How is the Fragile X genetic mutation passed on to the next generation?

Fragile X-associated disorders are passed down in families through expansions of the FMR1 gene. These disorders include a wide range of physical, intellectual, and behavioral symptoms that can affect family members in many different ways. Even for genetics professionals, FMR1 inheritance is complex and confusing, so it’s no surprise that families often have questions about the genetics of Fragile X. 


  • When a male passes the premutation on to his daughters, it usually does not expand to a full mutation.
  • A male never passes the Fragile X gene to his sons, since he passes only his Y chromosome to them, which does not contain a Fragile X gene.


  • A female with the FMR1 premutation will often pass on a larger version of the mutation to her children (more on this point below).
  • She also has a 50% chance of passing on her normal X chromosome in each pregnancy, since usually only one of her X chromosomes has the FMR1 mutation.

The chance of the premutation expanding to a full mutation is related to the size of the mother’s premutation—the larger the mother’s CGG repeat number, the higher the chance that it will expand to a full mutation if it is passed on.

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