Dominant Epistasis

In some cases, a dominant allele at one locus may mask the phenotype of a second locus. This is called dominant epistasis, which produces a segregation ratio such as 12:3:1, which can be viewed as a modification of the 9:3:3:1 ratio in which the AB class is combined with one of the other genotypic classes that contains a dominant allele. One of the best known examples of a 12:3:1 segregation ratio is fruit color in some types of squash. Alleles of a locus that we will call B produce either yellow (B) or green (bb) fruit. However, in the presence of a dominant allele at a second locus that we call A, no pigment is produced at all, and fruit are white. The dominant A allele is therefore epistatic to both B and bb combinations. One possible biological interpretation of this segregation pattern is that the function of the A allele somehow blocks an early stage of pigment synthesis, before neither yellow or green pigments are produced.