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

Colour blindness is an X-linked recessive trait.Man: $X^{c}Y$ Woman: $XX$ Progeny: $X^{c}X$ (carrier girls), $X^{c}X$ (carrier girls), $XY$ (normal boys), $XY$ (normal boys).Since the father gives the Y chromosome to his sons and the mother gives a normal X, the probability of a son being colour-blind is 0.