 3: 4.Example:Complete dominance at both gene pairs, but one gene, when homozygous recessive, is epistatic to the other.In Mouse coat colour.Gene pair A: colour dominant over albino.Gene pair ‘B’ agouti colour dominant over black.Interaction: homozygous albino is epistatic to aguoti and black.Agouti 9/16Black 3/16Albino 4/16