What is epistasis? What is the difference between dominant epistasis and recessive epistasis?

The Epistasis is the gene interaction in which a gene (the epistatic gene) able to disallow the phenotypical manifestation of another gene (the hypostatic gene). In the dominant epistasis the inhibitor allele is the dominant allele (for instance, I) of the epistatic gene so inhibition occurs in dominant homozygosity (II) or in heterozygosity (Ii). In recessive epistasis the inhibitor allele is the recessive allele of the epistatic gene (i) so inhibition occurs only in recessive homozygosity (ii).