KCET · BIOLOGY · STD 12 - 4. Principles of Inheritance and Variation

The gene for haemophilia is located on 'X' chromosome. Hence it is normally impossible for a

A haemophilic father to pass the gene to his daughter
B carrier mother to pass the gene to her daughter
C carrier mother to pass the gene to her son
D haemophilic father to pass the gene to his son.

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Answer & Solution

Correct Answer

(D) haemophilic father to pass the gene to his son.

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Detailed explanation

(D) haemophilic father to pass the gene to his son.,
In males, a single gene for defect is able to express itself as the Y chromosome has no allele for this gene. If a male is \(X ^{ h } Y\) then he is haemophilic. Also, affected males never transmit the mutant allele to their sons.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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