EPISTASIS

EPISTASIS

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.
   

DOMINANT EPISTASIS

In poultry white birds belong to two different varieties namely white leghorns or white wyandottes. Experiments reveal that the gene for white plumage of white leghorns is dominant over the gene for coloured plumage of coloured varieties.  But the gene for white plumage of white wyandottes is recessive to the gene for coloured plumage of coloured varieties.Therefore the gene which produces white plumage in white leghorns is different from the gene for white plumage in white wyandottes. A cross between a white leghorn and a white wyandotte gives an F1 of white birds withsmall dark flecks. When such birds are inbred, the F2 progeny segregates in the ratio of 13 white to 3 coloured birds. The experiment is explained below by postulating two genes C and I for the white leghorns Out of sixteen genotypes, that is iiCC, iiCc, iiCc produce coloured birds. The white leghorns obviously contain a gene I, which in the dominant state inhibits or suppresses the expression of the dominant colour gene C, resulting in white plumage.The recessive alleles of the inhibitor gene (ii) produce coloured birds due to expression of gene C.
 In other words gene I is epistaticto gene C. This is a case of dominant epistasis because even one dominant allele of gene I isable to express itself.

 

Recessive epistasis

• 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.
•  Epistaticeffect is usually only in one direction, from one particular gene pair to another.