Question

Why is the possibility of a human female becoming haemophilic extremely rare?

Answer 1

Explanation:

Hemophilia being a sex chromosome related recessive disorder has its gene present on X chromosome. ... For the disease to occur both the X chromosomes should have the affected genes which is a rare chance. For this to be possible, the father of the female must be diseased

Answer 2

Hemophilia as has been pointed out can vaguely be called as a loss of function mutation that is we need at least one copy of the correct gene to do the work.

In female carriers there is one correct, functioning X chromosome copy and one incorrect non functioning one.

Now we know X inactivation in humans is random, so in a carrier woman half of her liver cells will have a functioning X chromosome copy activated and half will have non  functioning X inactivated. Basically half her liver cells will make the required clotting factor which is enough to prevent the clinical manifestations of Hemophilia.

There are other mechanisms in which X linked recessive diseases do not manifest in heterozygous female carriers even after X inactivation; namely

1. If the defect in X chromosome is incompatible with life of the cell, then there will be selective growth of cells with the correct copy of the X chromosome and the all the cells of the body might have the correct copy activated, somewhat like preferential inactivation.

2. Earlier it was thought that all the genes in the inactivated X are silenced, but studies show that it is not the case and a good number of genes actually escape the inactivation and are functioning.

Many scientist share your doubt and believe that the X linked recessive or dominant  nomenclature is not appropriate as in heterozygous female patients there are multiple mechanisms that lead to expression of X linked traits like skewed X inactivation.