Question

Assertion(A): Human beings have 23 pair of chromosomes, one of which is the sex chromosomes and the remaining are the 22 autosomes. The X-linked disease is related to the mutation on sex chromosomes.
Reason(R): Colour blindness results from a mutation in X-chromosome.
A) Both A and R are correct and R is the correct explanation for A.
B) Both A and R are correct and R is not the correct explanation for A.
C) A is correct, but R is incorrect.
D) A is incorrect, but R is correct.

Answer 1

A chromosome is a well package of DNA which is well organized and is found inside the nucleus. Colour blindness is a chromosomal disorder wherein the person infected is unable to distinguish between certain colours.

Complete answer:
Let's get familiar with some of the details about chromosomes.
Chromosome is a DNA molecule, which carries the genetic material of an organism. Diploid eukaryotes like humans have 22 different types of autosomes, each present as two homologous pairs and have two sex chromosomes, giving 46 chromosomes in total. Chromosomes which are not sex chromosomes are known as autosomes.
A trait in which the gene is located on the X - chromosome is known as X - linked. X and Y are the two sex chromosomes seen in humans and other mammals. So an X - linked disease is related to the mutation or abnormality of sex chromosomes.
The condition called colour blindness, is a genetic disorder, which is the decreased ability to see colour or distinguish the colour. In humans, males are mostly affected by colour blindness, since the genes responsible for most common forms of colour blindness are on the X - chromosomes. Males only have one X chromosome and therefore they always express the genetic disorder, if they have the recessive gene.

Now, let's have a quick analysis on the options to find the correct answer.

Option A: Both A and R are correct. But, the fact that, "colour blindness results from a mutation in X chromosome" is not the reason for the fact that the "X-linked disease is related to the mutation on sex chromosomes". Therefore, this is the incorrect option.
Option B: Both A and R are correct and R is not the correct explanation for A. Both A and R are independent statements, which are linked together only with the fact about X - chromosomes, and R is not the reason for A. Therefore, this is the correct option.
Option C: Both A and R are correct. Therefore, this is the incorrect option.
Option D: Both A and R are correct. Therefore, this is the incorrect option.

Thus, the correct option is (B) Both A and R are correct and R is not the correct explanation for A.

Note: Usually males are affected with a X-linked or sex linked disease, because they have only one copy of X chromosome, which will carry the mutation. While in the case of females, the effect of mutation in one X chromosome gets masked by the other healthy copy of the X - chromosome.