Expert Answers
jerichorayel eNotes educator| Certified Educator

Epistasis is genetic phenomenon wherein it describes how gene interactions affect phenotypes which relates to the evolution of genetic systems. The expressions of a gene depends on the modifier genes which seems to controls what phenotype is to show - either create a new phenotype or masks a certain genes to show a particular phenotype. To better explain this effect, let us try to work out on a certain examples. 

Breeding two different kinds of pure breed of chickens with different types of combs

Wyandotte (Rose comb) x Brahama (Pea comb) 

F1 generation produces a new phenotype (walnut comb)
F2 generation produces a ratio of 9walnut:3rose:3pea:1single

Another effect is the masking of a certain gene so as to disable a certain phenotype. This type of interaction is called the dominant epistasis. 

orchid101 | Student

Epistatic interaction means that one gene hides the effect of the other. In other words it does not allow the other gene to express itself. This is a counterpart of dominance. While dominance works at interallelic but intragenic level, epistasis works at intergenic level. Epistasis can be of the following types---(1) due to recessive allele: a hides the effect of B, (2) due to a dominant allele: A hides the effect of B. This may be in one direction only meaning that either A hides the effect of B or vice versa.