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.