Question

Hemophilia is a blood clotting disorder that is a sex-linked trait carried on the X chromosome. If a woman with hemophilia and a man without the disorder have two sons and two daughters, how many of their children would have hemophilia?

a.none
b.one
c.two
d.four

Answer 1

four because the mens chromosome is XY and womens is XX

Answer 2

c.two

Haemophilia is X-linked and usually considered recessive - it is only manifested when in woman exists two X chromosomes carrying the haemophilia allele, or when in man exists one X chromosome, as in man there is only one X chromosome. If the mother has haemophilia, it means that she will always pass to her generation an X chromosome that has the allele for haemophilia. The father does not have haemophilia which means he does not have an X chromosome with the haemophilia allele, therefore, he will either pass an X chromosome without the haemophilia allele to a daughter or a Y chromosome to a son.
The two sons would have haemophilia because the only X chromosome they would get would be from their mother. The daughters, on the other hand, would not have haemophilia because they only get an X chromosome with the haemophilia allele - from their mother, while from their father they would get an X chromosome without the haemophilia allele.
Two of their children would have haemophilia.