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Home :: Fragile X Syndrome

Fragile X Syndrome

Fragile X syndrome is an inherited disorder associated with mental retardation and a particular appearance. It is caused by an error in a small piece of the DNA (genetic blueprint) for the FMR-1 gene. This gene is found on the X chromosome, one of the two chromosomes, X and Y, that determine gender.

Fragile X syndrome, characterized by learning disabilities and developmental delay, is believed to be the most prevalent inherited form of mental retardation. The fragile X gene is located on the X chromosome. Children and adults with unexplained mental retardation should be considered for fragile X testing. Affected individuals may also exhibit autistic-like behaviors, such as poor eye contact, speech abnormalities, and difficulty adapting to change. They may also be hyperactive.


Fragile X syndrome is transmitted through X-linked dominant (or semidominant) inheritance; that is, the abnormal gene is located on the X (sex) chromosome. Females have two X chromosomes, whereas males have one X chromosome and one Y chromosome. A female carrier of fragile X has the gene on one of her X chromosomes; the other X chromosome is normal. There is a 50% chance of a carrier passing the abnormal gene to each offspring. Some carriers exhibit no signs of the disorder because of the protective effects of the normal X chromosome; others may be mentally retarded or have learning disabilities and show other signs of fragile X syndrome.

Most male offspring who inherit the abnormal gene on their single X chromosome from their mothers show characteristics of fragile X syndrome; a few males who inherit the gene exhibit no signs of the syndrome and are known as "nonpenetrant, normal transmitting males." These males will pass the abnormal gene to all their daughters but to none of their sons because the disorder can't be transmitted from male to male.

Fragile X syndrome occurs in about 1 in 1,500 males and 1 in 2,500 females. It has been reported in almost all races and ethnic populations.

Signs and symptoms

The main problem in fragile X is mental impairment. This may range from a normal IQ to severe learning disabilities. Other symptoms include hyperactivity, attention deficit disorder, emotional and behavioral problems, anxiety and mood swings.

There may also be characteristic facial features, such as a long face and large ears. Other physical features include flat feet and hyperextensible joints.


Individuals with clinical signs of fragile X syndrome or relatives of an affected individual should undergo chromosomal analysis as well as deoxyribonucleic acid (DNA) analysis to determine whether they carry the fragile X gene. DNA analysis can differentiate affected males and nonpenetrant, normal transmitting males from normal males. It can also differentiate carrier females from non-carrier females. Prenatal diagnosis may also be available for most families.

Once a carrier has been identified in a family, other family members should be notified because they may also be carriers with an increased risk of having children with this disorder.


There is no cure for fragile X syndrome. Treatment focuses on relieving symptoms and maximizing a child's potential. This can include:

  • A special education program with modifications in classroom environment, teaching materials and curriculum
  • Psychological counseling for the affected child and family
  • Occupational therapy and speech-language therapy
  • Medication to treat hyperactivity, attention difficulties, violent outbursts and other behavioral problems

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Genetic counseling may help prospective parents with a family history of Fragile X syndrome. Genetic testing can help determine the level of risk in such a family. Accurate diagnosis is important because of the possibility of other family members inheriting either Fragile X syndrome or other problems related to an increased number of repeats in FMR1.

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Disclaimer: website is designed for educational purposes only. It is not intended to treat, diagnose, cure, or prevent any disease. Always take the advice of professional health care for specific medical advice, diagnoses, and treatment. We will not be liable for any complications, or other medical accidents arising from the use of any information on this web site. Please note that medical information is constantly changing. Therefore some information may be out of date.