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Diseases

Genetic and Rare Diseases Information Center (GARD)

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Fragile X syndrome

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Other Names for this Disease

  • Fra(X) syndrome
  • FraX syndrome
  • FRAXA syndrome
  • FXS
  • Marker X syndrome
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Inheritance

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How is fragile X syndrome inherited?

Fragile X syndrome (FXS) is inherited in an X-linked dominant manner. A condition is X-linked if the responsible gene is located on the X chromosome. The inheritance is dominant if having only one changed (mutated) copy of the responsible gene is enough to cause symptoms of the condition.

In women who carry an FMR1 gene premutation (approximately 55 to 200 CGG repeats), the repeats can expand to more than 200 repeats in their cells that develop into eggs. This means that women with a premutation (or a full mutation) have an increased risk to have a child with FXS. The size of the risk corresponds to the number of CGG repeats they have.[1] By contrast, men with premutations are not at risk for the repeats expanding to over 200 when passing the gene to offspring.[2][1] However, men with a premutation will pass the premutation on to all of their daughters and none of their sons.[1] This is because boys receive only a Y chromosome from their fathers.[2]
Last updated: 1/29/2015

References
  1. What Does it Mean to be a Carrier?. The National Fragile X Foundation. September 30, 2008; http://www.fragilex.org/html/carriers.htm. Accessed 6/15/2011.
  2. Fragile X syndrome. Genetics Home Reference. May 2007; http://ghr.nlm.nih.gov/condition/fragile-x-syndrome. Accessed 6/15/2011.


Other Names for this Disease
  • Fra(X) syndrome
  • FraX syndrome
  • FRAXA syndrome
  • FXS
  • Marker X syndrome
See Disclaimer regarding information on this site. Some links on this page may take you to organizations outside of the National Institutes of Health.