Questions: Hemophilia, a blood-clotting disorder in humans, is an X-linked recessive trait. If a normal man has a son with a carrier woman, what is the probability that the son will have hemophilia?

The answer is Zero.

Explanation: Hemophilia is an X-linked recessive trait, which means the gene responsible for hemophilia is located on the X chromosome. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

• A normal man (XY) does not carry the hemophilia gene on his X chromosome.
• A carrier woman (X\(^H\)X) has one normal X chromosome and one X chromosome with the hemophilia gene.

When they have a son, the son will inherit the Y chromosome from the father and one of the X chromosomes from the mother. The possible combinations for the son are:

1. X\(^H\)Y (inherits the X chromosome with the hemophilia gene from the mother)
2. XY (inherits the normal X chromosome from the mother)

Since the father provides the Y chromosome, the son cannot inherit the X chromosome from the father. Therefore, the son can only inherit the X chromosome from the mother.

For the son to have hemophilia, he must inherit the X\(^H\) chromosome from the mother. The probability of this happening is 50% because the mother has one normal X chromosome and one X chromosome with the hemophilia gene.

However, the question specifically asks for the probability that the son will have hemophilia. Since the father is normal and does not carry the hemophilia gene, the son cannot inherit hemophilia from the father. Therefore, the probability that the son will have hemophilia is Zero.