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Genetic Causes of Intellectual Disabilities: Down Syndrome

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

Genetic Causes of Intellectual Disabilities

Many intellectual disabilities (ID, formerly mental retardation) are caused by genetic abnormalities. The two most common genetic causes of intellectual disabilities are Down syndrome and Fragile X syndrome.

Down syndrome (or Down's syndrome in the UK)

Down syndrome is the most common genetic origin of intellectual disabilities (ID, formerly mental retardation). Down syndrome occurs in 1 out of every 800 births. Down syndrome derives its name from John Langdon Down. He was the one who first pinpointed the disorder. The condition caused by an extra chromosome. An error in cell division during prenatal growth results in an extra third chromosome 21. The extra chromosome is called Trisomy 21.

Down syndrome has a unique pattern of symptoms. These symptoms lead to the identification intellectual disabilities. The total brain volume is generally smaller than average. The shape of the brain is also different.

Doctors quickly recognize the physical features of Down syndrome at birth. The child's eyes usually have an upward slant. They are also white spots on the iris called Brush field spots. The child's ears have an unusual shape. The neck is shorter than usual. The shape of the face is full. The profile of the face tends to be flat. The palm of the hand may have a profound crease running crossways. A person who has Down syndrome may not have all of these physical features. Moreover, these traits are also found in children without the disorder. Therefore, physical features alone are insufficient for diagnosis.

The definitive diagnosis of Down syndrome requires a blood test. Blood tests reveal the abnormal Trisomy 21. The procedure is called a chromosomal karyotype. The procedure takes about two weeks. During this time, blood cells are grown. Then, they are sorted and tested for the extra chromosome.

People with Down syndrome have shorter lives than the general population. The average life expectancy is about 55 years. This relatively short life span is 20 years longer than it was two decades ago. The syndrome is not associated with a particular race. Nor is it limited to certain socioeconomic groups. The risk for Down syndrome increases with the mother's age.

Down syndrome causes a variety of deficits in intellectual and adaptive functioning. Each person with Down syndrome is unique. Some function quite well with good social skills. Others have significant challenges. The Down syndrome population is reputed to have friendly and loving temperaments. They often express affection openly. Many people find their loving and sweet natures endearing. However, these same characteristics make them vulnerable to victimization.

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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
      • Intellectual Disabilities Summary and Conclusion
    • Intellectual Disabilities Resources & References
      • Intellectual Disabilities Resources and References
      • Intellectual Disabilities Document Revision History
  • Book & Media Reviews

    • A Special Education
    • Living with Mild Cognitive Impairment
  • Videos

    • What is Intellectual Disability?
    • Let's Talk About Intellectual Disabilities: Loretta Claiborne
    • What Causes an Intellectual Disability?
    • What Is An Intellectual Disability?
    • What's disability to me? Mia's story.
    • Primary Care of Children and Young Adults with Down Syndrome
    • Dr. Tim Shriver: Intellectually Different ... Not Disabled
    • Living with Down Syndrome: Parents, Health Professionals and Personal Perspectives