The Dihybrid Test Cross

The dihybrid test cross applies the same logic as a monohybrid test cross to two genes at once: the F1 dihybrid (round-yellow, RrYy) is crossed with the homozygous double-recessive parent (wrinkled-green, rryy) instead of being self-pollinated. Because the recessive rryy parent contributes only one kind of gamete (ry), the phenotype of each test-cross offspring directly reveals which combination of alleles the heterozygous RrYy parent's gamete carried. The RrYy plant produces four kinds of gametes in equal numbers - RY, Ry, rY and ry - so the test cross yields four phenotype classes, round-yellow, round-green, wrinkled-yellow and wrinkled-green, in an equal 1:1:1:1 ratio (25% each). This 1:1:1:1 outcome is itself powerful evidence for independent assortment: if the two genes were instead compl …