Which two-allele genotype combination corresponds to blood type AB?

Which two-allele genotype combination corresponds to blood type AB?

• A. IA IA
• B. IA IB
• C. IB IB
• D. IA i

Answer: (B)

A key idea is that in the ABO blood group system, the IA and IB alleles are codominant, while i is recessive. This means one allele can express its antigen, and when both IA and IB are present, both A and B antigens appear on the surface of red blood cells. So, blood type AB occurs only when an individual inherits IA from one parent and IB from the other. That two-allele genotype (one IA and one IB) results in red blood cells that display both A and B antigens, and their plasma contains neither anti-A nor anti-B antibodies. The other combinations give different phenotypes: IA IA expresses only A antigens (type A), IB IB expresses only B antigens (type B), and IA i or IB i express only A or B antigens respectively, because i is recessive and does not produce antigens.

A key idea is that in the ABO blood group system, the IA and IB alleles are codominant, while i is recessive. This means one allele can express its antigen, and when both IA and IB are present, both A and B antigens appear on the surface of red blood cells.

So, blood type AB occurs only when an individual inherits IA from one parent and IB from the other. That two-allele genotype (one IA and one IB) results in red blood cells that display both A and B antigens, and their plasma contains neither anti-A nor anti-B antibodies.

The other combinations give different phenotypes: IA IA expresses only A antigens (type A), IB IB expresses only B antigens (type B), and IA i or IB i express only A or B antigens respectively, because i is recessive and does not produce antigens.