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Down Syndrome

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Also known as: Trisomy 21; DS

What is Down syndrome?

Down syndrome (DS) is a group of signs, symptoms, birth defects, and complications that arise from an error in cell division that results in an extra 21st chromosome. This error may occur before, or shortly after, conception and has a widespread effect on the physical and mental development of the affected person. DS is one of the most common and well known chromosome disorders and the most common genetic cause of learning disabilities. It is a congenital condition caused by an extra copy or piece of chromosome 21 in all or most of the affected person's cells.

Chromosomes hold the body's genetic blueprint. Most cells in the body contain 22 pairs of chromosomes and a 23rd set of either XX (in females) or XY (in males) for a total of 46 chromosomes. The 46 chromosomes come together when the woman's egg, containing a single set of 23 chromosomes, is fertilized by a single sperm, containing a single set of 23 chromosomes. The fertilized egg will then divide repeatedly as it grows and develops into a fetus.

In most cases of Down syndrome, random chance leads to the insertion of an extra copy of chromosome 21 in either the egg or sperm. This extra copy becomes part of the fertilized egg and is replicated in all of the embryo's cells. This form of Down syndrome is called trisomy 21, and it accounts for about 95% of DS cases.

Less commonly, an error may occur after conception, in the developing embryo. As the fetus grows, some cells have 47 chromosomes, while others have 46. This form of Down syndrome is called mosaic trisomy 21.

In a rare form of Down syndrome called translocation trisomy 21, a piece of chromosome 21 adheres to another chromosome before or at conception. Even though the fetus has 46 chromosomes, it still has an extra portion of chromosome 21 in its cells.

All individuals with additional chromosome 21 genetic material, regardless of the cause, will develop some of the features of Down syndrome.

In the United States, about 6,000 babies each year are born with Down syndrome. The risk of having an affected baby increases significantly as a woman ages. According to the National Institute of Child Health & Human Development, the risk increases from about 1 in 1,300 for women who are 25 years old to about 1 in 55 for women who are 42 and 1 in 25 for those who are 49. However, since younger women have the greatest number of babies, the majority of those with Down syndrome are born to women under 35.

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