This type of epistasis is sometimes called dominant suppression, because the deviation from 9:3:3:1 is caused by a single allele that produces a dominant phenotype, and the action of this allele is to suppress the expression of some other gene.

This 9:3:3:1 phenotypic ratio is the classic Mendelian ratio for a dihybrid cross in which the alleles of two different genes assort independently into gametes. Figure 1: A classic Mendelian example of independent assortment: the 9:3:3:1 phenotypic ratio associated with a dihybrid cross (BbEe × BbEe).

A chicken with a walnut comb has one of four possible gene combinations: RRPP, RrPP, RRPp, or RrPp. When both genes are absent, the result is a single comb. A chicken with a single comb has the only possible gene combination: rrpp.

The phenotypic ratio is 9:3:4.

A 9:3:3:1 Ratio is at ratio of phenotypes among offspring (progeny) that results when two dihybrids mate, e.g., AaBa × AaBa, where allele A is dominant to allele a, allele B is dominant to allele b, and the A and B loci otherwise have no impact on each other phenotypically (no epistasis) nor genotypically (no linkage).

The hypothesis that color and shape genes assort independently thus predicts that the F2 generation will display a 9:3:3:1 phenotypic ratio: nine individuals with round, yellow seeds, three with round, green seeds, three with wrinkled, yellow seeds, and one with wrinkled, green seeds.

Answer and Explanation: In the case of complete dominance of one allele over the other and no genetic linkage (i.e., independent assortment), a cross of two F1 dihybrid individuals will produce F2 offspring with a 9:3:3:1 ratio of phenotypes.