Which genotype corresponds to blood type AB?

• A. IAIB
• B. IAIA
• C. IBIB
• D. ii

Answer: (A)

AB blood type appears when both A and B antigens are present on the surface of red blood cells. This happens because the alleles IA and IB are codominant: having one IA and one IB allele (genotype IAIB) results in the expression of both A and B antigens. If someone has IAIA, only the A antigen is produced, giving type A blood; if someone has IBIB, only the B antigen is produced, giving type B blood. The i allele is recessive to both IA and IB, so ii yields type O, with no A or B antigens. So the genotype IAIB is the one that leads to AB blood type because it expresses both antigens.