Question 1
A healthy man marries a healthy woman. One of their sons has an X linked disease but the other one is healthy.
Which of the following statements are true?
a. The defect is X-linked dominant
b. The defect is X-linked recessive
c. None of their daughters will have the disease
d. 50% of their daughters will have the disease
If the disease was X-linked dominant and both parents are healthy, it means that none of them possesses the trait for the disease (they would both be recessive), so they would not be able to pass it onto their children (please refer to the Transmission genetics: Single Gene disorders lesson for more details).
So the disease is recessive and since we were told that it is an X-linked disease, it is X-linked recessive.
If the disease is X-linked recessive and the father is healthy, it means that he lacks the trait(he has a dominant X chromosome). This means that the mother has to be heterozygous for the trait(she has one recessive X chromosome that she gave to the sick son).
If you make the cross, you will see that none of their daughters will have the disease because all the girls will receive one healthy, dominant X chromosome from the father irrespective of what chromosome they receive from the mother
Making a cross of XY and Xx will give XX, Xx, XY and xy. So one of their daughters will be healthy(XX), the other is a carrier but still healthy(Xx), one son is