AAMC FL Practice Exam

How do dihybrid crosses typically segregate traits in the F2 generation?

1. 3:1 ratio

2. 9:3:3:1 ratio

3. 1:2:1 ratio

4. 1:1 ratio

The correct answer is: 9:3:3:1 ratio

In a dihybrid cross, two traits, each controlled by different genes, are examined simultaneously. When conducted between two individuals that are both heterozygous for two traits (for example, AaBb x AaBb), the expected phenotypic ratio in the F2 generation is a characteristic 9:3:3:1 ratio. This ratio arises due to the independent assortment of the alleles during gamete formation as outlined in Mendel's law of independent assortment. Specifically, the F1 generation (resulting from the cross of two homozygous parents) is typically heterozygous for both traits. When these heterozygous individuals are crossed, their gametes can combine in four possible ways for each trait. The combinations yield the following proportions in the offspring: - 9 offspring will exhibit both dominant traits, - 3 will exhibit the dominant trait for the first gene and the recessive trait for the second, - 3 will exhibit the recessive trait for the first gene and the dominant trait for the second, - 1 will exhibit both recessive traits. This distinct 9:3:3:1 ratio reflects the phenomenon where each trait segregates independently of the other when considering two traits simultaneously in