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 0 negative. The blood group of the three sons is A positive, B positive, and 0 positive. The blood group of the adopted son is

The answer is A) 0 positive. The key is that a child inherits one ABO allele from each parent. The father is AB, so he can pass on only A or B, never O. The mother is O, so she passes on only O. Every biological child must therefore be A or B, never O.

• A positive and B positive are both possible. Each can come from an AB father and an O mother (genotypes AO and BO).
• O positive is not possible as a biological child of this couple, because the AB parent cannot contribute an O allele. So this son must be the adopted child.
• Option D is wrong, because the data is enough once you check which genotype each parent can produce.

The Rh factor does not change the result. The O negative mother gives d, and the AB positive father can give D, so all three sons being positive is consistent with the couple's Rh genotypes.

This is a Mendelian inheritance question. Its Mains angle is the use of genetic evidence in disputed parentage and adoption cases, where the legal standard is the child's rights rather than the blood test alone.

Trap: aspirants often assume that any child with a blood group the parents cannot produce is adopted, and then pick between A and B without checking which group is impossible. Work out the impossible group first.