Hemophilia is a recessive sex-linked disorder present only in the x-chromosome.
Hence in females, it takes two faulty genes to develop the disease. If one of the two X-chromosomes (out of XX) is normal, the female is a carrier.
In males, if the X-chromosome (out of XY) is defective, he will develop hemophilia. In other words, there are no male carriers.
In the given case, father does not have hemophilia, so he has a normal X-chromosome. (XY)
The mother is a carrier, so she has one faulty gene out of two (Xx) where x is the hemophilia related gene.
A punnett square will explain all possible outcomes:
X YX XX XY (healthy)x Xx xY (carrier or has hemophilia)
(F) (M)
Where
x=gene-carrying hemophiliaXX=healthy female (25% out of 50% females)XY=healthy male (25% out of 50% males)Xx=female carrier (25% out of 50% females)xY=male with hemophilia (25% out of 50% males)
Now examine the given statements:
None of the children will have hemophilia.
(false because a son could develop hemophilia)
There is a 100 percent chance that a boy will have hemophilia.
(false because given a boy, 25% out of 50%=50% chance he will develop hemophilia)
There is a 50 percent chance that a girl will be a carrier.
(given a girl, 25% out of 50%=50% she will be a carrier, so TRUE)
There is a 25 percent chance that a girl will have hemophilia.
(given a girl, she will be either a carrier (50%) or healthy, so 0% that she develops hemophilia)