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Genetic and Rare Diseases Information Center (GARD)

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Down syndrome


* Not a rare disease
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Other Names for this Disease
  • Down's syndrome
  • Trisomy 21
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What causes Down syndrome?

Three genetic variations can cause Down syndrome. 

1) Trisomy 21. Most often, Down syndrome is caused by an extra chromosome 21 in all cells of the individual.[1][2]  In these cases, a pair of number 21 chromosomes fails to separate during the formation of an egg (or sperm); this is referred to as "nondisjunction". When that egg unites with a normal sperm to form an embryo, the embryo ends up with three copies of chromosome 21 instead of the normal two.[1][2] The extra chromosome is then copied in every cell of the baby's body.[3]

The cause of nondisjunction is unknown, but research has shown that it increases in frequency as a woman ages.  However, due to higher birth rates of younger women, 80% of children with Down syndrome are born to women under 35.  Nondisjunction is not known to be caused by environmental factors or anything the parents did or did not do before or during the pregnancy.[2]
2) Mosaic Trisomy 21. For some individuals, approximately 1-2% of cases, only some of the cells have an extra chromosome 21; this is called "mosaic trisomy 21".[1][4][2]  In this situation, the fertilized egg may have the right number of chromosomes, but due to a cell division error early in the development of the embryo, some cells acquire an extra chromosome 21.  Thus, an individual with mosaic trisomy 21 will typically have 46 chromosomes in some cells and 47 chromosomes (including an extra chromosome 21) in others.  The signs and symptoms for individuals with mosaic trisomy 21 may have a wider range of variability.[2]

3) Translocation Trisomy 21. Approximately 3-4% of individuals with Down syndrome have cells that contain 46 chromosomes; however, there is extra chromosome 21 material attached  (translocated ) onto another chromosome  - this is called "translocation trisomy 21".[1][2]  For parents of a child with Down syndrome due to a translocation, there may be an increased chance of Down syndrome in future pregnancies. This is because one of the two parents may be a balanced carrier of the translocated chromosome. However, it important to realize that not all parents of individuals with translocation trisomy 21 are balanced carriers.[1]

Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. This extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome.[1][2]  
Last updated: 12/22/2011

  1. Down syndrome. Genetics Home Reference (GHR). 2008; Accessed 3/9/2012.
  2. What Causes Down Syndrome?. National Down Syndrome Society. 2011; Accessed 12/22/2011.
  3. Down Syndrome. Genetic Science Learning Center. 2011; Accessed 12/22/2011.
  4. Down syndrome. March of Dimes. 2009; Accessed 3/9/2012.