Do you know the 3 Types of Down Syndrome?

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Types of Down Syndrome

There are three chromosomal patterns that result in Down syndrome: Trisomy 21 (nondisjunction), Mosaicism and Translocation. A blood test is required for chromosome analysis and the type of pattern would be determined at that time.

• Trisomy 21 (nondisjunction) is the first type which occurs in about 95 percent of individuals with Down syndrome. Trisomy 21 is caused by a faulty cell division that results in the baby having three #21 chromosomes instead of two.  Prior to or at conception, a pair of #21 chromosomes in either the egg or the sperm fails to separate properly, which result in the extra chromosome being replicated in every cell of the body (NADS).

• Mosaicism type is the rarest, with only 1% to 2% of individuals having it. Mosaicism occurs when nondisjunction of chromosome #21 takes place in one of the initial cell divisions after fertilization.  When this happens, there is a mixture of two types of cells, some containing 46 chromosomes and some 47.  The cells with 47 chromosomes contain an extra 21st chromosome. The percentage of cells with 47 chromosomes instead of 46 varies from person to person. Because of the “mosaic” pattern of the cells, the term mosaicism is used (NADS).

• Translocation accounts for only 3% to 4% of all individuals with Down syndrome.  In translocation a part of chromosome #21 breaks off during cell division and attaches to another chromosome.  The presence of an extra piece of the 21st chromosome causes the characteristics of Down syndrome.  Unlike Trisomy 21, which is the result of random error in the early cell division, translocation may indicate that one of the parents is carrying chromosomal material that is arranged in an unusual manner. If the chromosome test shows a translocation, it is recommended that the parents have their own chromosomes analyzed as the chance of having other children with Down syndrome may increase significantly. Therefore genetic counseling can be sought to ascertain more information when these circumstances occur (NADS;CDSS).

Regardless of the type of Down syndrome a person may have, a critical portion of the 21st chromosome is present in some or all of their cells.  This additional genetic material alters the course of development and causes the characteristics associated with the syndrome.

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Sources:

Canadian Down Syndrome Society. http://www.cdss.ca/information/general-information/types-of-down-syndrome.html

Do Something.org. https://www.dosomething.org/us/facts/11-facts-about-down-syndrome Accessed February 2, 2016

National Association for Down Syndrome. http://www.nads.org/resources/facts-about-down-syndrome/

National Down Syndrome Society. “Down Syndrome Facts.” National Down Syndrome Society. http://www.ndss.org/Down-Syndrome/Myths-Truths

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