Expected phenotypic ratios in Mendelian dihybrid crosses If an F1 individual is heterozygous for two independently assorting genes and is selfed (or mated inter se with a like F1), what phenotypic ratio is expected in the F2?

Introduction / Context:
The hallmark ratio for a Mendelian dihybrid cross under independent assortment is 9:3:3:1 in the F2 generation. This assumes complete dominance at each locus, no epistasis, and random segregation.

Given Data / Assumptions:

• One F1 is heterozygous at both loci (AaBb).
• Cross is an F1 self (AaBb × AaBb) or inter se with another AaBb.
• Genes assort independently; dominance is complete at each locus.

Concept / Approach:
Each locus segregates 3:1 for the dominant phenotype. The joint probability across two independent loci yields the product distribution: 9 double-dominant phenotypes, 3 dominant at A only, 3 dominant at B only, and 1 double-recessive phenotype.

Step-by-Step Solution:

Verification / Alternative check:
Punnett square or probability tree confirms the same ratio. A testcross (AaBb × aabb) would instead yield 1:1:1:1, highlighting the importance of the specified mating.

Why Other Options Are Wrong:

• 1:1:1:1: testcross outcome, not F1 self.
• Other ratios do not follow independent assortment with complete dominance.

Common Pitfalls:
Confusing F1 self with testcross; always check the mating design.

Final Answer:
9:3:3:1