Final answer:
The probability that their son will have the trait is 50%.
Step-by-step explanation:
The probability that their son will have the trait is 50%.
In case of a sex-linked recessive trait, if the father has the trait and the mother is a carrier, their son has a 50% chance of inheriting the trait. This is because the son receives one X chromosome from his mother, which may contain the recessive trait, and one Y chromosome from his father. If the X chromosome from his mother carries the trait, he will be affected.