Genotypes and phenotypes from three alleles with codominance and recessiveness

Steps:

• For three alleles (A¹, A², a) in diploid organisms, calculate genotypes using formula n(n+1)/2 where n=3, yielding 6: A¹A¹, A¹A², A¹a, A²A², A²a, aa.
• A¹ and A² are codominant, so A¹A² expresses both traits; a is recessive to both.
• Assign phenotypes: A¹A¹ (A¹ trait), A²A² (A² trait), A¹A² (A¹ + A² traits), A¹a (A¹ trait), A²a (A² trait), aa (recessive trait).
• Distinct phenotypes total 4: A¹ only, A² only, A¹ + A², recessive.

Why C is correct:

• Three alleles produce 6 genotypes per the multiple alleles formula; codominance and recessiveness yield 4 phenotypes as heterozygotes A¹a and A²a match homozygous dominant expressions.

Why the others are wrong:

• A: Assumes biallelic dominance (3 genotypes, 3 phenotypes), ignoring third allele.
• B: Counts only 4 genotypes, mistaking for two alleles, and undercounts phenotypes.
• D: Correct on 6 genotypes but overcounts phenotypes at 6, ignoring codominance merging some expressions.

Final answer: C