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Genetic and Rare Diseases Information Center (GARD)

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Chromosome 20 trisomy

Other Names for this Disease
  • Mosaic trisomy 20
  • Trisomy 20
  • Trisomy 20 mosaicism
  • Trisomy chromosome 20
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What causes chromosome 20, trisomy?

Chromosomal abnormalities usually result from an error that occurs when an egg or sperm cell develops. It is not always known why these errors occur, but it is thought that nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in a fetus or child. Egg and sperm cells should each contain 23 chromosomes. When they join together, they form a fertilized egg with 46 chromosomes. Sometimes, something goes wrong before fertilization. An egg or sperm cell may divide incorrectly, resulting in an egg or sperm cell with too many or too few chromosomes. When this cell with the wrong number of chromosomes joins with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality. If an egg or sperm cell has 24 chromosomes because either has an extra copy of chromosome 20, this can cause trisomy 20 (the resulting embryo will have 3 copies of chromsome 20). In most cases, an embryo with 3 full copies of chromosome 20 in all cells does not survive and the pregnant woman has a miscarriage, often very early in pregnancy.

Other types of errors can alter the structure of one or more chromosomes. Individuals with structural chromosomal abnormalities usually have the normal number of chromosomes. However, small pieces of a chromosome (or chromosomes) may be duplicated, and in the case of part of chromosome 20 being duplicated, it would be called partial trisomy 20 (because there are 3 copies of only part of chromosome 20 in each cell).

Errors in cell division involving chromosome 20 can also occur soon after fertilization, which can cause mosaicism, a condition in which an individual has cells with different genetic makeups. Individuals with the mosaic form of trisomy 20 have an extra chromosome 20 in some, but not all, of their cells. Some individuals with chromosomal mosaicism may be mildly affected or apparently not affected at all, but the severity of the condition may depends on the number of abnormal cells that are present. Other individuals may be severly affected.[1]
Last updated: 12/17/2010

  1. Chromosome abnormalities. March of Dimes. December 2009; Accessed 12/17/2010.