Read the given passage and answer the following questions:
Colour blindness is a sex-linked 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.
Given below are two statements: one is labelled Assertion (A) and the other is labelled Reason (R).
Assertion (A): Red-green colour blindness occurs far more often in males than in females.
Reason (R): Males have one X chromosome, whereas females have two.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A)
Assertion (A): Red-green colour blindness occurs far more often in males than in females. — True. A is about how often red-green colour blindness occurs in the two sexes. The passage gives the figures: about 8 per cent of males but only about 0.4 per cent of females.
Reason (R): Males have one X chromosome, whereas females have two. — True. R is about the number of X chromosomes. Males are XY and have one X; females are XX and have two.
Why R explains A? The colour-blindness gene is recessive and lies on the X chromosome. A male has no second X to cover a defective gene, so one copy is enough to make him colour blind. A female becomes colour blind only if both her X chromosomes carry the gene, which is much rarer. So the difference in X number is the cause of the difference in frequency, and R is the correct explanation of A.
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. 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.”