Autism Wiki
Fragile X Syndrome from Encyclopedia Britannica

Note the "pinched" part of the X chromosomes (source)

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability. It was so named because one part of the X chromosome has a defective piece that appears pinched and fragile when under a microscope. Fragile X syndrome affects about two to five percent of people on the autism spectrum.

It is important to have an autistic child checked for Fragile X, especially if the parents are considering having another child. If one child has Fragile X, there is a one-in-two chance that boys born to the same parents will have Fragile X. Other members of the family who may be contemplating having a child may also wish to be checked for the syndrome. This is important as a child with FXS may develop autism. However, not all children with FXS are autistic. Certain data from Fragile X Foundation may be quoted to establish linkage between FXS and autism:

  • In around 2% to 6% of all children diagnosed with autism, the cause is the Fragile X gene mutation,
  • Around one-third of all children diagnosed with fragile X syndrome are also autistic
  • Fragile X syndrome is the most common known single gene cause of autism

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