The three different forms of gene I are IA, IB and i., ABO blood grouping is controlled by these three alleles, hence it is an example of multiple allelism. Each person possesses any two of the three I gene alleles. lA and lB are completely dominant over allele (l) while IA and IB are co-dominant because both of them express themselves equally and independently when present together. As there are three alleles of gene I there would be following six types of allelic combinations of genotypes resulting in four phenotypic expressions as follows :
