ABO blood grouping is controlled by gene I, which has three alleles...

In $\mathrm{ABO}$ blood group system, inheritance of grouping is controlled by a single autosomal gene on chromosome-9 with three major alleles $\mathrm{A}, \mathrm{B} and$\mathrm{O}\left($\mathrm{I}$^{\mathrm{A}}, $\mathrm{I}$^{\mathrm{B}}\right. and \left.$\mathrm{I}$^{\mathrm{O}}\right)$. The$\mathrm{ABO}$blood group system has at least 6 genotypes. On the basis of presence or absence of antigens and antibodies, 4 blood groups (phenotypes) have been differentiated, ie,$A, B, A B and $O$ blood groups.