Out of the three alleles of gene I, the sugar polymers on the plasma membrane of RBCs is controlled by how many alleles?

The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by 'I' gene. It has three alleles I^A, I^B and i. Out of these only two alleles I^A and IB produce a slightly different form of the sugar, while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. I^A and I^B are completely dominant over i.