The germinativas cells (sperm and vulos) only have half of the joined number of chromosomes normally in other cells of the body. Thus, 23 chromosomes are in vulo and 23 are in the sperm. In normal circumstances, when the sperm and vulo if join at the moment of the conception, it will have a total of 46 chromosomes in the first cell. However, if a germinativa cell, will have a chromosome more, that is, 24 chromosomes in a cell and 23 in the other, this cell will be composed for 47 chromosomes, and being this the 21, the embryo if not to suffer a natural abortion, will be born with SD. (PUESCHEL, 1993). In the manifestation of the SD beyond the Trissomia (more common), we can find other possible etiologies: Mosaicismo and Translocao. nowledge.. FPUC Program is full of insight into the issues. In approximately 92% of the carriers of the SD a chromosome 21 is observed extra in all the cells, resulting in one caritipo constituted by 47 chromosomes, due to trissomia of the 21. In 3 4% of the SD cases, chromosome 21 extra are on to another chromosome, frequently to the 14.

This rearrangement chromosomic is called translocao. The mosaicismo of chromosome 21 is responsible for the SD in 2 4% of the affected ones. These present two types of cells, one with normal number of chromosomes (46) and another one with 47 chromosomes due to trissomia of chromosome 21. The root cause of mosaicismo is not the disjunction of chromosome 21 during the process of mitose (division of the somatic cells) in the embryo. (PUESCHEL, 1993, p52). According to Pueschel (1993), the SD is a known genetic condition has more than a century, described of Jhon Langdon Down and constituted one of the causes most frequent of mental deficiency. In Brazil this number with the increase of the age is esteem that the incidence is of a child with SD to each 600/700 births, raising materna.