Question

If a colourblind man marries a woman who is homozygous for normal colour vision, the probability of their son being colourblind is

• A. 0
• B. 0.5
• C. 0.75
• D. 1

Answer 1

The correct option is A 0

Colour blindness is an X-linked disorder in which the individual is usually unable to distinguish between red and green colours (red-green colorblindness), or rarely, blue colour.
The following cross can be performed according to the information given in the question.

Since the man is colour blind, he must possess a mutant allele for the trait on his X chromosome. Therefore his genotype would be X${}^c$Y. The mother is homozygous for normal coloured vision, hence she won’t possess any mutant allele on any of her X chromosomes. Her genotype would be XX. From the cross that has been attempted above, it can be concluded that all the sons born to the given couple would be normal. Hence, the probability of their son being colour blind is zero.