A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — blood group A or O — is dominant? Why or why not?

Yes, we can tell that blood group O is the recessive trait. Since the father has blood group A but the daughter has O, the father must carry a hidden O gene along with his A gene. The daughter got an O gene from each parent, making her blood group O.

Yes, this information is sufficient to determine dominance. Blood groups in humans are controlled by three alleles: Iᴬ (for A antigen), Iᴮ (for B antigen), and i (for no antigen, giving group O). Iᴬ and Iᴮ are dominant over i, which is recessive. In this case: 1. The mother has blood group O, so her genotype must be ii (both alleles recessive). 2. The daughter also has blood group O, so her genotype is ii — she received one 'i' from each parent. 3. For the father (blood group A) to pass an 'i' allele to his daughter, his genotype must be Iᴬi (heterozygous), not IᴬIᴬ (homozygous). 4. If A were recessive, a person with blood group A would be homozygous for the A allele, and could not pass an 'i' to the child. 5. Since the father contributed 'i', blood group A must be dominant over O, and the father is heterozygous (Iᴬi). Therefore, blood group O is recessive.