UPSC Prelims 2009 Q69: Science & Technology – Biotechnology & Genetics
- (a) Both children suffer from colour blindness
- (b) Daughter suffers from colour blindness while son does not suffer from it
- (c) Both children do not suffer from colour blindness
- (d) Son suffers from colour blindness while daughter does not suffer from it
Answer: (d) Son suffers from colour blindness while daughter does not suffer from it
Explanation
A colour-blindness genetics puzzle
Colour blindness is an X-linked recessive trait, meaning its inheritance pattern differs by the child's sex.
| Point | Reasoning |
|---|---|
| Mother's genotype | Colour-blind, so she carries the recessive allele on both X chromosomes (XᶜXᶜ) |
| Father's genotype | Not colour-blind, so his single X chromosome carries the normal allele (XY) |
| Son's inheritance | Sons receive their only X chromosome from the mother — since the mother's X is always the colour-blind allele, sons will be colour-blind |
| Daughter's inheritance | Daughters receive one X from each parent: the mother's colour-blind X, and the father's normal X — making daughters carriers but not colour-blind themselves |
Why sons and daughters differ here:
| Child | Outcome |
|---|---|
| Son | Colour-blind (only X comes from mother) |
| Daughter | Not colour-blind (gets a normal X from father too) |
Answer: (d) Son suffers from colour blindness while daughter does not suffer from it.
Why each option is right or wrong
- (a) Incorrect. Both children being colour-blind doesn't match the X-linked inheritance pattern given a non-colour-blind father.
- (b) Incorrect. This reverses the actual pattern — the daughter, not the son, would be the one more protected in this scenario.
- (c) Incorrect. Both children unaffected is wrong, since the son must inherit his only X (colour-blind) from the mother.
- (d) CORRECT. Since sons get their only X chromosome from the colour-blind mother, the son will be colour-blind, while the daughter, receiving a normal X from the father too, will not be.
Key point
With a colour-blind mother and non-colour-blind father, the SON (who gets his only X from the mother) will be colour-blind, while the DAUGHTER (who gets a normal X from the father too) will not be.
Remember
Colour-blind mother + unaffected father: son is colour-blind, daughter is not (X-linked recessive inheritance).
Common mistakes
- Forgetting that sons receive their only X chromosome from the mother, making them directly dependent on her genotype.
- Reversing the son/daughter outcome, assuming daughters are more likely affected.
- Not applying the basic X-linked recessive inheritance pattern correctly.
Exam tip
X-linked recessive traits: sons inherit their only X from the mother, so a colour-blind mother's sons are colour-blind; daughters need the condition from both parents to be affected.
Sources
- NCERT Class XII Biology — Principles of Inheritance and Variation (sex-linked inheritance: colour blindness)
- Anthony J. F. Griffiths et al. — Introduction to Genetic Analysis (W. H. Freeman)