Intergenic Gene Interactions

Intergenic (interlocus, or non-allelic) gene interactions take place between the alleles of different genes sitting at different loci, unlike intragenic interactions, which involve only the alleles of one gene. The commonest and most instructive case is epistasis: a gene interaction in which the alleles of one gene at one locus interfere with, suppress, or mask the phenotypic expression of a different gene's alleles at another locus. The gene doing the masking is called epistatic; the gene whose expression is blocked and prevented from showing is called hypostatic - when both genes are present together, it is the epistatic gene's effect, not the hypostatic gene's, that actually shows up in the phenotype. Dominant epistasis is illustrated by fruit colour in summer squash: one locus carries a dominant allele W for white fruit (which is epistatic and masks any colour outcome) and a recessive allele w for coloured fruit; a second, hypostatic locus carries a dominant allele G for yellow fruit and recessive g for green. Crossing a white fruit (WWgg) with a yellow fruit (wwGG) gives an all-white, heterozygous F1 (WwGg); selfing this F1 produces an F2 in the ratio 12 white : 3 yellow : 1 green, because the W allele - present in 12 of the 16 combinations - masks the G/g locus outright, and only the double-recessive homozygous ww genotypes (4/16) can show any colour at all, splitting further into 3/16 yellow (at least one G) and 1/16 green (gg). Beyond dominant epistasis, the same interlocus logic produces several other named, characteristically modified F2 ratios - recessive epistasis (9:3:4, flower colour of Antirrhinum), duplicate genes with cumulative effect (9:6:1, fruit shape in summer squash), complementary genes (9:7, flower …