The great-great maternal grandmother of a boy was a carrier for colour-blindness, an X-linked disorder. His great uncle on his mother's side was colourblind but his great uncle's father was unaffected. The boy's mother has 2 brothers (1 colourblind, 1 unaffected) and 1 sister (unaffected). The boy's grandmother on his mother's side had 1 brother who was colourblind and 3 sisters. Two of these sisters were unaffected and one was a carrier. The boy's great grandmother on his mother's side had 4 sisters. The boy has one unaffected sister and he is colourblind. What is the probability of the boy's sons being colourblind if he marries a non-carrier?