Bernstein (1925) demonstrated that blood group is controlled by three genes located at the same locus on homologous chromosomes. This condition is called multiple allele. The three alleles are LA, LB and LO. The LA allele carries antigen A and the LB allele carries antigen B.
If A (AO) group man is married to B (BO) group woman, the offspring will have genotype-
AB, A (AO), B (BO), O
Blood ratio of children: A t B t AB t O = 1 : 1 : 1 : 1