Which genotype corresponds to the AB phenotype?

Which genotype corresponds to the AB phenotype?

• A. IAIA
• B. IAIB
• C. IBIB
• D. OO

Answer: (B)

In the ABO system, a person’s red blood cells show A and/or B antigens based on which alleles they inherit. The IA and IB alleles are codominant, so having one IA and one IB allele means both A and B antigens are expressed on the cell surface, yielding the AB phenotype. If someone has two IA alleles, they display only the A antigen; two IB alleles give only the B antigen; and two O alleles (often written ii or OO) give no A or B antigens, producing type O. Therefore, the genotype that results in AB antigen expression is IAIB.

In the ABO system, a person’s red blood cells show A and/or B antigens based on which alleles they inherit. The IA and IB alleles are codominant, so having one IA and one IB allele means both A and B antigens are expressed on the cell surface, yielding the AB phenotype. If someone has two IA alleles, they display only the A antigen; two IB alleles give only the B antigen; and two O alleles (often written ii or OO) give no A or B antigens, producing type O. Therefore, the genotype that results in AB antigen expression is IAIB.