Codominance with Recessive Allele Yields 6 Genotypes and 4 Phenotypes

Steps:

• Identify three alleles: A¹, A² (codominant), a (recessive), forming diploid combinations.
• List genotypes: A¹A¹, A¹A², A¹a, A²A², A²a, aa (6 total).
• Assign phenotypes: A¹A¹ and A¹a show A¹ trait; A²A² and A²a show A² trait; A¹A² shows both traits (codominance); aa shows recessive trait.
• Count distinct phenotypes: A¹ only, A² only, A¹+A², a only (4 total).

Why C is correct:

• With three alleles, diploid genotypes follow (n(n+1)/2) formula for n=3, yielding 6; codominance creates unique A¹A² phenotype, plus three others from dominance/recessiveness, totaling 4 phenotypes.

Why the others are wrong:

• A: Assumes simple dominance like one dominant allele, ignoring codominance and extra genotypes.
• B: Counts only codominant/recessive like two alleles, missing A¹a and A²a genotypes.
• D: Treats all genotypes as phenotypically unique, but A¹a matches A¹A¹ and A²a matches A²A².

Final answer: C