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

The inheritance of ABO blood groups is determined by alleles IA, IB, and i. A parent with AB blood group has the genotype IAIB, while a parent with O blood group has the genotype ii. Their biological offspring can only inherit either an IA or IB allele from the first parent and an i allele from the second, resulting in genotypes IAi (Type A) or IBi (Type B). Consequently, biological children of an AB and O couple can only have blood groups A or B. In this scenario, the couple has three sons with blood groups A positive, B positive, and O positive. Since an O positive child cannot be the biological offspring of an AB parent (who cannot pass an 'i' allele if they provide A or B, and has no 'O' phenotype to give), the son with blood group O positive must be the adopted child.