Overview of Down Syndrome

Down syndrome, or Trisomy 21, occurs when a baby is born with an extra copy of chromosome 21 in some or all of his or her cells. The extra genetic material changes the body's development process and causes mental retardation. It also can lead to numerous medical complications, including heart defects, leukemia, and vision problems. Some children have only mild complications and for others, the disorder is more severe.

Types of Down Syndrome

Down syndrome is classified into three types.

  1. Trisomy 21 accounts for approximately 90 percent of all Down syndrome cases and occurs when developing egg and sperm cells do not divide correctly. Babies with trisomy 21 have an extra chromosome in all cells.
  2. Mosaic Down syndrome results from an error in cell division after fertilization, resulting in an extra chromosome in only some of the child's cells.
  3. Translocation Down syndrome can occur prior to or at conception. In this case, when the cell divides, part of chromosome 21 becomes stuck, or translocated, onto another chromosome. Babies with translocation Down syndrome do not have the extra chromosome 21, but they do have extra genetic material from chromosome 21 attached to another chromosome. This happens in 3–4 percent of cases and is the only type of Down syndrome that can be inherited.

Incidence and Prevalence of Down Syndrome

According to the National Down Syndrome Society, Down syndrome occurs in 1 out of every 733 live births in the United States and the condition affects more than 350,000 people. The Mayo Clinic estimates that up to 6,000 babies are born with Down syndrome each year. Incidence of Down syndrome does not vary according to race, ethnicity, or socioeconomic factors.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 27 Aug 2008

Last Modified: 14 Sep 2015