Color-blindness is a sex-linked trait passed from mother to offspring via the X-chromosome. A woman (who is not color-blind herself) has two sons who are color-blind. She recently learned she is pregnant with her third son. What is the probability her third son will also be color-blind?
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She is a carrier, which we will call Xx. Because the child is a son, he CANNOT have inherited the gene from his father. There is statistically a fifty percent chance of him inheriting his mother's X or x. It only takes one. Therefore he has a 50% chance.