The chromosomal abnormality involved in most cases of Down syndrome is trisomy-21, or
the presence of three copies of the 21st chromosome. As a result, the affected person
has 47 chromosomes in all body cells instead of the normal 46, although how this causes
the condition's symptoms is not yet known. Scientists assume that the reason for the
abnormal chromosomal assortment is the fertilization of an ovum having 24 chromosomes by
a sperm with a normal assortment of 23, but they have also found that the sperm can carry
the extra chromosome as well. The abnormal ovum or sperm is derived from a germ cell in
which the pair of 21st chromosomes holds together and passes into the same sperm or ovum
instead of separating. In the type of Down syndrome called translocation, the extra
chromosome 21 material is attached to one of the other chromosomes; when some, but not
all, of the body's cells carry an extra chromosome 21, the condition is a type of Down
syndrome called mosaicism.
Because of the extra chromosome 21, children with Down syndrome often have some
characteristic physical features, such as a small head, a flat face, slightly upward
slanted eyelids, skin folds at the inner corners of the eyes, small nose and mouth, and
small hands and feet. Most of these characteristic do not interfere with the child's
functioning, a doctor primarily uses the characteristics for diagnostic purposes. These
physical features are variable, and children with Down syndrome are usually more like
other children than they are different.
Individuals with Down syndrome also often have certain medical conditions such as weak
muscles, neurological impairments, heart disease, intestinal abnormalities, poorly
functioning thyroid gland, eye abnormalities, hearing problems, and skeletal problems.
Almost all children with Down syndrome are mentally retarded, in the mild to moderate
range. The degree of mental retardation varies considerably. Through medical treatment
the disorders and infections accompanying Down syndrome have no effect on an almost
normal life span.
The overall incidence of Down syndrome is approximately one in 700 births, but the risk
varies with the age of the mother. The incidence of Down syndrome in children born to
25-year-old mothers is approximately 1 in 1200; the risk increases to approximately 1 in
350 for 35-year-olds and approximately 1 in 120 for women older than 40 years. Prenatal
tests can be used to detect chromosome abnormality causing Down syndrome.
SOURCES
1) "Down Syndrome," Microsoft(R) Encarta(R) 96 Encyclopedia. (c) 1993-1995 Microsoft
Corporation. All rights reserved. (c) Funk & Wagnalls Corporation. All rights
reserved.
2) "Down Syndrome," Encyclopedia of Sleep and Dreaming.
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