When one gene, hides the effect of other it is called epistasis. The effective gene is called epistatic gene, while the affected gene is called hypostatic gene. Lethal gene is mutant form of a gene that eventually results in the death of an organism if expressed in the phenotype. Most lethal genes are recessive, eg, sickle cell anaemia. Multiple alleles are three or more alternative forms of a gene (alleles) that occupy the same locus, eg, ABO system of blood groups. The complementary genes are two pairs of non-allelic dominant genes (ie, present on separate gene loci), which interact to produce only one phenotypic trait, but neither of them if present alone produces the phenotypic trait in the absence of other.