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

a. Why is human ABO blood group gene considered a good example of multiple alleles?
b. Work out a cross up to generation only, between a mother with blood group A and the father with blood group B.Explain the pattern of inheritance exhinited.

a. In human ABO blood group, there are 4 possible phenotypes for this character, i.e.,AB,A,B,O. More than two alleles govern the human blood group.The four blood groups result from various combination of 3 different alleles for the enzyme.
b. The cross exhibits co-dominance. When the two alleles are present together,both the alleles express each other equally forming the blood group AB.