If all females have Barr bodies on one of their X chromosomes, why don't individuals who are heterozygous for

Question

a s*x-linked trait express that trait? For instance, if a woman is a carrier for color blindness, why won't the Barr body ever mask the "normal" chromosome instead of the "defective" one, causing her to be color blind even though she's just a carrier?

Answer 1

The X chromosome with the "defective" gene will be expressed in some cells.  However, because some of the retinal cells will have the normal functioning gene on the other X chromosome expressed, she will still be able to see color.

If, by chance, all of her retinal cells have the "defective" X activated, then she would be color blind.

Answer 2

Firstly, the Barr Body is not "on" the chromosome, it it the chromosome itself.

Females do suffer from s*x-linked disorders, but at a much lower frequency than males because, in the heterozygotes, some cells will express the Wild Type gene, while others will express the mutant. In most cases, unless a given tissue has significant portion of it;s cells expressing the mutant gene, you will get a normal phenotype. For example, there are well documented cases of women who are colour-blind in one eye and not in the other.

For another example: http://trc.ucdavis.edu/biosci10v/bis10v/...