Mary has type A blood and her husband (John) has type B blood. John's parents both had type AB. Mary and John have three children, one with type A, one with type AB, and one with type B. Relatives suspect that one of the children was adopted. What are Mary and John's blood genotypes, which child was adopted?

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Since Mary has a Type A blood, she is either AA or AO while John is either BB or BO. Since both John's parents are Type AB, he can only have the either A or B as an allele and not O so he is BB. Since John can only give the B allele, all of his children must have the B allele in their blood types. So the adopted one is the child with type A blood.

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