In ABO blood group system, inheritance of grouping is controlled by a single autosomal gene on chromosome-9 with three major alleles A,B and O(IA,IB and IO). The 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,AB and O blood groups.