An X-linked recessive gene produces red-green color blindness in humans. A woman with normal color vision whose father was color-blind marries a color-blind man. What is the probability that their son will be color-blind

50% chance

Explanation:

If the woman's father was color blind then that means that he has the recessive gene on his X chromosome. The Y chromosome does not carry this trait. Now, she has inherited on of her mother's X chromosome and her father's recessive gene on the only X chromosome that he can provide. Since she is not color blind she has inherited a normal/dominant trait from her mother as well. Her male offspring will inherit either one normal or one recessive. So in this case there is a 50% chance of being a color blind male.