Question

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?

 

Answer 1

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