Down syndrome is a chromosomal anomaly which usually causes delays in physical and intellectual development. Children with Down syndrome are not defective, they are just different. For some unexplained reason, an error in cell development results in forty-seven chromosomes rather then the normal forty-six. The extra gene material slightly changes the orderly development of the body and the brain. There are about 6,000 babies born with Down syndrome each year in the United States. There are thousands more born throughout the world. The national population of individuals with Down syndrome is estimated to be around 250,000. The exact cause and prevention of Down syndrome is unknown. .
Well before the genetic link to Down syndrome was discovered, John Lagdon Down described it in 1866 as a distinct set of characteristics. He distinguished Down syndrome from other conditions by noting some of the common features associated with it, such as thin, straight hair, a small nose, and a broad face. Down is also credited with naming the condition "mongolism." It was called this because most infants with this condition have a somewhat Oriental appearance. This term is no longer used and we refer to this condition as simply Down syndrome. By the early 1930s, some researchers began to suspect that Down syndrome might be caused by a chromosomal abnormality. In 1959, Jerome Lejune discovered that cells grown from individuals with Down syndrome had an extra chromosome. Later the exact location of the extra chromosome was found to be at the number 21 chromosome.
There are three major types of Down syndrome. The first and most common type is called Nondisjunction Trisomy 21. This type occurs in about 95% of the babies born with Down syndrome. This type of Down syndrome probably results from some genetic material sticking together or failing to disjoin during development of the egg or sperm or during fertilization.