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