Which genotype produces AB phenotype?

• A. IAIA
• B. IAi
• C. IAIB
• D. ii

Answer: (C)

AB phenotype arises when red blood cells display both A and B antigens. This happens because the IA and IB alleles are codominant, so each allele’s product is expressed when present together. Having one IA allele and one IB allele (IAIB) means both A and B antigens are produced, giving the AB blood type. If only IAIA or IAi is present, only A antigens appear; if IBIB or IBi is present, only B antigens appear; and if ii is present, neither A nor B antigens are produced, giving type O. So the genotype with one IA and one IB allele produces the AB phenotype.