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?
No. There are two possibilities:
Case I: If the father's blood group is A, and is the dominant trait, his genotypes will be IAIA and IAIO ; and the mother's blood group O being a recessive trait, her genotype will be IOIO. So, the daughter can receive one recessive allele IO from the father and another from the mother to have genotype IOIO and the blood groupO.
Case II: If the father's blood group A is a recessive trait, his genotype will be IAIA ; the mother's blood group O being a dominant trait, her genotype will be IOIO and IOIA. So, the daughter can receive one dominant allele IO from the mother and one recessive allele IA from the father to have genotype IOIA and blood group O.