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Three Genetic Variants Cited as Causes of Down Syndrome

written by: Deidra Alexander•edited by: Elizabeth Wistrom•updated: 6/12/2012

Most people with this disorder, named after British doctor John Down, have trisomy 21 type. Read about two other causes of Down syndrome, along with diagnostics, related illnesses, and treatments here.

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    Although Down syndrome is a disorder that many know about, there is a lot of misunderstanding about some of the basics of the disorder. Down syndrome is a chromosomal disorder where an extra or partial set of genetic material, chromosome 21, is present in every cell of a person’s body. This is a simple enough definition but when it comes to who is more likely to have this disorder or who will birth a child with it, facts become complicated.

    With this basic definition of the term comes a prevailing notion that only women over a certain age can have a child with the disorder. This is untrue as woman of any age can have a child with this condition, only the risk increases with age. Besides age, a predisposition for reproducing a child with Down syndrome increases the likelihood of presentation of symptoms. There are three types of Down syndrome with only one of them inheritable; translocation, and mosaic trisomy and trisomy 21, both brought on by spontaneous events in reproduction.

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    Translocation Down Syndrome

    In a rare number of cases (>4%), some persons do not actually have Down syndrome but have characteristics due to a shift in material from chromosome 21 onto another chromosome.. At any time that there is extra material present on otherwise regular cells, abnormalities will present but with this type, Down syndrome is not actually diagnosable.

    Translocation, like all other types of Down syndrome, can happen before or during conception. For instances where genetics is the cause, one parent has to be a carrier. Depending upon the part of chromosome 21 moved, when one has this variant of the disorder and they reproduce the chances of them having a child with Down syndrome or simply passing on the trait are based on the exact shifting of the extra genetic material. This rate of risk inheriting Down syndrome through translocation is at 3% in male carriers and 12% in women.

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    Mosaic Trisomy Down Syndrome

    With this form of the disorder, only some cells carry an extra chromosome - creating a composite of trisomy cells and normal cells. This can happen two ways:

    • In the early stages of gestation, three chromosome 21 were present but at some point one was lost.

    • In the same stage of development, the fertilized ovum had only two chromosome 21 and one duplicated in the reproduction process.

    In a person with this type of Down syndrome, you will see certain physical features depending on which chromosomes are actually affected.

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    Trisomy Down Syndrome

    The majority of Down syndrome cases are trisomy 21. Instead of having the two sets of genetic cells, three copies exist in the person, with the extra set originating from either the egg or the sperm at or before conception.

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    Testing for Down Syndrome

    During pregnancy, routine fetal screening if an irregularity in development is seen further testing will be ordered to determine if the baby has Down syndrome. A follow-up of test indications is completed by conducting another round of testing to clarify results. Amniocentesis, chorionic villus sampling, and percutaneous umbilical cord blood sampling are invasive (the first and second of the former) and non-invasive methods for obtaining blood samples from the unborn child to be sure of any problems.

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    Associate Disorders, Life Expectancy, and Treatment

    Regardless of the causes of Down syndrome, these individuals are more likely to develop the following associated disorders:

    • Leukemia
    • Heart conditions
    • Susceptibility to infectious disease
    • Dementia (at a higher risk of onset in their older age than those without Down syndrome)
    • Hearing loss
    • Thyroid problems
    • Impaired vision
    • Skeletal issues
    • Sleep apnea
    • Corpulence

    Life expectancy for those with Down syndrome is typically 50+ years, dependent upon the number and severity of associated conditions.