Human ABO blood type is determined by a single gene that comes in 3 distinct alleles: IA , IB , and i. The IA and IB alleles are each dominant to the i allele, but are codominant with each other, meaning that both phenotypes are expressed in the heterozygote. The Punnett square below shows a cross between two parents with different blood types. Drag the genotypes and phenotypes from the left to correctly complete the Punnett square.