Gene of which one of the following is present exclusively on the X-ch…

The correct answer is Option (3) → Red and green colour blindness

Option (1) Thalassemia — Incorrect. Thalassemia is an autosomal recessive blood disease. Its genes lie on chromosome 16 (α) and chromosome 11 (β).

Option (2) Phenylketonuria — Incorrect. Phenylketonuria is an autosomal recessive disorder: the enzyme that changes phenylalanine into tyrosine is missing.

Option (3) Red and green colour blindness — Correct. It is a sex-linked recessive disorder. Its genes lie on the X chromosome, so it is far more common in males, who have only one X.

Option (4) Sickle cell anaemia — Incorrect. Sickle-cell anaemia is an autosome-linked recessive disease, caused by a change in the β-globin gene.

NCERT Extract:
Mendelian Disorders – “… Myotonic dystrophy) (b) Autosomal recessive trait (for example: Sickle-cell anaemia) 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. This is because the genes that lead to red-green colour blindness are on the X chromosome. Males have only one X chromosome and females have two. The son of a woman who carries the gene has a 50 per cent chance of being colour blind.”