Many genes can undergo changes, called mutations, which, in some cases, can make a person more likely to get certain diseases. Diseases caused by mutations in genes on the X chromosome are called X-linked diseases. Remember, males do not experience XCI because they only have one X chromosome. So, if males have a disease-causing gene on their X chromosome, it will be active and more likely to cause disease. However, XCI helps protect females from X-linked diseases. Imagine a girl has a healthy copy of a gene on one X chromosome and a mutant copy of the same gene on her other X chromosome. If the X chromosome with the mutant copy is turned off due to XCI, then the X chromosome with the healthy copy will stay active and express the gene properly. This does not mean she would not get sick, but it will increase her chances of not getting the diseases related to the mutant gene.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00134