Why is human ABO blood group gene considered a good example of multiple alleles?

ABO blood group is controlled by the I gene. The I gene has three alleles: IA, IB, and i. Thus, the ABO blood group is a good example of multiple allelism. For example, IAIA (mother) x IBIB (father) progeny: IAIB. So, the blood group of the progeny will have the AB blood group. This shows a codominance pattern of inheritance.