a center, called a nucleus, in which genetic code of each human being is stored. This genetic material, known as genes, carries the codes responsible for all our inherited characteristics from the mother and the father. Genes are grouped along rod-like structures called chromosomes. Normally, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. In Down syndrome, however, the cells usually contain not 46, but 47 chromosomes. The extra chromosome, being a number 21. This excess genetic material, in the form of additional genes along the 21st chromosome, results in Down syndrome. Because 95 percent of all cases of Down syndrome occur because there are three copies of the 21st chromosome, it is often referred to as "trisomy 21." Chromosomes may be studied by examining blood or tissue cells. Individual chromosomes are identified, stained and numbered from largest to smallest. The visual display of the chromosomes is known as a karyotype. Down syndrome is usually caused by an error in cell division called non-disjunction. However, two other minor types of chromosomal abnormalities, mosaicism and translocation, are also causes of Down syndrome. Regardless of the type of Down syndrome that a person may have, all people with Down syndrome have an extra, critical portion of the number 21chromosome present in all, or some, of their cells. This additional genetic material alters the course of development and causes the characteristics associated with the syndrome. Non-disjunction is a faulty cell division that results in an embryo with three number 21 chromosomes instead of two. Prior to, or at, conception, a pair of number 21 chromosomes, in either the sperm or the egg, fail to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This faulty cell division is responsible for 95 percent of all cases of Down syndrome. Why nondisjunction occurs is currently...
