Identify the disorder which is a sex-linked recessive and with occurrence of about 8% in males.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Colour blindness
Red-green colour blindness is a sex-linked recessive disorder. The genes lie on the X chromosome. A male has only one X, so a single recessive allele shows. A female needs two. This is why it occurs in about 8 per cent of males but only about 0.4 per cent of females.
(1) Haemophilia — Incorrect. It is also X-linked recessive, but it is much rarer, and NCERT gives no such figure for it. Affected females are rarely born alive.
(2) Colour blindness — Correct. It is sex-linked, recessive and occurs in about 8 per cent of males.
(3) Phenylketonuria — Incorrect. It is an autosomal recessive trait, so it is not sex-linked.
(4) Sickle-cell anaemia — Incorrect. It is an autosome-linked recessive trait, so it is not sex-linked.
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.”