Which alleles determine ABO blood groups?

• A. IA, IB, and i
• B. IA and IB only
• C. i only
• D. IA and i

Answer: (A)

ABO blood groups are determined by three alleles at a single gene locus: IA, IB and i. The IA and IB alleles code for functional enzymes that attach specific sugars to the surface of red blood cells, producing A and B antigens respectively. The i allele is nonfunctional, so no antigen is added and type O results. IA and IB are codominant, so a person with IAIB has both A and B antigens and is type AB. When the i allele is present with IA or IB (IAi or IBi), only the corresponding A or B antigen is produced, giving type A or type B. If two i alleles (ii) are present, neither antigen is formed, giving type O.