FXS is more common and severe in males because they have only one X chromosome. Females have two X chromosomes, which means they have two different FMR1 genes. A healthy FMR1 gene can counteract a mutated gene, so many women carrying the FXS mutation do not show any symptoms, but their sons might if they inherit the mutation instead of the healthy gene. FXS also tends to worsen with each generation because the different mutations or permutations become gradually more severe.

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