what is epistasis? Genetics..?

This is when one of two (or several genes) has been changed in some way so the phenotype (what is seen on the organism) is called the epistatic gene. Epistasis masks the expressed of the other gene and therefore the other gene will not be physically seen on the organism.

Epistasis (EPI=above and STASIS=remaining) most commonly refers to a case in which one gene is more "important" than, or masks the expression of, another.

Example:  The B allele in guinea pigs makes black fur, so any pig with a dominant allele is black.  However, the P gene determines the deposition of pigment, so that without a P allele, pigment is not deposited.  Not matter how many B alleles ther are, there is no black fur without a P allele.  Guinea pigs with the genotype BBpp are, therefore, white, and those that are BBPp are black.

The P gene is epistatic to the B gene.

Or, nobody sees his widow's peak because he's bald;  the gene for baldness is epistatic to the gene for widow's peak.