Question

what is epistasis and define the resesive epistasis and dominant epistasis

Answer 1

the interaction of genes that are not alleles, in particular the suppression of the effect of one such gene by another.

Answer 2

Answer:

Epistasis : One gene masks the action of another non-allelic gene.

Recessive epistasis: The recessive allele of one gene masks the effects of either allele of the second gene.

A famous example of recessive epistasis is the color of fur in mice.  There are two genes responsible for the phenotypes (agouti, black, or albino (white)).  One pair of genes is responsible for determining if the mouse will be agouti or black.  Agouti is dominent and will be expressed if the mouse carries an AA or Aa gene pairing.  Black fur is recessive and will be expressed if the recessive genes are present, aa.  These phenotypes, however, can only be expressed if another gene pairing is "turned on".  The expression of pigmentation is determined by another gene pairing.  If the second gene pairing for coloration is present, CC or Cc, the agouti or black fur color will be expressed.  If the recessive coloration gene pairing, cc, is present the expression of the agouti or black fur color can not be expressed even though it is present.  In this case, the mouse will be albino

Dominant epistasis: The dominant allele of one gene masks the effects of either allele of the second gene.

Fruit and flower color in plants is a common example.