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Genetic and Rare Diseases Information Center (GARD)

Other Names for this Disease
  • Fra(X) syndrome
  • FRAXA syndrome
  • FXS
  • Marker X syndrome
  • Martin-Bell syndrome
More Names
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How is fragile X syndrome inherited?

Fragile X syndrome is inherited in an X-linked dominant pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes (the Y chromosome is the other sex chromosome). The inheritance is dominant if one copy of the altered gene in each cell is sufficient to cause the condition. In women, the FMR1 gene premutation (approximately 55 to 200 repeats) on the X chromosome can expand to more than 200 CGG repeats in cells that develop into eggs that contribute to offspring. This means that women with a premutation (or a full mutation) have an increased risk of having a male or female child with fragile X syndrome. The magnitude of this risk is related to the number of CGG repeats identified.[1] By contrast, the premutation in men does not expand to more than 200 repeats as it is passed to the next generation, and the vast majority of males with a full mutation have fragile X syndrome.[2][1] Men pass the premutation on to all of their daughters and none of their sons.[1] Their sons receive a Y chromosome, which does not include the FMR1 gene.[2] However, there is a risk for fragile X syndrome in a male premutation carrier's grandchildren through his daughters.[1] This is why fragile X syndrome is typically passed on through the mother to her son.
Last updated: 6/15/2011

  1. What Does it Mean to be a Carrier?. The National Fragile X Foundation. September 30, 2008; Accessed 6/15/2011.
  2. Fragile X syndrome. Genetics Home Reference. May 2007; Accessed 6/15/2011.