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People with the disorder are living longer and more productive lives than in the past. But they may also face a higher risk of Alzheimer’s disease at an early age.

This is the VOA Special English Health Report.

Human genes are normally organized along 46 chromosomes in our cells, 23 from each parent.

Understanding Down syndrome

But some people are born with an extra copy of the 21 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 1860s. An estimated 350 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 100 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 1980s they lived an average of just 25 years. Today the life expectancy for someone with Down syndrome is 60 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 25 percent of those 35 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 65.

Down syndrome is the most common genetic cause of mental retardation. Many 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 1200 births at age 25. At 35 it rises to one in 350 births. And at 45 the rate is one in 30.

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

chromosome: a circular strand of DNA in bacteria that contains the hereditary information necessary for cell life 染色體

leukemia: a malignancy of blood-producing tissues, characterized by proliferating immature white blood cells and infiltration of the spleen, liver, and other organs 白血病