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Genetic Causes of Intellectual Disabilities: Fragile X Syndrome

Tammy Reynolds, B.A., C.E. Zupanick, Psy.D. & Mark Dombeck, Ph.D.

Fragile X syndrome is the second leading genetic cause of intellectual disabilities (ID, formerly mental retardation). However, it is the leading genetic cause in males. It involves a genetic anomaly of the FRM1 gene. This is located on the X (sex-determining) chromosome. This gene normally occurs as multiple copies. However, with Fragile X, the code is repeated in excess. People can be carriers of Fragile X without any symptoms. In other words, family members may have excessive repeats of the gene. However, the repeats are not enough to cause ID. Fewer repeats mean fewer problems.

Fragile X is the most common genetic cause of intellectual disabilities in males. If females have a working copy of the FRM1 gene, the abnormal copy is not expressed. Even so, Fragile X remains a significant cause of ID in both females and males.

There are several physical features of Fragile X. The face is elongated and the ears are large. The forehead tends to be larger than normal. The jaw has a pronounced shape. People with Fragile X are generally large in stature, but with poor muscle tone. Males tend to have abnormally large testicles. This becomes evident during adolescence.

Behavioral features include a reluctance to make eye contact. They may engage in odd, stereotyped movements. Other behavioral traits include hyperactivity, tremors, and poor coordination. Their social and communication skills are not well developed. They tend to be withdrawn and shy. Similar behavioral symptoms are characteristic of autism. People can receive a diagnosis of both ID and autism. There are three subtypes of Fragile X. The first is easily detected. It is expressed at birth. The cognitive deficits are more severe. This type is shares many symptoms of autism. This includes social avoidance, repetitive movements, and severe delays in speech and language.

A second type of Fragile X affects a person's sense of balance. They also have tremors and memory deficits.

The third type is Fragile X-related Premature Ovarian Failure (POF). It is limited to females. The ovaries do not function correctly. This leads to infertility and early menopause.

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Genetic Causes of Intellectual Disabilities: Williams Syndrome

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  • Articles

    • Introduction to Intellectual Disabilities
    • Causes of Intellectual Disabilities
    • Diagnosis of Intellectual Disabilities
      • The Diagnosis of Intellectual Disabilities
      • Psychological Tests and Intellectual Disabilities
      • Psychological Tests and Intellectual Disabilities Continued
      • Tests of Adaptive Functioning
      • Diagnostic Criteria for Intellectual Disabilities: DSM-5 Criteria
      • The American Association on Intellectual and Developmental Disabilities (AAIDD) Diagnostic Criteria for Intellectual Disability
      • Comparing the APA and the AAIDD Diagnostic Criteria for Intellectual Disability
      • Intellectual Disability and Severity Codes
      • Intellectual Disability and Other Psychiatric Disorders
    • Historical & Contemporary Perspectives of Intellectual Disabilities
      • Historical And Contemporary Perspectives on Intellectual Disabilities
      • Early Medical Explanations of Intellectual Disability
      • History of Stigmatizing Names for Intellectual Disabilities
      • History of Stigmatizing Names for Intellectual Disabilities Continued
      • Reducing the Stigma of Intellectual Disabilities: The Evolution of Modern Medical Explanations
      • Paving the Way to a Modern Conception of Intellectual Disability: Advancements in Intelligence Testing
      • Advancements in Genetic Research
      • Social and Political Controversies Associated with Intellectual Disabilities
      • Reproductive Rights for People with Intellectual Disabilities
    • Intellectual Disabilities & Supportive Rehabilitation
      • Intellectual Disabilities and Supportive Rehabilitation: Developing an Individualized Support Plan (ISP)
      • Educational Supports and Individual Educational Plans (IEPs)
      • The Choice of Educational Settings: The Pros and Cons of Mainstreaming Children With Intellectual Disabilities
      • Effective Teaching Methods for People With Intellectual Disabilities
      • Applied Behavioral Analysis (ABA) and Intellectual Disabilities
      • Physical Therapy and Sensory Skills Training
      • Individualized Support Plans: Adaptive Functioning & Life Skills
      • Social Skills Training
      • Supported Employment and Integrated Work Sites
      • Supported Housing and Community Integration
      • Therapies for Intellectual Disabilities and Outdated/Unproven Treatments
    • Support for Families of People with Intellectual Disabilities
      • Additional Support Services for People with Intellectual Disabilities and Their Families: Community Supports
      • Additional Support Services: Financial Supports
      • Additional Support Services: Family Supports
      • Additional Support Services: Advocacy and Legal Supports
    • Intellectual Disabilities Summary & Conclusion
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    • Intellectual Disabilities Resources & References
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