UPSC Prelims 2009 · General Studies
Question 69: Science and Technology
In the context of genetic disorders, consider the following: A woman suffers from colour blindness while her husband does not suffer from it. They have a son and a daughter. In this context, which one of the following statements is most probably correct ?
AnswerThe answer is (d).
Colour blindness is an X-linked recessive trait. A colour-blind woman has the gene on both X chromosomes. Every son gets her X chromosome and the father's Y, so all sons are colour-blind. Every daughter gets one X from the mother (with the gene) and one from the normal father, so she is a carrier but not colour-blind. Hence the son is affected and the daughter is not.
Why each option is right or wrong
- (a) Wrong: the daughter gets a normal X from her father and is not affected.
- (b) Wrong: it is the other way round.
- (c) Wrong: the son must inherit the mother's affected X and so is colour-blind.
- (d) Right: the son is colour-blind and the daughter is a carrier.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2009, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.