10 Points for Genetics?

the probability of a boy with haemophilia is 50%. Since the woman's mother is a haemophiliac, the woman is a carrier. The man is not a carrier (if he was he would be a haemophiliac because he only has one X chromosome). In a punnett square you would have X*X, XX, X*Y and XY (* = haemophiliac allele). Therefore, out of two possible boys, one would be a haemophiliac... 50%.