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What is Down syndrome?

Down syndrome is a genetic condition that causes delays in physical and intellectual development resulting from the presence of an extra chromosome – individuals with Down syndrome have 47 instead of the usual 46.

The presence of the additional genetic material alters development and causes characteristics that are associated with Down syndrome – including low muscle tone, small stature, an upward slant to the eyes and a single deep crease across the center of the palm.

How common is Down syndrome?

Down syndrome occurs in approximately one in every 691 live births, making it the most frequently occurring chromosomal disorder. There are about 400,000 Americans diagnosed with Down syndrome and approximately 6,000 babies are born with Down syndrome each year in the United States

What are the different types of Down syndrome?

There are three types of Down syndrome: trisomy 21 (nondisjunction), translocation and mosaicism.

Trisomy 21 (nondisjunction) accounts for 95% of known cases of Down syndrome. This type of Down syndrome results in an embryo with three copies of chromosome 21 instead of two and happens when a pair of 21st chromosomes in either the sperm or the egg fails to separate before or during conception. This causes the extra chromosome to be replicated in every cell of the body.

Translocation accounts for 4% of all cases of Down syndrome. In this occurrence, part of chromosome 21 breaks off during cell division and attaches to another chromosome – typically chromosome 14. The total number of chromosomes remains at 26, but the extra part of 21 causes characteristics of Down syndrome to appear.

Mosaicism occurs when nondisjunction of chromosome 21 takes place in only one of the initial cell divisions instead of all. This causes a mixture of two types of cells, some containing 46 chromosomes while others contain 47. Mosaicism accounts for only 1% of all cases of Down syndrome and research has indicated that individuals with mosaic Down syndrome may have fewer characteristics than those with the other types present.