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 I^A, I^B and I^O. Since there are three different alleles, six different combination of these three alleles are possible I^A I^A ,I^A I^O, I^B I^B, I^B I^O, I^A I^B and I^O I^O or ii, i.e., six genotypes. Four phenotypes are A, B and AB and O.