A man heterozygous for blood type A marries a women heterozygous for blood type B the chance that their first child will have type O blood is?

To determine the chance that their first child will have type O blood, we can use a Punnett square. The man, being heterozygous for blood type A (genotype AO), can pass on either the A or O allele, while the woman, being heterozygous for blood type B (genotype BO), can pass on either the B or O allele. The possible combinations for their child are AO, BO, AB, and OO. Therefore, there is a 25% chance that their first child will have type O blood (genotype OO).