Recessive epistasis -coat color in mice

Due to the phenomenon of dominance a recessive allele remains obscure in the hybrid. But when two different genes which are not alleles, both affect the same character in such a way that the expression of one masks, inhibits or suppresses the expression of the other gene, it is called epistasis. The gene that suppresses is said to be epistatic, and the gene which remains obscure is hypostatic.

If the recessive genotype  aa suppresses the expression of alleles the locus B, a  is said to exhibit recessive epistasis over the B . Only if the the dominant allele is present at the A locus

can the alleles of the hypostatic B locus can be expressed. The genotypes AB and A-bb produce to additional phenotypes. The 9:3:3:1 ratio becomes 9:3:4ratio. Epistasis due to recessive genes is called recessive epistasis.

 

 In mice albinism (white coat) is produced by a recessive gene aa.

There is a different gene B which in the dominant state (BB and Bb) produces grey coat colour called agouti, and when recessive (bb) leads to black coat colour. The recessive gene for albinism (aa) is found to be epistatic to the gene for agouti(BB and Bb), and also to its recessive, homozygous allele (bb) for black.  The presence of thedominant allele (AA) of the epistatic gene allows expression of gene B so that agouti (BB andBb) and black (bb) coat colours can be produced.The 9 : 3 : 4 ratio obtained is a modification of the classical 9 : 3 : 3 : 1 in which the last two classes (3 : 1) are phenotypically identical and are therefore added up together.In human beings also the recessive gene for albinism shows epistasis in a similar manner.Epistatic effect is usually only in one direction, from one particular gene pair to another.