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

No (Case I : If father's blood group A is dominant trait, his genotypes will be `I^(A)I^(A)` and `I^(A)I^(O)` ; and mother's blood group O being recessive trait, her genotype will be `I^(O)I^(O)` . So, daughter can receive one recessive allele `I^(O)` from father and another recessive allele`I^(O)` from mother to have genotype `I^(O)I^(O)` and hence blood group O. Case II : If father's blood group A is recessive trait, his have genotype will be `I^(A)I^(A)` ; and mother's blood group O being dominant trait, her genotypes will be `I^(O)I^(O)` and `I^(O)I^(A)` . In this case, daughter can receive a dominant allele `I^(O)` from mother and a recessive allele `I^(A)` from father to have genotype `I^(O)I^(A)` and hence blood group O)