A colourblind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their sons being colourblind?

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.

When a colourblind man marries a woman with normal sight who has no history of colour blindness in her family. In their progeny, all the daughters would be a colourblind carrier, and all sons would be normal.