UPSC Prelims 2011 · GS Paper I
Question 66: Science and Technology
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
AnswerThe answer is (a).
A couple with blood groups AB and O can only have children of group A (genotype AO) or B (BO), because the O parent gives only the O allele and the AB parent gives either A or B. A child of group O cannot be their biological child. The A and B sons can be the twins, so the O-positive son must be the adopted one. The Rh factor is consistent, since an Rh-positive father can have Rh-positive children with an Rh-negative mother.
Why each option is right or wrong
- (a) Right: group O cannot be born to an AB parent, so the O-positive son is adopted.
- (b) An A-positive child is possible for this couple, so he can be a biological son.
- (c) A B-positive child is possible for this couple, so he can be a biological son.
- (d) The data are enough: only the O son cannot be biological.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2011, General Studies Paper I, Series A. The paper on upsc.gov.in.