Final answer:
The ABO blood group system is controlled by a single gene with three common alleles: IA, IB, and i. Blood type A is determined by IAIA or IAi, blood type B is determined by IBIB or IBi, blood type AB is determined by IAIB, and blood type O is determined by ii.
Step-by-step explanation:
The ABO blood group system is controlled by a single gene with three common alleles: IA, IB, and i. The IA allele encodes the A blood group antigen, the IB allele encodes the B antigen, and the i allele encodes the O antigen. Both A and B alleles are dominant to the O allele.
A person with two A alleles (IAIA) has blood type A, a person with two B alleles (IBIB) has blood type B, and a person with one A allele and one B allele (IAIB) has blood type AB. A person with two O alleles (ii) has blood type O.
Therefore, the different genotypes for blood type in humans are: IAIA or IAi for blood type A, IBIB or IBi for blood type B, IAIB for blood type AB, and ii for blood type O.