| Down syndrome or trisomy 21 (British
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| | environmental factors, and random chance.
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| Down's syndrome) is a genetic condition
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| | Down syndrome occurs in all human
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| resulting from the presence of all or
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| | populations, and analogous effects have
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| part of an extra 21st chromosome. Down
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| | been found in other species such as
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| syndrome is characterized by a
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| | chimpanzees and mice. Recently,
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| combination of major and minor
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| | researchers have been able to create
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| abnormalities of body structure and
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| | transgenic mice with most of human
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| function. Among features present in
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| | chromosome 21 (in addition to the normal
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| nearly all cases are impairment of
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| | mouse chromosomes). People with Down
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| learning and physical growth, and a
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| | Syndrome often have a simian crease on
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| recognizable facial appearance usually
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| | the palmar surface of the hand.[6] The
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| identified at birth. It is named after
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| | extra chromosomal material can come about
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| John Langdon Down, the British doctor who
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| | in several distinct ways. A normal human
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| first described it in 1866.
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| | karyotype is designated as 46,XX or
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| Individuals with Down syndrome have lower
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| | 46,XY, indicating 46 chromosomes with an
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| than average cognitive ability, normally
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| | XX arrangement for females and 46
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| ranging from mild to moderate
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| | chromosomes with an XY arrangement for
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| retardation. Some individuals may have
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| | males.
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| low intelligence overall, but will
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| | Trisomy 21
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| generally have some amount of
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| | Trisomy 21 (47,XX,+21) is caused by a
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| developmental disability, such as a
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| | meiotic nondisjunction event. With
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| tendency toward concrete thinking or
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| | nondisjunction, a gamete (i.e., a sperm
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| naiveté. There is also a small number of
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| | or egg cell) is produced with an extra
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| individuals with Down syndrome with
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| | copy of chromosome 21; the gamete thus
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| severe to profound mental retardation.
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| | has 24 chromosomes. When combined with a
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| The incidence of Down syndrome is
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| | normal gamete from the other parent, the
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| estimated at 1 per 800 to 1 per 1000
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| | embryo now has 47 chromosomes, with three
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| births.
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| | copies of chromosome 21. Trisomy 21 is
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| The common physical features of Down
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| | the cause of approximately 95% of
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| syndrome also appear in people with a
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| | observed Down syndromes, with 88% coming
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| standard set of chromosomes. They include
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| | from nondisjunction in the maternal
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| a simian crease (a single crease across
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| | gamete and 8% coming from nondisjunction
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| one or both palms), almond shaped eyes,
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| | in the paternal gamete.[8]
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| shorter limbs, speech impairment, and
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| | Mosaicism
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| protruding tongue. Health concerns for
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| | Trisomy 21 is generally caused before
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| individuals with Down syndrome include a
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| | conception, and all cells in the body are
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| higher risk for congenital heart defects,
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| | affected. However, when some of the cells
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| gastroesophageal reflux disease,
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| | in the body are normal and other cells
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| recurrent ear infections, obstructive
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| | have trisomy 21, it is called Mosaic Down
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| sleep apnea, and thyroid dysfunctions.
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| | Syndrome (46,XX/47,XX,+21). This can
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| Early childhood intervention, screening
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| | occur in one of two ways: A
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| for common problems, medical treatment
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| | nondisjunction event during an early cell
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| where indicated, a conducive family
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| | division in a normal embryo leads to a
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| environment, and vocational training can
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| | fraction of the cells with trisomy 21; or
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| improve the overall development of
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| | a Down syndrome embryo undergoes
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| children with Down syndrome. While some
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| | nondisjunction and some of the cells in
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| of the genetic limitations of Down
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| | the embryo revert back to the normal
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| Syndrome cannot be overcome, education
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| | chromosomal arrangement. There is
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| and proper care, initiated at any time,
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| | considerable variability in the fraction
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| can improve quality of life
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| | of trisomy 21, both as a whole and among
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| Characteristics
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| | tissues. This is the cause of 1–2% of
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| Example of white spots on the iris known
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| | the observed Down syndromes.There is
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| as Brushfield’s Spots.Individuals with
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| | evidence that mosaic Down syndrome may
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| Down syndrome may have some or all of the
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| | produce less developmental delay, on
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| following physical characteristics:
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| | average, than full trisomy 21.
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| oblique eye fissures with small skin
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| | Robertsonian translocation
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| folds on the inner corner of the eyes,
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| | The extra chromosome 21 material that
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| muscle hypotonia, a flat nasal bridge, a
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| | causes Down syndrome may be due to a
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| single palmar fold (simian crease), a
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| | Robertsonian translocation. In this case,
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| protruding tongue (due to small oral
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| | the long arm of chromosome 21 is attached
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| cavity, poor muscle tone, and an enlarged
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| | to another chromosome, often chromosome
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| tongue near the tonsils), a short neck,
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| | 14 (45,XX,t(14;21q)) or itself (called an
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| white spots on the iris known as
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| | isochromosome, 45,XX,t(21q;21q)). Normal
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| Brushfield spots, excessive flexibility
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| | disjunction leading to gametes have a
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| in joints, congenital heart defects,
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| | significant chance of creating a gamete
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| excessive space between large and second
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| | with an extra chromosome 21.
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| toe, and a single flexion furrow of the
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| | Translocation Down syndrome is often
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| fifth finger. Most individuals with Down
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| | referred to as familial Down syndrome. It
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| syndrome have mental retardation in the
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| | is the cause of 2-3% of observed cases of
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| mild (IQ 50–70) to moderate range (IQ
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| | Down syndrome.[8] It does not show the
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| 35–50), with scores for children with
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| | maternal age effect, and is just as
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| Mosaic Down syndrome (explained below)
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| | likely to have come from fathers as
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| some 10–30 points higher. In addition,
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| | mothers.
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| individuals with Down syndrome can have
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| | Duplication of a portion of chromosome 21
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| serious abnormalities affecting any body
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| | Rarely, a region of chromosome 21 will
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| system.
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| | undergo a duplication event. This will
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| Down syndrome is a chromosomal
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| | lead to extra copies of some, but not
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| abnormality characterized by the presence
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| | all, of the genes on chromosome 21
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| of an extra copy of genetic material on
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| | (46,XX,dup(21q)). If the duplicated
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| the 21st chromosome, either in whole
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| | region has genes that are responsible for
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| (trisomy 21) or part (such as due to
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| | Down syndrome physical and mental
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| translocations). The effects of the extra
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| | characteristics, such individuals will
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| copy varies greatly from individual to
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| | show those characteristics. This cause is
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| individual, depending on the extent of
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| | very rare and no rate estimates are
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| the extra copy, genetic background,
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| | possible.
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