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?

Explanation

No, this information is not enough to tell us which of the traits, blood group A or blood group O, is dominant. This is because:
(i) if the blood group A is dominant trait and blood group O is recessive trait, the daughter can have blood group O, and
(ii) even if the blood group A is recessive trait but blood group O is dominant trait, the daughter can still have blood group O.
Let us discuss these two possibilities in detail.
Possibility 1: When blood group A is dominant trait but blood group O is recessive trait
When father's blood group A is dominant trait, it can have two genotypes: `I^(A)I^(A) and I^(A)I^(O)` . And when mothers' blood group O is recessive trait it can have one genotype : `I^(O)I^(O)` (because it should have two recessive alleles). Now, if one recessive allele `I^(O)` comes from and one recessive allele `I^(O)` comes from mother, then the daughter can also have the genotype `I^(O)I^(O)` which can give her blood group O.
Possibility 2: When blood group A is recessive trait but blood group O is dominant trait
When father's blood group A is recessive trait, it can have only one genotype: `I^(A)I^(A)` (because it should have two recessive alleles). And when mother 's blood group O is dominant trait, then it can have two genotypes : `I^(O)I^(O) and I^(O)I^(A)`. Now, if one dominant allele `I^(O)` comes from the mother and one recessive allele `I^(A)` comes from the father the daughter will have the genotype `I^(O)I^(A)` which will again give her blood group O.