Consider a cross between a man with an X-linked recessive disorder and a woman who is a carrier for the disorder. Approximately what percentage of their children are predicted to express the disorder?

The X-linked recessive disorder is carried on the X chromosome. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

1. The man with the disorder has the affected X chromosome (XaY).
2. The woman is a carrier, which means she has one affected X chromosome and one normal X chromosome (XaX).

When they have children, each parent gives one of their sex chromosomes:

From the father (XaY):

• If he passes on his X chromosome (Xa), the child will have the disorder gene.
• If he passes on his Y chromosome, the child will not have the disorder gene.

From the mother (XaX):

• If she passes on her affected X chromosome (Xa), the child will have the disorder gene.
• If she passes on her normal X chromosome (X), the child will not have the disorder gene.

So, the possible combinations are:

• Xa from the father and Xa from the mother: The child will be a girl with the disorder (XaXa).
• Xa from the father and X from the mother: The child will be a girl, carrier but not affected (XaX).
• Y from the father and Xa from the mother: The child will be a boy with the disorder (XaY).
• Y from the father and X from the mother: The child will be a boy without the disorder (XY).

In conclusion, 50% of the children (XaXa and XaY) are predicted to express the disorder.