LECTURE NOTES ON 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 with
small 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 epistatic
to gene C.
 This is a case of dominant epistasis because even one dominant allele of gene I is
able to express itself.