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?

(i) If A is considered to be dominant over O then the following combinations are possible:
Male: AO and Female: 00

	A	O
O	AO	00
O	AO	00

(For understanding)
Daughter can be OO and hence have blood group O.

(ii) If O is considered to be dominant over A then the following combinations are possible:
Male: AA and Female: 00 or OA

	A	A
O	OA	OA
O	OA	OA
O	OA	OA
A	AA	AA

(For understanding)
In this case daughter can be OA and hence have blood group O.
Thus from the information given in the question we cannot ascertain which blood group is dominant as in both cases we can see that the that the daughter can have the blood group O.

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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?