In a dihybrid cross with independent assortment, what phenotypic ratio is expected in the F2 generation?

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

Answer: (C)

Two genes that assort independently, with both parents heterozygous for two traits, produce a distinctive phenotypic pattern in the F2 generation. Each gene segregates into two alleles with equal chance, and the possible gametes from a AaBb individual are AB, Ab, aB, and ab, each occurring 1/4 of the time. When you cross AaBb with AaBb, you get 16 equally likely genotype combinations. Grouping these by phenotype shows:

• A- B- (dominant for both traits): 9 of 16

• A- bb (dominant for the first trait only): 3 of 16

• aa B- (dominant for the second trait only): 3 of 16

• aa bb (recessive for both): 1 of 16

Thus the expected phenotypic ratio is 9:3:3:1.