About Down Syndrome

Overview

Down Syndrome is a genetic disorder caused when abnormal cell division results in extra genetic material from chromosome 21. This genetic disorder, which varies in severity, can cause lifelong intellectual disability and developmental delays, and in some people it causes health problems. Down syndrome is the most common genetic chromosomal disorder and cause of learning disabilities in children.  Better understanding of Down syndrome and early interventions can greatly increase the quality of life for children and adults with this disorder and help them live fulfilling lives.

Causes & Types

Human cells normally contain 23 pairs of chromosomes. One chromosome in each pair comes from your father, the other from your mother.  Down syndrome results when abnormal cell division involving chromosome 21 occurs. These cell division abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome. Any one of three genetic variations can cause Down syndrome:

Trisomy 21: About 95 percent of the time, Down Syndrome is caused by trisomy 21 — the child has three copies of chromosome 21 (instead of the usual two copies) in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.
Mosaic Down Syndrome: In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
Translocation Down Syndrome: Down Syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. These children have the usual two copies of chromosome 21, but they also have additional material from chromosome 21 attached to the translocated chromosome.
There are no known behavioral or environmental factors that cause Down syndrome.