Blood Group Genetics

The ABO blood group system is governed by a single gene I with three alleles: IA, IB, and i, where IA and IB are codominant and both are dominant over i. A blood group AB person has the genotype IAIB and can produce gametes carrying either IA or IB. A blood group O person is homozygous recessive ii and produces only i gametes. Combining these, the children can be IAi (group A) or IBi (group B), so only A and B blood groups appear. The option AB and O is wrong because neither IAIB nor ii genotype can be formed from these parents, since the mother cannot donate IA or IB and the father cannot donate i. The choice listing all four groups overstates the possibilities, as AB and O are impossible here. Group O only is incorrect because every child receives at least one dominant antigen-producing allele from the AB father. As explained in NCERT Class 12, Chapter 5 (Principles of Inheritance and Variation), this multiple-allelism example highlights codominance and recessiveness together. As a sanity check, each child must inherit one allele from each parent, and the only valid combinations are IAi and IBi, confirming A and B as the sole outcomes.