A man with type AB blood is married to a woman with type O blood. They have two biological children, and one adopted child. The children’s blood types are: A, B, and O. Which child was adopted?

The adopted child is the one with type O blood.  

Explanation:

Available data:

The man´s genotype for blood type is AB (Codominant type of blood) and the woman´s genotype for blood type is OO, as this type of blood is expressed by two recessive alleles.

Cross:

Parental)  AB   x   OO

Gametes) A   B   O   O

Punnet square)      A      B

                        O    AO   BO

                        O    AO   BO

F1) Half of the biological progeny will be AO and the other half BO.  

   There are no possibilities that any of the biological child could be OO. This means that the adopted child´s blood type is O and the biological children´s blood type are A and B.

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