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Genes are the building blocks of the human body, which normally possesses 46 chromosomes in 23 pairs. Any irregularities in these microscopic materials have dramatic effects upon the development of the body.
The medical term for 95 percent of Down Syndrome cases is Trisomy 21--"tri-" meaning three, "-somy" referring to the chromosome, and "21," referring to the 21st pair. Trisomy 21 is simply the presence of three, rather than two, of chromosome 21. Other DS cases involve different effects on the 21st chromosome--but the effect on the body as a whole is similar. (A more severe genetic disorder is Trisomy 18, which is almost always fatal in infancy.)
Prior to conception, during the creation of the egg or the sperm, a woman's or man's pair of chromosomes normally split so that only one chromosome is in each egg or sperm. In Trisomy 21, the twenty-first chromosome pair does not split and a double-dose goes to the egg or sperm.
Children born with Down Syndrome also have all the other genes given to them by their parents. This gives these children a combination of features typical of the disorder--some degree of mental retardation and physical characteristics such as a flattened bridge of the nose, a single crease in the palm, and decreased muscle tone--as well as the individual features of their parents.
There is no known cause of Down Syndrome, but the extra 21st chromosome causes an extra dose of some proteins, most of which are unknown. What is known is that, because Trisomy 21 is genetic and is found in every strand of human DNA, Down Syndrome is incurable.
But it is not insurmountable. The negative effects of Down Syndrome can be lessened by regular medical care and the love and attention of a family.