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

A colour-blind man (${\mathrm{X}}^{\mathrm{c}}\mathrm{Y}$) marries a woman with normal sight (XX) who has no history of colour blindness in her family. 

Their offspring: 

Carrier daughters: ${\mathrm{X}}^{\mathrm{C}}{\mathrm{X}}^{\mathrm{c}}$ ,  ${\mathrm{X}}^{\mathrm{C}}{\mathrm{X}}^{\mathrm{c}}$ 

Normal sons:  XY,  XY

Assuming that the carrier daughter (${\mathrm{X}}^{\mathrm{C}}{\mathrm{X}}^{\mathrm{c}}$)is married to a man with normal sight (XY), the possibility of their son being colour blind is 25% or 0.25.