What is the genotype for blood type O?

• A. IAIA
• B. IBIB
• C. ii
• D. IAIB

Answer: (C)

The way ABO blood types are determined hinges on three alleles: I^A, I^B, and i. The I^A and I^B alleles add A or B antigens to the surface of red blood cells; the i allele is recessive and does not produce A or B antigens. When you inherit i from both parents, you have no A or B antigens on the cells, so the observable blood type is O. This happens with the genotype ii. Other combinations include two copies of I^A or one I^A and one i for type A, two copies of I^B or one I^B and one i for type B, and one I^A with one I^B for type AB. Therefore, ii is the genotype that yields blood type O.