UPSC Prelims 2011 Q66: Science & Technology – Biotechnology & Genetics
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
- (a) O positive
- (b) A positive
- (c) B positive
- (d) Cannot be determined on the basis of the given data
Answer: (a) O positive
Explanation
A blood-group genetics puzzle: finding the adopted son
The parents are AB positive and O negative; the three sons are A positive, B positive, and O positive.
| Point | Reasoning |
|---|---|
| Possible children of AB × O parents | Under ABO inheritance, an AB parent contributes either an A or B allele; an O parent contributes only an O allele. So children can only be group A or group B — never group O, and never AB |
| Which child doesn't fit | Since group O is genetically impossible for a biological child of an AB and an O parent, the O positive son cannot be their biological child |
| Conclusion | The twins born to them must be the biologically consistent A positive and B positive sons; the O positive son must be the adopted one |
ABO inheritance basics:
| Parent 1 | Parent 2 | Possible children |
|---|---|---|
| AB | O | A or B only (never O, never AB) |
| A | B | A, B, AB or O possible |
Answer: (a) O positive.
Why each option is right or wrong
- (a) CORRECT. Since an AB × O pairing can never biologically produce a group O child, the O positive son must be the adopted one.
- (b) Incorrect. Group A is a biologically possible outcome of this pairing, so the A positive son is not the one that must be adopted.
- (c) Incorrect. Group B is also a biologically possible outcome, so the B positive son is not the one that must be adopted.
- (d) Incorrect. The data is sufficient: ABO inheritance rules clearly rule out group O as a biological outcome of this parent pairing.
Key point
An AB-blood-group parent and an O-blood-group parent can only have A or B blood-group children, NEVER group O — so the O positive son in this puzzle must be the adopted child.
Remember
AB-group × O-group parents → only A or B children possible, NEVER O. The O-positive son here must be adopted.
Common mistakes
- Not knowing that group O can never result from an AB × O parent pairing.
- Assuming blood group puzzles need more information than the ABO inheritance rule alone provides.
- Confusing the Rh factor (positive/negative) with the ABO grouping relevant to this puzzle.
Exam tip
AB × O parents can only produce A or B children, never O or AB. Any 'O' child in such a family must be adopted.
Sources
- NCERT Class XII Biology — Principles of Inheritance and Variation (ABO blood groups)
- Nobel Prize — The Nobel Prize in Physiology or Medicine 1930 (Karl Landsteiner: discovery of human blood groups), press release