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
- A.O positive
- B.A positive
- C.B positive
- D.Cannot be determined on the basis of the given data
▶ Answer & Explanation
Correct answer: A. O positive
Blood group inheritance follows Mendelian genetics. An offspring inherits one allele from each parent. The AB blood group parent can only pass on an A or a B allele. The O negative parent can only pass on an O allele. Therefore, their biological children can only have blood groups A or B. An offspring with blood group O must inherit an O allele from both parents. Since neither parent has blood group O, a child with blood group O cannot be their biological offspring. Therefore, the adopted son must be the one with the O positive blood group.
Source: UPSC gs1 2011