Final answer:
Children of parents with O blood and AB blood can have blood types A, B, or O, depending on the allele contributed by the AB parent. The offspring cannot have AB blood type because the O blood type parent can only pass on the recessive i allele.
Step-by-step explanation:
If parents have O blood and AB blood, their children could have blood types A, B, or O, but not AB. The person with O blood has a genotype of ii, which means both alleles are recessive. The person with AB blood has a genotype of IAIB, where both alleles are dominant and expressed due to codominance. When these genotypes come together, the possible genotypes of their offspring are IAi (blood type A), IBi (blood type B), or ii (blood type O).
For instance, if the parent with type AB blood contributes an IA allele and the parent with type O contributes an i allele, the child will have blood type A. Similarly, if the parent with type AB blood contributes an IB allele and the parent with type O contributes an i allele, the child will have blood type B. It is impossible for the child to have AB blood because the parent with type O only carries i alleles and therefore cannot contribute either an IA or IB allele necessary for AB blood type.