Codominance Requires AB Phenotype for Heterozygosity

Steps:

• Examine the pedigree diagram to identify blood types of F1 generation members.
• Recall that ABO alleles are codominant (IA and IB) with O recessive (i); AB phenotype requires IAIB genotype.
• Check each F1 member's phenotype: only AB mandates heterozygosity for codominant alleles.
• Eliminate options with A, B, or O phenotypes, as they allow homozygous or heterozygous recessive possibilities.

Why B is correct:

• Member B has AB blood type, which by codominance definition must be IAIB (heterozygous for codominant alleles).

Why the others are wrong:

• A: A phenotype could be IAIA (homozygous) or IAi (heterozygous recessive).
• C: B phenotype could be IBIB (homozygous) or IBi (heterozygous recessive).
• D: O phenotype is ii (homozygous recessive).

Final answer: B