Colour blindness is a hereditary disease in which the gene for this disease is located on the X-chromosome. So, if a colour-blind woman marries a normal man, then it will produce all colour blind $\left(\mathrm{X}^{\mathrm{C}} \mathrm{Y}\right)$ sons. In the case of a carrier woman, the probability of a colour-blind son and a normal son is 50:50.