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Fragile X Syndrome: Other FAQs

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Basic information for topics, such as “What is it?” and “How many people are affected?” is available in the Condition Information section. In addition, Frequently Asked Questions (FAQs) that are specific to a certain topic are answered in this section.

Are there specific disorders or conditions associated with Fragile X syndrome?
How is Fragile X syndrome inherited?
How does the FMR1 gene change as it is passed from parent to child?

Are there specific disorders or conditions associated with Fragile X syndrome?

Among the other conditions associated with Fragile X syndrome are the following:

  • Autism spectrum disorder. From 30% to 50% of people with Fragile X also meet the criteria for autism spectrum disorder.1
  • Mitral valve prolapse. In mitral valve prolapse (pronounced MY-truhl valv PROH-laps), a heart condition, the valve that separates the upper and lower left chambers of the heart does not work properly. This condition is usually not life-threatening, but in severe cases, surgery might be required to correct the problem.
  • Seizures. Between 6% and 20% of children with Fragile X syndrome have seizures. Seizures associated with the syndrome are more common in boys than in girls.2

How is Fragile X syndrome inherited?

The gene for Fragile X is carried on the X chromosome. Because both males (XY) and females (XX) have at least one X chromosome, both can pass on the mutated gene to their children.

  • A father with the altered gene for Fragile X on his X chromosome will pass that gene on only to his daughters. To his sons he will pass on a Y chromosome, which doesn’t transmit Fragile X syndrome. Therefore, a father with the altered gene on his X chromosome and a mother with normal X chromosomes would have daughters with the altered gene for Fragile X, while none of their sons would have the mutated gene.
  • A father can pass on the premutation form of the FMR1 gene to his daughters but not the full mutation. Even if the father himself has a full mutation of this gene, it appears that sperm can carry only the premutation. Scientists don’t understand how or why fathers can pass on only the milder form of Fragile X to their daughters. This remains an area of focused research.
  • Mothers pass on only X chromosomes to their children, so if a mother has the altered gene for Fragile X, she can pass that gene to either her sons or her daughters. If a mother has the mutated gene on one X chromosome and has one normal X chromosome, and the father has no mutations, all the children have a 50-50 chance of inheriting the mutated gene.
  • These 50-50 odds apply for each child the parents have. Having one child with the FMR1 mutation does not increase or decrease the chances of having another child with the mutated FMR1 gene. This is also true for the severity of the symptoms. Having one child with mild symptoms does not mean that the other children will have severe symptoms, and having a child with severe symptoms does not mean that the other children will have mild symptoms.

How does the FMR1 gene change as it is passed from parent to child?

The repeats in the promoter part of the FMR1 gene are unstable, and sometimes the number of repeats increases from one generation to the next.
A premutation gene is less stable than a full mutation gene. So as it passes from parent to child, a premutation gene might expand to become a full mutation gene. The chances of expansion depend on the number of repeats in the promoter of the premutation gene:

Normal

FMR1 genes that have 5 to 44 CGG repeats in the promoter are considered normal. When these genes are passed from parent to child, the number of repeats does not increase or decrease.4

Intermediate

FMR1 genes with 45 to 54 CGG repeats in the promoter are considered intermediate, or borderline. An intermediate gene may expand from one generation to the next, depending on which parent has the gene.

Mother to Child

About 14% of the time, when a mother passes an intermediate gene to her child, the CGG repeats increase to a number seen with premutations. Research shows that an intermediate gene will not become a full mutation gene in one generation, and so a mother with an intermediate gene will not have a child with a full mutation.

Father to Child

When intermediate genes are transmitted from father to child, they are generally stable and do not increase to premutations.4

Premutations

Premutation (55 to 199 CGG repeats) FMR1 genes can expand to a full mutation from one generation to the next. The risk of expansion depends on which parent has the gene and the number of repeats in that gene.

Mother to Child

An FMR1 gene from the mother with 100 CGG repeats is very likely to expand to a full mutation when passed to the child. An FMR1 gene from the mother with 70 to 79 CGG repeats has about a 30% chance of expanding to a full mutation in one generation.5

Father to Child

Premutations passed from father to child have almost no chance of expanding to full mutations.5


  1. Eunice Kennedy Shriver National Institute of Child Health and Human Development. (2009). NIH Research Plan on Fragile X Syndrome and Associated Disorders (NA). Retrieved June 8, 2012, from http://www.nichd.nih.gov/publications/pubs/Documents/NIH_Research_Plan_on_Fragile_X_and_Assoc_Disorders-06-2009.pdf (PDF - 440 KB) [top]
  2. Berry-Kravis, E., Raspa, M., Loggin-Hester, L., Bishop, E., Holiday, D., & Bailey, D. B. (2010). Seizures in fragile X syndrome: Characteristics and comorbid diagnoses. American Journal of Intellectual and Developmental Disabilities, 115, 461-472. [top]
  3. Schultz-Pedersen, S., Hasle, H., Olsen, J. H., & Friedrich, U. (2001). Evidence of decreased risk of cancer in individuals with fragile X. American Journal of Medical Genetics, 103, 226-230. [top]
  4. Sherman, S., Pletcher, B. A., & Driscoll, D. A. (2005). Fragile X syndrome: Diagnostic and carrier testing. Genetics in Medicine, 7, 584–587. [top]
  5. Nolin, S. L., Brown, W. T., Glicksman, A., Houck, G. E., Jr., Gargano, A. D., Sullivan, A., et al. (2003). Expansion of the Fragile X CGG repeat in females with premutation or intermediate alleles. American Journal of Human Genetics, 72, 454-464. [top]

Last Updated Date: 12/19/2013
Last Reviewed Date: 10/29/2013
Vision National Institutes of Health Home BOND National Institues of Health Home Home Storz Lab: Section on Environmental Gene Regulation Home Machner Lab: Unit on Microbial Pathogenesis Home Division of Intramural Population Health Research Home Bonifacino Lab: Section on Intracellular Protein Trafficking Home Lilly Lab: Section on Gamete Development Home Lippincott-Schwartz Lab: Section on Organelle Biology