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Fragile X syndrome (FXS) is a genetic condition that causes intellectual disability, behavioral and learning challenges and various physical characteristics.

Female carriers have a 50% chance of passing the mutation to each of her children, while males will pass it to all of his daughters (and and none of his sons).

Though Fragile X syndrome occurs in both genders, males are more frequently affected than females, and generally with greater severity. In fact, some females may not experience any of the behavioral, cognitive, or physical features that appear more widely in males.

Exactly why males are more often and more severely affected is unknown. But it’s believed to be due to the fact that females have two X chromosomes and males have only one — can a damaged FMR1 gene on one X chromosome be compensated for by a healthy FMR1 gene on the other? Because males have only one X, if they receive a damaged FMR1 gene, it’s the only one they have.