Down syndrome is a disorder that includes a combination of birth defects; among children, some degree of mental retardation, characteristic facial features and, often, heart defects, increased infections, problems with vision and hearing, and other health problems. The severity of all of these problems varies greatly among affected individuals. Down syndrome is one of the most common genetic birth defects, affecting approximately one in 800 to 1,000 babies. It generally is caused by an extra chromosome, the structures in cells that contain the genetic information (genes). According to the National Down Syndrome Society, there are approximately 350,000 individuals with Down syndrome in this country. Life expectancy among adults with Down syndrome is about 55 years, though life span varies depending on the individual and his or her medical condition.
Normally, what causes Down syndrome is that each egg and sperm cell contains 23 chromosomes. The union of these creates 23 pairs, or 46 chromosomes in total.
Sometimes, an accident occurs when an egg or sperm cell is forming, causing it to have an extra chromosome number 21. When this cell contributes the extra chromosome 21 to the embryo, Down syndrome results. All of the features and birth defects associated with Down syndrome result from having this extra chromosome 21 in each of the body's cells. Down syndrome also is called trisomy 21 because of the presence of three number 21 chromosomes. Occasionally, the extra chromosome 21 is attached to another chromosome in the egg or sperm; this may result in what is called translocation Down syndrome. This is the only form of Down syndrome that can be inherited from a parent. In such cases, the parent has a rearrangement of chromosome 21, called a balanced translocation, which does not affect his or her health. Rarely, a form...