altonweb.com/cs/downsyndrome/index.htm?page=derayeh.html). Technically, Down syndrome is caused by having an extra chromosome present on the twenty-first pair leaving you with forty-seven chromosomes instead of only forty-six. Some or all of the following traits normally accompany Down syndrome; mental retardation, characteristic facial features, vision and hearing problems, and heart defects. "In Trisomy 21, the presence of an extra set of genes leads to over expression of the involved genes- (http://www.ds-health.com/trisomy.htm). Down syndrome has different levels of severity, as well as different types, which include: Nondisjunction Trisomy 21, Translocation, and Mosaicism.
At an overwhelming 95%, Trisomy 21 is the most common type of Down syndrome found in children born to mothers of any age (Selikowitz 34). With Trisomy 21 each cell contains an extra chromosome on the 21st pair, which is the result of one parent giving two chromosomes instead of one. Until recently it was believed that the extra chromosome was inherited from the mother, but it is now known that the extra chromosome can be inherited from either parent. As of right now the most significant factor found that causes Trisomy 21 is the mother's age. The risk of having a baby with Trisomy 21 increases from 1 in 1231 at the maternal age of 20 to 1 in 274 at the maternal age of 35 (stray-Gundersen 17). The only type of Down syndrome in which a mother's age is a factor is Trisomy 21. The other 8% of babies with Down syndrome will have one of the other two types, Translocation or Mosaicism (Stray-Gundersen 10).
Translocation is extremely different from nondisjunction Trisomy 21. In Translocation there are 23 correct pairs of chromosomes, but a defect is still there. Translocation Down syndrome is not very common, only occurring in about 4% of Down syndrome cases. Translocation can usually happen in one of two ways: Spontaneously during fertilization, or it can be inherited from a parent.