Epistasis

===Epistasis===
is the interaction of two or more genes, with more than one allele, in the development of a single phenotype. Unlike genetics describes by Mendel, traits that are effected by more than one gene do not follow the 9:3:3:1 phenotypic ratio, but rather a complex hierarchical relationship. 
A phenotype is considered epistatic (1) whenever two or more loci interact to create new phenotypes; (2) whenever an allele at one locus masks the effects of alleles at one or more other loci; or (3) whenever an allele at one locus modifies the effects of alleles at one or more other loci.
At the phenotypical level, epistasis is an exceptio from Hardey-Weinber Equilibrium since its ratio deviates from expected independent assortment, while the genotype may or may not be in  be in independent assortment. 
===Example of Epistasis ===
Some cats have a gene called tabby that has three alleles:  The T allele produces the common tabby pattern with parallel stripes of black fur (solid colored hairs) with agouti fur (banded hairs) between the black stripes.  The Ta allele produces the abyssinian color pattern in which all the fur is agouti.  The t allele produces what is known as 'classic tabby' which  has a more complex pattern of black and agouti fur with swirls rather than fairly straight lines.  The t allele is recessive to the other two.  T and Ta have what is known as incomplete dominance.  An individual with one of each (e.g. TTa) usually has black striping on the face, legs, and tail but not on the body which is all agouti.