Question

A colour blind son is born to normal parents. It shows that
A) The father was heterozygous for colour blindness
B) The mother was genotypically homozygous
C) The mother was heterozygous for colour blindness
D) Both parents carried a recessive gene for the disorder.

Answer 1

Colour blindness is a sex-linked or X-linked recessive disorder. That means the genes for this disorder are carried on the X chromosome, and in the heterozygous condition, an individual will be phenotypically normal.

Complete answer:
In humans, the males have the sex chromosomes XY. If the father has the colour blindness gene on his X chromosome, he will have the disorder. Having only one copy of X means the father cannot be heterozygous for colour blindness. Option A is incorrect.

Colour blindness is a recessive gene. If the mother was homozygous for it, with both X chromosomes carrying the defect, she would be colour blind herself. Option B is incorrect.

If the mother is heterozygous for the disorder, and the father is normal, male offspring have a fifty percent chance of inheriting the defective X chromosome from her. If the son is born colour blind and his father has normal vision, it means the mother was heterozygous. C is the correct answer.

Again, if both parents carried a recessive gene for this disorder, having only one copy anyway, the father would express the recessive gene. But the premise is that he has normal vision, therefore D is not the correct answer.

Hence the correct answer is option ‘C’.

Note: There are many disorders which are X-linked. Some of these may even be fatal for the male offspring who express only the one recessive gene. The female often has some regulation from the healthy X chromosome. An example of another such disorder is haemophilia, a clotting disorder.