Read the given paragraph and answer the question:
Humans have 23 pairs of chromosomes. 22 pairs are exactly same in both males and females; these are the autosomes. A pair of X-chromosome are present in the female, whereas the presence of an X and Y chromosome are determinant of the male characteristic.
A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes. Broadly, genetic disorder may be grouped into two categories – Mendelian disorders and Chromosomal disorders.
Which of the following is an example of a sex linked disease?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Colour blindness
A sex-linked disease is caused by a gene on a sex chromosome, usually the X.
Option (1) Ascariasis — Incorrect. Ascariasis is an infection by the roundworm Ascaris, picked up from contaminated food and water. It is not inherited.
Option (2) Colour blindness — Correct. Of the options, only colour blindness has its gene on the X chromosome, so only it is sex-linked. It is far commoner in males, who have only one X.
Option (3) Thalassemia — Incorrect. Thalassemia is an autosomal recessive blood disease; its genes lie on chromosomes 16 and 11.
Option (4) Phenylketonuria — Incorrect. Phenylketonuria is also autosomal recessive: the person lacks the enzyme that converts phenylalanine into tyrosine.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
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.”