Down Syndrome  

Introduction
Down Syndrome, also known as Down’s Syndrome  (or Trisomy 21  in Europe),  is a genetic disorder that affects roughly one in 800 live births around the world.   Some 400,000 people in the United States have the condition.  Generally random, its causes remain unknown and untreatable.    Down Syndrome, which almost always includes a degree of developmental delay, varies widely in severity, but DS children have many talents and can flourish, becoming active members of their community when given the opportunity and encouragement.

Basic Information
  The condition known today as Down Syndrome was first described under the name of ‘Mongolism’ by an English physician, John Langdon Down  in 1866.  Its root cause, a genetic abnormality, was not discovered until 1959.  

Down Syndrome is the result of an error in cell division at, or soon after, the mother’s egg is fertilised.   Within the nucleus of a typical human cell the genetic information is carried in coded form on 23 pairs of chromosomes, spiral threads made up of DNA.  Cell division is an extremely complex process, and potentially subject to many mishaps.  In Down Syndrome one of several errors in the division process results in chromosome 21 being duplicated, the extra DNA subtly interfering with the biochemistry of normal cell function.  

In roughly 93 percent of cases the symptoms of Down Syndrome are caused by a simple duplication of chromosome 21.  However, there are two other less-common forms of DS, which carry somewhat different consequences.  

Translocation DS is the result of DNA from chromosome 21 becoming attached to another chromosome.  The practical implication of this is that Translocation DS is an inheritable condition, meaning that a couple whose offspring have it are at significantly greater risk of having a second DS baby.   For this reason it is important that DS parents undergo genetic testing to determine the nature of their condition.  

When the duplication of chromosome 21 DNA occurs not at fertilisation, but subsequently during early cell division, the result can be that only some cells and not others carry the abnormality.   This results in ‘Mosaic Down Syndrome’, a condition found in 2-4 percent of cases.   Because the effects depend on which cells possess the extra DNA, the symptoms of Mosaic DS are very diverse, but generally less severe that conventional DS.  

Effects of Down Syndrome   [this section under revision]

  Links to some useful DS web sites

National Down Syndrome Society: Welcome:www.ndss.org/

National Association for Down Syndrome: www.nas.com/

Down Syndrome: Health Issues - Medical Essays and Information: www.ds-health.com/  

Down's Heart Group (UK): www.dhg.org.uk

Down's Syndrome Association (UK): www.downs-syndrome.org.uk/

Down Syndrome Information Network - Home Page: www.down-syndrome.info/

Down Syndrome: For New Parents: www.downsyn.com/

Down Syndrome Educational Trust - Home page: www.downsed.org/

Down Syndrome Sites on the Internet: www.ds-health.com/ds_sites.htm