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

(b) Work out a cross up to F1 generation only, between a mother with blood group A (Homozygous) and the father with blood group B (Homozygous). Explain the pattern of inheritance exhibited.

(a) ABO blood group is considered a good example of multiple alleles as it is controlled by I gene which has 3 alleles I^A, I^B and i. The three alleles govern the same character and hence show multiple allelism.

(b) Homozygous mother having blood group A will have a genotype of- I^AI^A
Homozygous father having blood group B will have a genotype of- I^BI^B
 A cross between


Thus the progeny will havethe blood group AB (genotype I^AI^B) as the alleles A and B are co-dominant to each other that is they express themselves independently even in the presence of other. Thus the progeny resembles both the parents. The pattern of inheritance is called co-dominance pattern of inheritance.