$\quad$ ABO blood groups in humans are controlled by the gene I. It has three alleles $-\mathrm{I}^{\mathrm{A}}, \mathrm{I}^{\mathrm{B}}$ and $\mathrm{i}$. Since there are three different alleles, six different genotypes are possible. How many phenotypes can occur?

In ABO blood group system, inheritance of grouping is controlled by a single autosomal gene on chomosome 9 with three major alleles $\mathrm{A}, \mathrm{B}$ and $\mathrm{O}\left(\mathrm{I}^{\mathrm{A}}, \mathrm{I}^{\mathrm{B}}\right.$ and $\left.\mathrm{I}^{\mathrm{O}}\right)$. The $\mathrm{ABO}$ blood group system has at least 6 genotypes. On the basis of presence or absence of antigens and antibodies, four blood groups (phenotypes) have been differentiated-A, B, AB and $O$ blood groups.