How many different genotypes and phenotypes are possible in the ABO blood grouping types in human beings?

ABO blood group system is controlled by the gene I. The gene I has three alleles IA, IB and IO. Since there are three different alleles, six different combination of these three alleles are possible IA IA ,IA IO, IB IB, IB IO, IA IB and IO IO or ii, i.e., six genotypes. Four phenotypes are A, B and AB and O.