8.1: Epistasis and Other Gene Interactions
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Some dihybrid crosses produce a phenotypic ratio that differs from 9:3:3:1, such as 9:3:4, 12:3:1, 9:7, or 15:1. Note that each of these modified ratios can be obtained by summing one or more of the 9:3:3:1 classes expected from our original dihybrid cross. In the following sections, we will look at some modified phenotypic ratios obtained from dihybrid crosses and what they might tell us about interactions between genes.
Recessive epistasis
Epistasis (which means “standing upon”) occurs when the phenotype of one locus masks, or prevents, the phenotype of another locus. Thus, following a dihybrid cross fewer than the typical four phenotypic classes will be observed with epistasis. As we have already discussed, in the absence of epistasis, there are four phenotypic classes among the progeny of a dihybrid cross. The four phenotypic classes correspond to the genotypes: A_B_, A_bb, aaB_, and aabb . If either of the singly homozygous recessive genotypes (i.e. A_bb or aaB_ ) has the same phenotype as the double homozygous recessive ( aabb ), then a 9:3:4 phenotypic ratio will be obtained. For example, in the Labrador Retriever breed of dogs (Figure \(\PageIndex{5}\)), the B locus encodes a gene for an important step in the production of melanin. The dominant allele, B is more efficient at pigment production than the recessive b allele, thus B _ hair appears black, and bb hair appears brown. A second locus, which we will call E , controls the deposition of melanin in the hairs. At least one functional E allele is required to deposit any pigment, whether it is black or brown. Thus, all retrievers that are ee fail to deposit any melanin (and so appear pale yellow), regardless of the genotype at the B locus (Figure \(\PageIndex{6}\)).
The ee genotype is therefore said to be epistatic to both the B and b alleles, since the homozygous ee phenotype masks the phenotype of the B locus. The B/b locus is said to be hypostatic to the ee genotype. Because the masking allele is in this case is recessive, this is called recessive epistasis .
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 A_B_ 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 (Figure \(\PageIndex{7}\)). 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 (Figure \(\PageIndex{8}\)). 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.
Figure \(\PageIndex{8}\): Genotypes and phenotypes among the progeny of a dihybrid cross of squash plants heterozygous for two loci affecting fruit color. (Original-Deyholos-CC:AN)