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Deep
inside each cell in our body is all the genetic information needed
for life. Human genes are normally organized along forty-six
chromosomes -- twenty-three from each parent.
David Petersohn of Liberty, Missouri, reads with his daughter
Darcie, who has Down syndrome
But as
a result of a mistake in cell division, some people have three
copies of the twenty-first chromosome. There are supposed to be
just two.
About
one in every seven hundred babies has this extra copy. The name for
this condition is Down syndrome. A British doctor named John
Langdon Down first described it in the eighteen
sixties.
Republican vice presidential candidate Sarah
Palin has a son who was born in April with Down syndrome. The
Alaska governor and her husband also have four other children who
do not have it.
Many
babies with Down syndrome have low muscle tone, so they need extra
support when they are held. Their heads are smaller than average
and they can have unusually shaped ears. Also, their eyes often
angle upward.
People
with Down syndrome often have other conditions. These include
problems with their heart and with their breathing and hearing. But
a lot of these conditions are
treatable.
As a
result, people with Down syndrome are living longer. In nineteen
eighty-three, they lived an average of just twenty-five years.
Today the average life expectancy is
fifty-six.
But
that longer life has led to a sad discovery. People with Down
syndrome may have an increased risk for Alzheimer's disease at an
early age.
The
National Down Syndrome Society says an estimated twenty-five
percent of those thirty-five and older show signs of
Alzheimer's.
In the
general population, this brain-wasting disease is usually not found
until people are over the age of sixty-five. It slowly destroys
memory, thinking and reasoning
skills.
Down
syndrome is the most common genetic cause of mental retardation.
Most people with Down syndrome are mildly to moderately retarded.
Many are able to attend regular classes with other students. Later,
as adults, many hold jobs and lead independent
lives.
An
estimated three hundred fifty thousand people in the United States
have Down syndrome. There are tests that can be done to look for it
during pregnancy.
The
risk of Down syndrome is higher for older mothers. The rate for
those under thirty is one in one thousand births. In women age
forty-four, like Sarah Palin, that number is one in
thirty-five
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