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.
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.