Which one of the following is not the genetic disease?

The correct answer is A. Night blindness is a nutritional disorder caused by deficiency of vitamin A (retinol), which is needed to regenerate rhodopsin in the rod cells of the retina. It is treated through diet or supplementation and is not inherited through a faulty gene like the others.

Why the other options are genetic disorders:

• Albinism is an autosomal recessive disorder caused by a defect in the enzyme tyrosinase, so melanin is not produced.
• Haemophilia is an X-linked recessive disorder affecting clotting factors, mainly VIII (haemophilia A) and IX (haemophilia B).
• Colour blindness is usually X-linked recessive, affecting the red or green cone pigments.

Core concept: the difference between inherited (Mendelian) disorders and deficiency diseases. This ties to Biology in GS-III and to nutrition and public health, where malnutrition-related blindness such as xerophthalmia is a recurring Mains theme.

Trap: candidates pick colour blindness or haemophilia because they are the well-known sex-linked examples, and they forget that night blindness is a symptom of vitamin A deficiency, not a gene defect.