Each human somatic cell contains 23 pairs of different chromosomes. Missing or additional chromosomes will have devastating clinical consequences on the affected ones. Down syndrome is one of the most common genetic disorder in mankind history (1). There are three genetic mechanisms which cause this disease. In this essay, I shall state out the causes and proceed to explain the clinical symptoms of the disorder and the current methods of treatment.
             The causes of Down syndrome are full trisomy 21 in 94% of cases. Mosaicism (2.4%) and Robertsonian translocations (3.3%) account for the remainder of cases (2). Regardless of the type of Down syndrome, all people with Down syndrome have an extra, critical portion of 21st chromosome present in their cells (4).
             In the first case of Down syndrome, there is an entire extra chromosome 21 in all cells. Nondisjunction during meiosis is thought to be responsible for the presence of the extra chromosome(3). In meiotic nondisjunction, two chromosome homologs migrate to the same daughter cell instead of disjoining normally and migrating to different daughter cell, hence produce monosomic and trisomic offspring (3). In Down syndrome, one cell has two 21st chromosomes instead of one, so this will result three chromosomes in the fertilized egg. Hence the scientific name, trisomy 21 (3). Why nondisjunction occurs is currently unknown, but there is definitely connection with advancing maternal age (3).
             In the case of Mosaicism, these individuals have a mixture of cell lines, some of which have a normal set of somatic cells and others which have trisomy 21(3). There are two possibilities that contribute to mosaic Down syndrome. The most likely explanation is that the initial zygote had three 21st chromosomes, which normally would result in simple trisomy 21, but during the course of cell division, the extra copy of the 21st chromosome was not passed on to both cells. In this way, a second cell group...