Epistatic Interactions Help
Dominant vs. Recessive Epistasis
When epistasis is operative between two gene loci, the number of phenotypes appearing in the offspring from dihybrid parents will be less than four. There are six types of epistatic ratios commonly recognized, three of which have three phenotypes and the other three have only two phenotypes.
Dominant Epistasis (12 : 3 : 1)
When the dominant allele at one locus, for example, the A allele,produces a certain phenotype regardless of the allelic condition of the other locus, then the A locus is said to be epistatic to the B locus.Furthermore, since the dominant allele A is able to express itself in the presence of eitherB or b, this is a case of dominant epistasis.Only when the genotype of the individual is homozygous recessive at the epistatic locus (aa) can the alleles of the hypostatic locus (B or b) be expressed. Thus, the genotypes A-B- and A-bb produce the same phenotype, whereas aaB- and aabb produce two additional phenotypes. The classical 9 : 3 : 3 : 1 ratio becomes modified into a 12 : 3 : 1 ratio.
Recessive Epistasis (9 : 3 : 4)
If the recessive genotype at one locus (e.g., aa) suppresses the expression of alleles at theB locus, theAlocus is said to exhibit recessive epistasis over the B locus. Only if the dominant allele is present at the A locus can the alleles of the hypostatic B locus be expressed. The genotypes A-B- and A-bb produce two additional phenotypes. The 9 : 3 : 3 : 1 ratio becomes a 9 : 3 : 4 ratio.
Dominant-and-Recessive Interaction (13 : 3)
OnlytwoF2phenotypesresultwhenadominantgenotypeatonelocus(e.g.,A-)andtherecessivegenotypeattheother (bb) produce the same phenotypic effect. Thus A-B-, A-bb, and aabb produce one phenotype and aaB- produces another in the ratio 13 : 3 (see Table 4-1).
Duplicate Genes with Cumulative Effect (9 : 6 : 1)
If the dominant condition (either homozygous or heterozygous) at either locus (but not both) produces the same phenotype, the F2 ratio becomes 9 : 6 : 1. For example, where the epistatic genes are involved in producing various amounts of a substance such as pigment, the dominant genotypes of each locus may be considered to produce one unit of pigment independently. Thus, genotypes A-bb and aaB- produce one unit of pigment each and therefore have the same phenotype. The genotype aabb produces no pigment, but in the genotype A-B- the effect is cumulative and two units of pigment are produced.
Duplicate Dominant Genes (15 : 1)
The 9 : 3 : 3 : 1 ratio is modified into a 15 : 1 ratio if the dominant alleles of both loci each produce the same phenotype without cumulative effect.
Duplicate Recessive Genes (9 : 7)
In the case where identical phenotypes are produced by both homozygous recessive genotypes, the F2 ratio becomes 9 : 7. The genotypes aaB-, A-bb, and aabb produce one phenotype. Both dominant alleles, when present together, complement each other and produce a different phenotype.
Practice problems for these concepts can be found at:
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