What Causes Down Syndrome?

Down syndrome occurs because of an abnormality characterized by an extra copy of genetic material on all or part of the 21st chromosome. Every cell in the body contains genes that are grouped along chromosomes in the cell’s nucleus or center. There are normally 46 chromosomes in each cell, 23 inherited from your mother and 23 from your father. When some or all of a person’s cells have an extra full or partial copy of chromosome 21, the result is Down syndrome.

The most common form of Down syndrome is known as Trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46. This is caused by an error in cell division called nondisjunction, which leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception. Trisomy 21 accounts for 95% of Down syndrome cases, with 88% originating from nondisjunction of the mother’s egg cell.

The remaining 5% of Down syndrome cases are due to conditions called mosaicism and translocation. Mosaic Down syndrome results when some cells in the body are normal while others have Trisomy 21. Robertsonian translocation occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome (usually chromosome 14). The presence of this extra part of chromosome 21 causes Down syndrome characteristics. Although a person with a translocation may appear physically normal, he or she has a greater risk of producing a child with an extra 21st chromosome.


Chromosomal Changes That Can Cause Down Syndrome

Research shows that three types of chromosomal changes can lead to Down syndrome.

  1. Complete trisomy 21
    • In this case, an error during the formation of the egg or the sperm results in either one having an extra chromosome. So after the egg and sperm unite, the resulting cells will also have three copies of chromosome 21. The complete extra copy of chromosome 21 is in all of the person’s cells-or a complete trisomy. Complete trisomy 21 is the cause of about 95% of Down syndrome cases.
  2. Mosaic trisomy 21
    • Not every cell in the body is exactly the same. In about 1% of Down syndrome cases, most of the cells in the body have the extra chromosome, but some of them don’t. This is called “mosaicism”. Mosaic trisomy 21 can occur when the error in cell division takes place early in development but after a normal egg and sperm unite. It can also occur early in development when some cells lose an extra chromosome 21 that was present at conception. The symptoms of someone with mosaic trisomy 21, may vary from those of someone with complete trisomy 21 or translocation trisomy 21, depending on how many cells have the extra chromosome.
  3. Translocation trisomy 21
    • In this type of chromosomal change, only part of an extra copy of chromosome 21 is in the cells. The extra part of the chromosome gets “stuck” to another chromosome and gets transmitted into other cells as the cells divide. This type of change causes about 4% of Down syndrome cases. There are no distinct cognitive or medical differences between people with translocation trisomy 21 and those with complete trisomy 21.

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