Trisomy

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A Trisomy  is a type of polysomy inwhich instead of the usual two chromosomes, there are three copies of a particular chromosome. A trisomy is a type of aneuploidy (an abnormal number of chromosomes). 

The majority of human trisomies result in miscarriage, however there are a number of types that survive to birth. These include; Trisomy 13 (Patau Syndrome), Trisomy 18 (Edward's syndrome) and Trisomy 21 (Down Syndrome). Autosomal trisomy shows various levels of mental retardation and is associated with physical traits.

Trisomy of sex chromosomes also occurs. XXX (Triple X syndrome) and XXY (Klinefelter syndrome).

Trisomy is where there is an extra of a particular chromosome due to nondisjunction during meiosis. Trisomy often leads to miscarriage although there are a number of chromosomal abnormalities in which the foetus survives, mainly 13, 18, 21,  X and Y. The chance of trisomy occuring increases with the age of the mother.

Main non-lethal trisomies in humans:

Occassionally you can also get a tetrasomy in which there are four copies of a chromosome instead of two.

Trisomy, an additional copy of a chromosome, is caused by nondisjunction; two homologous chromosomes migrate to the same pole rather than to the opposite pole. This can occur in Meiosis I and Meiosis II, althought it is much more common in Meiosis I (70%) compared to Meiosis II (30%). Depending on what the additional chromosome is, the conditions will vary. For example, Trisomy 21 will result in Down Syndrome whereas Trisomy 18 will cause Edwards Syndrome. Trisomys generally occur prior to fertilastation (prezygotic event), however they can happen after fertilisation (postzygotic event) during mitosis. If the nondisjunction happens as a posyzygotic event, not all of the somatic cells will contain the additional chromosome and this is condition is called a chromosomal mosiacism[2].

In recent years, there has been incredibly interesting research into the possibility of inserting an inducible Xist transgene into pluripotent down syndrome embryonic stem cells. This gene can then be induced to create a chromosome 21 Barr body in order to compensate for the extra chromosome 21 genes[3][4].

References

  1. http://downsyndrome.about.com/od/whatcausesdownsyndrome/a/othtrisomies_ro.htm
  2. Miller, O & Therman, E (2001) Human Chromosomes. 4th Edn. Springer - Verlag: New York. Page 157 Passarge, E (2007) Color Atlas of Genetics. 3rd Edn. Thieme: New York. Page 412
  3. Jiang J, Jing Y, Cost G, Chiang J C, Kolpa H J, Cotton A M, Carone D M, Carone B R, Shivak D A, Guschin D Y, Pearl J R, Rebar E J, Byron M, Gregory P D, Brown C J, Urnov F D, Hall L L, Lawrence J B. Translating dosage compensation to trisomy 21. Nature. 2013 August 15. 500: 296-303
  4. Jiang J, Jing Y, Cost G, Chiang J C, Kolpa H J, Cotton A M, Carone D M, Carone B R, Shivak D A, Guschin D Y, Pearl J R, Rebar E J, Byron M, Gregory P D, Brown C J, Urnov F D, Hall L L, Lawrence J B. Translating dosage compensation to trisomy 21. Nature. 2013 August 15. 500: 296-303
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