Down Syndrome

In 1959 French geneticist Jerome Lejeune recognized that individuals with Down syndrome have forty-seven chromosomes instead of the usual forty-six. Later, it was determined that it is an extra copy of chromosome 21 that causes the condition. It is not yet clear how the extra chromosome causes the clinical features, although it is believed that an "extra dose" of one or more of the genes on the chromosome is responsible.

Down syndrome is a chromosome disorder that causes mental retardation. Characteristic features of Down syndrome include a flattened midface, small ears and mouth, and short, broad hands.

There are three types of Down syndrome: trisomy 21, mosaic Down syndrome, and translocation Down syndrome. In 94 percent of cases, the extra copy of chromosome 21 stands alone (is not attached to any other chromosomes) and is present in every cell of the body. This is called trisomy 21, trisomy meaning three.

Trisomy 21 occurs due to a chromosome packaging error. Usually when the body makes its sex cells (egg or sperm cells) during meiosis, it packages up one chromosome from each pair. However, sometimes an error (nondisjunction) occurs, causing both chromosomes from a pair to get packaged together. If the sex cell with the extra chromosome is fertilized by a sex cell with the usual chromosome number, the resulting embryo will have a trisomy. If the extra chromosome is chromosome 21, the embryo will have Down syndrome. About 75 percent of embryos with trisomy 21 abort spontaneously before birth. Nondisjunction occurs by chance in the making of both egg and sperm cells, but it happens more often in egg cells as women get older. Thus, the chance of having a baby with Down syndrome increases with increasing maternal age.

Translocation Down syndrome, which accounts for 3 percent to 4 percent of cases, occurs when the extra copy of chromosome 21 is attached to another chromosome. In about one-fourth of the cases where a person has translocation Down syndrome, he or she inherited the translocation from a parent. Therefore it is important to test the parents' chromosomes in these cases, for purposes of future family planning.

The third type of Down syndrome is the mosaic type, which occurs in 2 percent to 3 percent of cases. In mosaic Down syndrome, a person has some cells with an extra copy of chromosome 21 and some cells with the usual two copies. People with mosaic Down syndrome may or may not have milder symptoms than people with "full" trisomy 21.