Which genotype yields the AB phenotype?

Which genotype yields the AB phenotype?

• A. OO
• B. IAIB
• C. IAIA
• D. IBIB

Answer: (B)

ABO blood type involves two codominant alleles, IA and IB, and a recessive i allele. The AB phenotype appears when red blood cells express both A and B antigens, which happens only if you have one IA allele and one IB allele. In that genotype, both A and B antigens are produced and displayed. If you have IAIA or IAi, you express only A antigen; if IBIB or IBi, only B antigen; and if ii, neither antigen is present, giving type O. So having one IA and one IB allele uniquely yields the AB phenotype.

ABO blood type involves two codominant alleles, IA and IB, and a recessive i allele. The AB phenotype appears when red blood cells express both A and B antigens, which happens only if you have one IA allele and one IB allele. In that genotype, both A and B antigens are produced and displayed. If you have IAIA or IAi, you express only A antigen; if IBIB or IBi, only B antigen; and if ii, neither antigen is present, giving type O. So having one IA and one IB allele uniquely yields the AB phenotype.