In a dihybrid cross AaBb x AaBb, what phenotype ratio is expected?

• A. 3:1
• B. 9:3:3:1
• C. 1:1:1:1
• D. 2:2:2:2

Answer: (B)

Two genes that assort independently and show complete dominance produce a characteristic dihybrid phenotype ratio of 9:3:3:1. In AaBb x AaBb, each parent can produce four kinds of gametes in equal frequency: AB, Ab, aB, ab. The phenotypes depend on whether each gene is dominant (A- or B-) or recessive (aa or bb). The chance of both traits being dominant is (3/4) for A- times (3/4) for B-, giving 9/16. The chance of one dominant and the other recessive is (3/4)×(1/4) for each of the two mixed classes, giving 3/16 for each. The chance of both recessive is (1/4)×(1/4) = 1/16. So the expected phenotype frequencies are 9:3:3:1, matching the dihybrid cross outcome.