X chromosome inactivation

[Nelsson]

Hi! when a X chromosome inactivates in a female fetus, creating a Barr body, this happens independently and random. The random selection of which of the X chromosomes( maternal and paternal) to be inactivated in each cell, results in a female "mosaic", where the genes in the paternal X chromosome is expressed in some cells, and the maternal one i other cells.
What if this female person is heterozygot for a hereditary sex-linked disease on the X-chromosome, in what way does this affect the person? If approximately half of her cells express the allele causing the disorder and the other half expressing the "normal" allele, will she the be sick or not?

[Darby]

Most of those conditions are considered recessive for the reason you're asking about, but it's a system-wide response. Some cells express the disease, but not enough to affect the whole individual.

[Cat]

The second X chromosome is not completely inactive. Don't forget about pseudoautosomal regions!