Q.ABO blood group in man is controlled by :

ABO blood grouping is controlled by a single gene with three alleles (IA, IB, i) -- the textbook example of multiple allelism in humans.

The ABO blood group gene (I gene) has three alleles: IA (produces antigen A), IB (produces antigen B), and i (produces no antigen). Since more than two alleles exist for this single locus in the population (any individual carries only two of the three, but the population pool has three), it is called multiple allelism; IA and IB are codominant to each other and both dominant over i, giving the four phenotypes A, B, AB and O. It is not sex-linked (a) -- the I gene is autosomal, located on chromosome 9, not on the X or Y chromosome. It …