Official Answer Key & Detailed Solution
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Official UPSC Answer KeyUPSC Prelims 2009 • Science and Technology
Correct Answer: Option (D) — Son suffers from colour blindness while daughter does not suffer from it.
Concept & Option Breakdown
Standard Reference Verified- Red-green colour blindness is an X-linked recessive genetic disorder.
- The affected mother has the homozygous recessive genotype: $X^c X^c$.
- The unaffected father has the normal hemizygous genotype: $X^Y$.
- Inheritance for the son: He inherits the Y chromosome from his father and an $X^c$ chromosome from his mother ($X^c Y$). Because males have only one X chromosome, the recessive allele is expressed, meaning 100% of sons will be colour blind.
- Inheritance for the daughter: She inherits a normal $X$ chromosome from her father and an $X^c$ from her mother ($X X^c$). Since the trait is recessive, she will be an unaffected carrier and will not suffer from colour blindness.
- Thus, the son suffers from colour blindness while the daughter does not.
Elimination Heuristic & Exam Approach
In X-linked recessive traits, an affected mother will always pass the mutant X allele to all her sons, causing 100% of sons to exhibit the trait.
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