Prelims 2011 · archive
EasyA 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
Answer & explanation
Answer: (a) O positive
The answer is (a) O positive. The ABO gene has three alleles: IA, IB and i. A child of an AB parent and an O parent must get either IA or IB from the AB parent and i from the O parent. So the couple's biological children can only be group A (IAi) or group B (IBi), never O. The O-positive son cannot be their biological child, so he is the adopted one.
- ✓ (a) AB (IAIB) × O (ii) gives only IAi (group A) and IBi (group B). A biological child cannot be group O, so the O-positive son must be the adopted child. One parent is Rh-positive, so Rh-positive twins are possible and the Rh factor creates no conflict.
- ✗ (b) Group A is a possible child of this couple (IAi). The A-positive and B-positive sons fit as the twins, so neither of them needs to be the adopted son.
- ✗ (c) Group B is also possible (IBi) from an AB × O couple. Together with group A, it accounts for the twin boys.
- ✗ (d) The data does settle it. Group O is the only one of the three groups that the parents' genotypes cannot produce, so the adopted son is fixed as the O-positive boy.
Remember · AB × O parents can have only A or B children, never AB or O. A child whose blood group the parents cannot produce is not their biological child.
📘 Read it in NCERT: Class 12 Biology, Ch 4 (practise this chapter)
Sources
- NCERT Class 12 · Biology, Chapter 4 “The gene (I) has three alleles IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar.”
Question: UPSC Civil Services (Preliminary) 2011, General Studies paper. Explanation: Minimalist IAS, checked 25 Oct 2026 (how we verify). ·