The human ABO blood type trait displays four phenotypes: A, B, AB, and O. It is controlled by a single gene which has three possible alleles: A, B, and O. The A and B alleles are codominant, and the O allele is recessive to both A and B. How many different possible genotypes exist for this gene?
There are six possible genotypes for the ABO blood type gene:
1. AA
2. AO
3. BB
4. BO
5. AB
6. OO
These genotypes result in the four phenotypes: A, B, AB, and O.