This is the VOA Special English Health Report.

Human genes are normally organized along forty-six chromosomes in our cells, twenty-three from each parent.

A school for people with Down syndrome in Mexico City

But some people are born with an extra copy of the twenty-first chromosome. This third copy is a result of a mistake in cell division. The name for this condition is Down syndrome. 

A British doctor named John Langdon Down first described it in the eighteen sixties. An estimated three hundred fifty thousand people in the United States have Down syndrome.

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. A lot of these conditions, though, are treatable.

About one in every one hundred people with Down syndrome will develop leukemia, a cancer of the blood. But the National Down Syndrome Society says many of these cases are curable as well.

As a result, people with Down syndrome are living longer. In the early nineteen eighties they lived an average of just twenty-five years. Today the life expectancy for someone with Down syndrome is sixty years.

But with that longer life, people with Down syndrome may have an increased risk for Alzheimer's disease at an early age. An estimated twenty-five percent of those thirty-five and older show signs of the brain-wasting disease. It slowly destroys memory, thinking and reasoning skills. Alzheimer's is usually not found in the general population until people are over the age of sixty-five.

Down syndrome is the most common genetic cause of mental retardation. Most people with Down syndrome are mildly to moderately retarded. Many, however, are able to attend regular classes with other students. And later, as adults, many are able to hold jobs and lead independent lives.

There are tests that can be done to look for Down syndrome during pregnancy.

The risk of having a baby with Down syndrome increases with the mother's age. The rate is one in every one thousand two hundred births at age twenty-five. At thirty-five it rises to one in three hundred fifty births. And at forty-five the rate is one in thirty.

And that's the VOA Special English Health Report, written by Caty Weaver. For more health news, go to voaspecialenglish.com. I'm Steve Ember.