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

The correct option is A Four

ABO blood groups in humans are controlled by the gene I. It has three alleles-I${}^A$, I${}^B$ and i. Since there are three different alleles, six different genotypes are possible. There are four phenotypes possible because humans are diploid and can have any two alleles at one time. Also, I${}^A$ and I${}^B$ when present together shows co-dominance, resulting in AB type and when they are present with I, they show dominance.

Hence, the correct answer is ‘Four’