Statement I: Human beings have 23 pair of chromosomes, one of which is the sex chromosome and the remaining 22 autosomes. The X-linked diseases are related to mutations on a sex chromosome. Statement II: Colour blindness results from a mutation in X-chromosome.

Statement I is true as human cells contain 23 pairs of chromosomes, consisting of 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males) [c1, t9]. X-linked diseases are indeed caused by mutations in genes located on the X chromosome [t2, t6]. Statement II is also true because red-green colour blindness is a classic example of an X-linked recessive disorder resulting from a mutation on the X chromosome [t5, t7, t10]. However, Statement II is not the correct explanation for Statement I. Statement I provides a general definition of human chromosomal structure and the nature of X-linked diseases, whereas Statement II merely provides a specific example of such a disease. The existence of colour blindness does not explain why humans have 23 pairs of chromosomes or the general mechanism of sex-linked mutations; rather, it is a consequence of that biological framework.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 8: Heredity > 8.2.4 Sex Determination > p. 132
2. [2] https://en.wikipedia.org/wiki/X-linked_recessive_inheritance
3. [3] https://www.nei.nih.gov/eye-health-information/eye-conditions-and-diseases/color-blindness/causes-color-vision-deficiency