Q.A plant heterozygous for two genes ($RrYy$) is test-crossed with a doubly recessive plant ($rryy$). Work out the expected phenotypic ratio of the offspring, and explain why this ratio differs from the 9:3:3:1 ratio obtained in the F2 of a dihybrid cross.

The phenotypic ratio is also 1:2:2:4:1:2:1:2:1 — identical to the genotypic ratio.

When both gene pairs of a dihybrid cross show codominance/incomplete dominance, every genotype produces its own distinct phenotype, so the phenotypic ratio equals the genotypic ratio.

A normal dihybrid F2 cross (with complete dominance at both loci) gives a 9-class genotypic ratio that collapses into the familiar 9:3:3:1 phenotypic ratio because dominant homozygotes and heterozygotes look alike. But when each of the two genes instead shows codominance or incomplete dominance (as in flower colour or coat colour crosses without a dominant/recessive relationship), heterozygotes produce a phenotype distinguishable from either homozygote. In that case, all nine genotypic c …