Final answer:
There are four possible ABO blood types: A, B, AB, and O, controlled by a single gene with three alleles, with IA and IB being codominant and both dominant over allele i.
Step-by-step explanation:
There are four ABO phenotypes (blood types), namely, A, B, AB, and O. These phenotypes are determined by a single gene on chromosome 9 with three common alleles: IA, IB, and i. The alleles IA (for A antigen) and IB (for B antigen) are codominant, meaning that if both are present, as in IAIB, the individual will express blood type AB showcasing both antigens. When only one of these alleles is present alongside the recessive i allele (as in IAi or IBi), the blood type will be A or B respectively. If only the i alleles are present (ii), the resulting blood type is O, which does not present any A or B antigens. The presence of multiple alleles and the concept of codominance are important in understanding the inheritance patterns and possible offspring blood types in the ABO blood group system.