Colour blindness is a:

The correct answer is Option (2) → Sex linked dominant disorder

Note: The given answer is as per the official NTA Answer Key. However, the correct answer should be Option (1) Sex linked recessive disorder, as explained below. The original NTA paper also printed “Autosomal recessive disorder” as an option in both Option (3) and Option (4). Therefore, Option (4) has been changed to “Autosomal dominant disorder” so that every option is different.

The faulty gene is on the X chromosome and shows its effect only when no normal copy is present, so it is far commoner in males than in females.

Why the answer given by NTA is incorrect

Option (1) Sex linked recessive disorder — Correct as per NCERT. A carrier mother has normal vision but passes the gene to half her sons.

Option (2) Sex linked dominant disorder — The answer given by NTA, but incorrect. A dominant X-linked gene would show in every female carrying it, so carrier women with normal vision could not exist.

Option (3) Autosomal recessive disorder — Incorrect. The gene is on the X chromosome, not an autosome; sickle-cell anaemia and phenylketonuria are autosomal recessive.

Option (4) Autosomal dominant disorder — Incorrect. The gene is not on an autosome, and a dominant gene would not give the excess of affected males.

NCERT Extract:
Mendelian Disorders – “… Colour Blidness : It is a sex-linked recessive disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females.”