This is the VOA Special English Health Report.

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.

And that's the VOA Special English Health Report, written by Caty Weaver. I'm Steve Ember.