Retrieved January 18, 2014, from http://down-syndrome.emedtv.com/down-syndrome/down-syndrome-effects.html Down syndrome (Trisomy 21). (n.d.). Retrieved January 18, 2014, from Lpch.org website: http://www.lpch.org/DiseaseHealthInfo/HealthLibrary/genetics/downs.html Groneberg, J. G. (n.d.). Down syndrome. Retrieved January 18, 2014, from http://jennifergrafgroneberg.wordpress.com/down-syndrome/ O’Connor, C. (2008).
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A child with trisomy 21 has three copies of chromosome 21 — instead of the usual two copies — in all of his or her cells.”(Para.3) so from this we can conclude that this is the most common type of Down syndrome. Next there is Mosaic Down syndrome according to the Mayo Clinic Staff (2011) “In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.” (Para.4) we can conclude from this that this is a rare type of Down syndrome and is a mixture of normal cells and the cells that have more than two chromosomes so it’s like a mosaic because the cells are mixed. Lastly there is Translocation Down syndrome. According to Mayo Clinic Staff (2011) “Down syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception.
Rarely is it inherited, Down syndrome is caused by three types of chromosomal abnormalities: Trisomy 21, Mosaic Down syndrome, and Translocation Down syndrome. The most common variation, Trisomy 21, occurs in about 95% of cases. According to “Down Syndrome”, this common variation is caused by “an error during the formation of the egg or sperm” and results in “either one having an extra chromosome.” After the sperm and egg mate, the cells produced will have three copies of chromosome 21. In addition, Mosaic Down syndrome, which accounts for 3% of cases, leads to some cells being affected with three copies of chromosome 21, with others unaffected. Finally, Translocation Down syndrome results when an “extra part or a whole extra chromosome 21 is present, but it is attached or “trans-located” to a different chromosome rather than being a separate chromosome 21” (“Facts about Down Syndrome”).
Instead of the normal number of 46 chromosomes in eachcell, the individual with Down syndrome has 47 chromosomes. This condition is called trisomy 21. The second type is called translocation since the extra 21 chromosome is attached or translocated on to another chromosome, usually on chromosome 14, 21 or 22. If translocation is found in a child with Down syndrome, it is important to examine the parents’ chromosomes, since in at least one-third of the cases, a parent may be a carrier of the translocation. This form of chromosome error is found in 3 - 4 percent of the individuals with Down syndrome.
In this form some cells have an extra chromosome 21 while some cells have the regular two. Sometimes problems occur before fertilization, as in the case of trisomy 21 and translocation, when an egg or sperm cell divides incorrectly leaving it with an extra copy of chromosome 21 or apiece of chromosome 21 breaks off and attaches to another chromosome. Unlike trisomy 21 and translocation down syndrome, mosaic down syndrome occurs after fertilization. The likelihood of having a child with Down syndrome increases as the mother gets older. According to the American Pregnancy Association the risk at the mothers age of 20 is 1:1600, 25 is 1:1300, 30 is 1:1000, 35 is 1:365, 40 is 1:90 and 45 is 1:30 (APA).
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