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Downs Syndrome

Down Syndrome encompasses a number of chromosomal differences, of which trisomy 21 (an aneuploid) is the most common, causing highly variable degrees of learning difficulties as well as physical disabilities. It is named for John Langdon Down, the British doctor who first described it in 1866. While Down Syndrome is the medically recognized term in the US, some support groups and organizations use Down's Syndrome. more...

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Down Syndrome is a naturally occurring chromosomal irregularity. The sole characteristic shared by all persons with Down Syndrome is the presence of extra genetic material associated with the 21st chromosome. The effects of that extra genetic material varies greatly from individual to individual 5, depending on the extent of the extra material, genetic background, environmental factors, and random chance.

The incidence of Down Syndrome is estimated at 1 per 800 births, making it the most common human aneuploid. The maternal age effect influences the chance of conceiving a baby with the syndrome. At age 20 to 24, it is 1/1490, while at age 40 it is 1/106, and at age 49 is 1/11. (Hook EB., 1981). Genetic counseling and genetic testing such as amniocentesis are usually offered to families who may have an increased chance of having a child with Down Syndrome. Many children with Down Syndrome are born to women under the age of 35, mainly because that is the prime reproductive ages for women.

The term 'Down Syndrome' was first used in 1961 by the editor of The Lancet 2. It was originally called mongolism or mongolian idiocy, after a perceived resemblance observed by John Langdon Down between the faces of some of his patients with Down Syndrome and the Mongoloid race. This usage is now viewed by medical professionals as offensive and medically meaningless, and is not commonly used today. Professor Jérome Lejeune proved in 1959 that Down Syndrome is a chromosomal irregularity.

While most children with Down Syndrome have a lower than average cognitive function, some have earned college degrees with accommodations, and nearly all will learn to read, write and do simple math. The common clinical features of Down Syndrome include any of a number of features that also appear in people with a standard set of chromosomes. They include a "simian crease" - a single crease across one or both palms, almond shaped eyes, shorter limbs, heart and/or gastroesophageal defects, speech impairment, and perhaps a higher than average risk of incidence of Hirschsprung's disease. Young children with Down Syndrome are also more prone to recurrent ear infections and obstructive sleep apnea.

Early educational intervention, screening for common problems such as thyroid functioning, medical treatment where indicated, a conducive family environment, vocational training, etc., can improve the overall development of children with Down Syndrome. On the one hand, Down Syndrome shows that some genetic limitations can not be overcome; on the other, it shows that education can produce excellent progress whatever the starting point. The commitment of parents, teachers and therapists, to individual children, has produced previously unexpected positive results.


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New clues to origins of Downs' syndrome - older women have a greater risk of giving birth to a Downs' child because they are more likely to carry them
From Science News, 5/11/91 by Carol Ezzell

Older mothers are known to face a greater risk than younger women of giving birth to a baby with Down's syndrome, a genetic disorder characterized by mental retardation. But researchers have not been able to explain why. A new study of 158 families with children affected by Down's now supports one contending theory -- that older women are more likely to carry a Down's baby to term.

Human geneticist Stylianos E. Antonarakis of Johns Hopkins University in Baltimore headed an international team of 18 other researchers that determined the extra chromosome responsible for Down's is incorporated more often during the first of the human egg's two stages of meiotic development. Both younger and older women were equally likely to have a chromosome error in the first stage, regardless of their age, the researchers reported last week at a meeting of the American Pediatric Society and Society for Pediatric Research in New Orleans.

The team used DNA tags to trace the origin of the extra chromosome-21 in the Down's children. They found, as expected from their previous studies, that 95 percent of the extra chromosomes came from the mother and only 5 percent from the father. When they traced further, they found that three-fourths of all maternal chromosome errors resulted from the first stage of meiosis, the cell division process that halves the genetic content of eggs and sperm so that children don't get double the proper number of genes.

Although the results are difficult to interpret, Antonarakis says, "what was surprising was that we didn't see a difference in maternal age according to the stage of meiosis where the chromosome error occurs." If mothers of all ages have similar chromosome errors leading to Down's, he reasons, older mothers might have more Down's babies because their bodies fail to recognize an egg with an extra chromosome as abnormal. In other words, "It's possible that older mothers could carry a Down's baby to birth better," he told SCIENCE NEWS.

But Antonarakis cautions that his group did not study tissue from fetuses spontaneously aborted by the women, to see if younger mothers were more likely to miscarry a Down's child than older mothers. "It's possible that in aborted fetuses there is a stronger correlation between maternal age and the stage of meiosis leading to Down's," he says.

David H. Ledbetter, a geneticist from the Baylor College of Medicine in Houston, agrees that further studies will be needed to confirm that younger women are more likely to miscarry Down's babies. "The only way to directly test it would be to compare the ages of women who have children with Down's syndrome and those who spontaneously abort Down's syndrome fetuses," he says.

COPYRIGHT 1991 Science Service, Inc.
COPYRIGHT 2004 Gale Group

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