"The DNA text is divided into 46 volumes or chromosomes. Half of these volumes are inherited from our mothers, and half from our fathers. Twenty-two of the maternally inherited volumes have a corresponding volume that is paternally inherited. The remaining volumes correspond to the X and Y chromosomes. Females have 2 X chromosomes, 1 inherited from each parent. Males have 1 X chromosome, which is maternally inherited, and a Y chromosome, which is passed down from father to son. Information from only one X chromosome is used per cell, so in females one X chromosome needs to be switched off or inactivated. The imaginative name that scientists have come up with for this inactivation process is “X-inactivation”. The X chromosomes are usually inactivated randomly so that the maternal and paternal X chromosomes would each be used in about half of a woman’s cells."

"In females with normal random X inactivation, the maternally inherited gene is active in ~50% of her cells, with the paternally inherited gene being active in the rest. This means that even if a female inherits a problematic FMR1 gene, its effects would only be felt in half of her cells. Since a male has only one copy of the gene, the effects would be felt in all of his cells. That is why boys are more likely to have more severe symptoms of fragile X syndrome than girls. For reasons that we don’t really understand, a small number of females show non-random or “skewed” X-inactivation where one chromosome is more likely to become inactivated than the other. IF THE CHROMOSOME WITH THE AFFECTED FMR1 GENE IS THE ONE THAT IS MORE FREQUENTLY INACTIVATED, THEN THE SYMPTOMS OF FRAGILE X SYNDROME ARE LIKELY TO BE MILD OR EVEN COMPLETELY ABSENT. In contrast, if the affected FMR1 gene is more frequently on the active X chromosome, then symptoms may be more severe."

"How does this discussion of genomes, chromosomes and genes relate to fragile X syndrome? Well, fragile X syndrome is caused by a problem in a single gene or chapter in our genome. This gene, known as fragile X mental retardation 1 or FMR1, is located on the X chromosome. Females, since they have 2 X chromosomes, have 2 versions or alleles of the FMR1 gene, one from each parent."