Suppose, there's a disease "D", which is an X-linked, dominant disease. Now, there's a female who possesses the allele for this disease in one of her two X chromosomes. (XDX) It's easy to say that since the allele for the disease is dominant, she'll suffer from it. Now, it's a well known fact that in females, one of the X chromosomes gets inactivated ( Barr Body formation or lyonization). My question is, how can we be sure that she will suffer from the disease? We can never be 100% sure! What if the X chromosome, which has the allele for the disease, is the one that gets inactivated ? That allele will never be expressed and she'll never suffer from the disease.