That extra chromosome

Special Issue | | The Editors

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.