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Understanding The Karyotype Of Individuals With Down Syndrome

written by: Finn Orfano • edited by: Elizabeth Stannard Gromisch • updated: 9/11/2012

People with Down Syndrome, or Trisomy 21, have a karyotype that consists of 47 chromosomes rather that the 46 found in neurotypical humans. A karyotype is a genetic profile that medical professionals use to detect chromosomal abnormalities. Several variations of Down Syndrome can be diagnosed.

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    A Down Syndrome karyotype shows the chromosomal makeup of a person who is affected with the disorder. When performing a karyotype test on an individual, medical experts can thoroughly screen for Down Syndrome and other genetic conditions. The karyotype of a person with Down Syndrome can be described as follows:

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    Comparing Karyotypes

    Down Syndrome is also known as Trisomy 21 due to the fact that the body cells of a person with the condition contain three copies of chromosome 21. A human being with a typical karyotype has cells that contain 46 chromosomes, which are grouped into 23 pairs. In contrast, the karyotype of a person with Down Syndrome reveals cells with a 47th chromosome, brought about by the extra copy of chromosome 21.

    When looking at a picture generated by a karyotype test for Down Syndrome, one can see the presence of three 21st chromosomes rather than the traditional pair. Karyotypes can be performed on unborn babies after the 12th week of pregnancy, and the results will accurately show the presence or absence of chromosomal disorders such as Down Syndrome.

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    Down Syndrome Variations

    Though the majority of Down Syndrome cases involve a karyotype that shows three distinct 21st chromosomes (trisomy), some variations of this genetic profile are possible. Translocation Down Syndrome occurs when the third 21st chromosome fuses together with another chromosome, typically the 14th. Though people with translocation Down Syndrome share symptoms and characteristics with people who have trisomy Down Syndrome, a karyotype test can be done to distinguish these types from one another.

    Another type of Down Syndrome is known as mosaic. Mosaic Down Syndrome occurs when three copies of chromosome 21 are present in some cells and only two copies are present in others. This disorder can also be detected through a karyotype.

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    Karyotype Testing

    Karyotype testing involves a sample of blood, amniotic fluid, skin cells, or an outer portion of the placenta known as chorionic villi. The sample is cultured in a laboratory, the chromosomes are sorted and counted, and a picture of the chromosomes is created. The results of a karyotype test are more detailed than those produced by some other types of genetic tests. The number and structure of the chromosomes is examined, and abnormalities caused by chromosome deletions or translocations are detected.

    A karyotype test for Down Syndrome can be completed within 3 to 14 days. Tests performed with blood cells generally produce faster results than those performed with other types of human tissue.

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    A Down Syndrome karyotype can provide valuable information for expectant parents, who can then get an early start in researching the characteristics of this genetic disorder. Though the results of a karyotype test may take a while to receive, the test is beneficial for those with an increased risk of having a child with Down Syndrome.

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    References

    1) Karyotype and Down Syndrome--http://downsyndrome.about.com/od/diagnosingdownsyndrome/a/whatkaryotype_ro.htm

    2) Down Syndrome Characteristics and Etiology--http://www.cerebromente.org.br/n04/doenca/down/down_i.htm

    3) Trisomy 21--http://www.ds-health.com/trisomy.htm