Discover the Down Syndrome


What is down syndrome

Down syndrome or trisomy 21 (Britishbackground, environmental factors, and
Down's syndrome) is a genetic conditionrandom chance. Down syndrome occurs in
resulting from the presence of all orall human populations, and analogous
part of an extra 21st chromosome. Downeffects have been found in other species
syndrome is characterized by asuch as chimpanzees and mice. Recently,
combination of major and minorresearchers have been able to create
abnormalities of body structure andtransgenic mice with most of human
function. Among features present inchromosome 21 (in addition to the normal
nearly all cases are impairment ofmouse chromosomes). People with Down
learning and physical growth, and aSyndrome often have a simian crease on
recognizable facial appearance usuallythe palmar surface of the hand.[6] The
identified at birth. It is named afterextra chromosomal material can come
John Langdon Down, the British doctorabout in several distinct ways. A normal
who first described it in 1866.human karyotype is designated as 46,XX
Individuals with Down syndrome haveor 46,XY, indicating 46 chromosomes with
lower than average cognitive ability,an XX arrangement for females and 46
normally ranging from mild to moderatechromosomes with an XY arrangement for
retardation. Some individuals may havemales.
low intelligence overall, but willTrisomy 21
generally have some amount ofTrisomy 21 (47,XX,+21) is caused by a
developmental disability, such as ameiotic nondisjunction event. With
tendency toward concrete thinking ornondisjunction, a gamete (i.e., a sperm
naiveté. There is also a small numberor egg cell) is produced with an extra
of individuals with Down syndrome withcopy of chromosome 21; the gamete thus
severe to profound mental retardation.has 24 chromosomes. When combined with a
The incidence of Down syndrome isnormal gamete from the other parent, the
estimated at 1 per 800 to 1 per 1000embryo now has 47 chromosomes, with
births.three copies of chromosome 21. Trisomy
The common physical features of Down21 is the cause of approximately 95% of
syndrome also appear in people with aobserved Down syndromes, with 88% coming
standard set of chromosomes. Theyfrom nondisjunction in the maternal
include a simian crease (a single creasegamete and 8% coming from nondisjunction
across one or both palms), almond shapedin the paternal gamete.[8]
eyes, shorter limbs, speech impairment,Mosaicism
and protruding tongue. Health concernsTrisomy 21 is generally caused before
for individuals with Down syndromeconception, and all cells in the body
include a higher risk for congenitalare affected. However, when some of the
heart defects, gastroesophageal refluxcells in the body are normal and other
disease, recurrent ear infections,cells have trisomy 21, it is called
obstructive sleep apnea, and thyroidMosaic Down Syndrome (46,XX/47,XX,+21).
dysfunctions.This can occur in one of two ways: A
Early childhood intervention, screeningnondisjunction event during an early
for common problems, medical treatmentcell division in a normal embryo leads
where indicated, a conducive familyto a fraction of the cells with trisomy
environment, and vocational training can21; or a Down syndrome embryo undergoes
improve the overall development ofnondisjunction and some of the cells in
children with Down syndrome. While somethe embryo revert back to the normal
of the genetic limitations of Downchromosomal arrangement. There is
Syndrome cannot be overcome, educationconsiderable variability in the fraction
and proper care, initiated at any time,of trisomy 21, both as a whole and among
can improve quality of lifetissues. This is the cause of 1–2% of
Characteristicsthe observed Down syndromes.There is
Example of white spots on the irisevidence that mosaic Down syndrome may
known as Brushfield’sproduce less developmental delay, on
Spots.Individuals with Down syndrome mayaverage, than full trisomy 21.
have some or all of the followingRobertsonian translocation
physical characteristics: oblique eyeThe extra chromosome 21 material that
fissures with small skin folds on thecauses Down syndrome may be due to a
inner corner of the eyes, muscleRobertsonian translocation. In this
hypotonia, a flat nasal bridge, a singlecase, the long arm of chromosome 21 is
palmar fold (simian crease), aattached to another chromosome, often
protruding tongue (due to small oralchromosome 14 (45,XX,t(14;21q)) or
cavity, poor muscle tone, and anitself (called an isochromosome,
enlarged tongue near the tonsils), a45,XX,t(21q;21q)). Normal disjunction
short neck, white spots on the irisleading to gametes have a significant
known as Brushfield spots, excessivechance of creating a gamete with an
flexibility in joints, congenital heartextra chromosome 21. Translocation Down
defects, excessive space between largesyndrome is often referred to as
and second toe, and a single flexionfamilial Down syndrome. It is the cause
furrow of the fifth finger. Mostof 2-3% of observed cases of Down
individuals with Down syndrome havesyndrome.[8] It does not show the
mental retardation in the mild (IQmaternal age effect, and is just as
50–70) to moderate range (IQ 35–50),likely to have come from fathers as
with scores for children with Mosaicmothers.
Down syndrome (explained below) someDuplication of a portion of chromosome
10–30 points higher. In addition,21
individuals with Down syndrome can haveRarely, a region of chromosome 21 will
serious abnormalities affecting any bodyundergo a duplication event. This will
system.lead to extra copies of some, but not
Down syndrome is a chromosomalall, of the genes on chromosome 21
abnormality characterized by the(46,XX,dup(21q)). If the duplicated
presence of an extra copy of geneticregion has genes that are responsible
material on the 21st chromosome, eitherfor Down syndrome physical and mental
in whole (trisomy 21) or part (such ascharacteristics, such individuals will
due to translocations). The effects ofshow those characteristics. This cause
the extra copy varies greatly fromis very rare and no rate estimates are
individual to individual, depending onpossible.
the extent of the extra copy, genetic



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