A married couple adopted a male child. A few years later, twin boys were born to them. The blood group of the couple is AB positive and O negative. The blood group of the three sons is A positive, B positive, and O positive. The blood group of the adopted son is

1. The parents have blood groups AB and O. Let's consider the ABO blood groups first. A parent with blood group AB has alleles IA and IB. A parent with blood group O has alleles i and i. 2. The possible blood groups of their biological children (the twin boys) are determined by combining one allele from each parent: IAi (resulting in blood group A) and IBi (resulting in blood group B). 3. The couple has three sons with blood groups A positive, B positive, and O positive. 4. Since the biological children of this couple can only have blood groups A or B, the son with blood group O positive cannot be their biological child. 5. The adopted male child is one of these three sons. As the child with blood group O positive cannot be biologically theirs, this must be the adopted son. 6. The Rh factor (positive/negative) information is consistent but not needed to determine the ABO group of the adopted son in this case.