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.