Editing Trisomy

{{Short description|Abnormal presence of three copies of a particular chromosome}}
{{more citations needed|date=August 2012}}
{{Infobox medical condition (new)
| name            = Trisomy
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| image           = Trisomy Detection in GeneMarker.jpg
| caption         = Example of trisomy 21 ([[Down syndrome]]) detected via [[qPCR]] [[STR analysis|short tandem repeat analysis]]
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| field           = [[Medical genetics]]
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A '''trisomy''' is a type of [[polysomy]] in which there are three instances of a particular [[chromosome]], instead of the normal two.<ref name="titleCRC - Glossary T">{{cite web |url=http://cll.ucsd.edu/glossary/glossary_t.html |title=CRC - Glossary T |access-date=2007-12-23 |url-status=dead |archive-url=https://web.archive.org/web/20100616231910/http://cll.ucsd.edu/glossary/glossary_t.html |archive-date=2010-06-16 }}</ref> A trisomy is a type of [[aneuploidy]] (an abnormal number of chromosomes).

==Description and causes==

Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one of each type of chromosome inherited from each parent. In such organisms, [[meiosis]] creates gamete cells (eggs or sperm) having only one set of chromosomes. The number of chromosomes is different for different species, with humans having 46 chromosomes (23 pairs) and human gametes 23 chromosomes.

If the chromosome pairs fail to separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes ([[non-disjunction]]). If such a gamete is fertilized with a normal gamete, the resulting embryo may have a total of three copies of the chromosome.

===Terminology===
The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2''n''+1 if one chromosome shows trisomy, 2''n''+1+1 if two show trisomy, etc.<ref name=Rieger>{{cite book|author1=Rieger, R. |author2=Michaelis, A. |author3=Green, M.M. |year=1968|title=A glossary of genetics and cytogenetics: Classical and molecular|url=https://archive.org/details/glossaryofgeneti00rieg |url-access=registration |publisher=Springer-Verlag|location=New York|isbn=9780387076683}}</ref>
*"Full trisomy", also called "primary trisomy",<ref name=Rieger/> means that an entire extra chromosome has been copied.
*"Partial trisomy" means that there is an extra copy of part of a chromosome.
*"Secondary trisomy" - the extra chromosome has quadruplicated arms (the arms are identical; it is an "isochromosome").<ref name=Rieger/> 
*"Tertiary trisomy" - the extra chromosome is made up of copies of arms from two other chromosomes.<ref name=Rieger/>

Trisomies are sometimes characterised as "autosomal trisomies" (trisomies of the non-sex chromosomes) and "sex-chromosome trisomies."  Autosomal trisomies are described by referencing the specific chromosome that has an extra copy.{{Citation needed|date=March 2009}}  Thus, for example, the presence of an extra [[chromosome 21]], which is found in [[Down syndrome]], is called trisomy 21.

==Human trisomy==
[[File:Trisomie 21 Genom-Schema.gif|thumb|[[Karyotype]] of a human with Trisomy 21 (Down syndrome)]]
Trisomies can occur with any [[chromosome]], but often result in [[spontaneous abortion]] (miscarriage) rather than live birth. For example, [[Trisomy 16]] is most common in human pregnancies, occurring in more than 1%, but the only surviving embryos are those having some normal cells in addition to the trisomic cells ([[Mosaic (genetics)|mosaic]] trisomy 16).<ref name="T Hassold et al">{{cite journal |pmc=1801507 |title=Recombination and maternal age-dependent nondisjunction: molecular studies of trisomy 16 |year=1995 |volume=57 |issue=4 |pmid=7573048 |last1=Hassold |first1=T |last2=Merrill |first2=M |last3=Adkins |first3=K |last4=Freeman |first4=S |last5=Sherman |first5=S |pages=867–74 |journal=American Journal of Human Genetics}}</ref> Furthermore, even these embryos usually suffer [[miscarriage]] in the first trimester.

The most common types of human [[autosome|autosomal]] (non-sex chromosome) trisomy that survive to birth are:

*Trisomy 21 ([[Down syndrome]])
*Trisomy 18 ([[Edwards syndrome]])
*Trisomy 13 ([[Patau syndrome]])
*[[Trisomy 9]]
*[[Trisomy 8]] (Warkany syndrome 2)

Of these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with Trisomy 13 can survive, giving rise to [[Patau syndrome]]. Autosomal trisomy can be associated with birth defects, [[intellectual disability]] and shortened life.

''Trisomy of [[sex chromosomes]]'' can also occur and include:<ref name="O'Connor">{{cite journal | url=http://www.nature.com/scitable/topicpage/chromosomal-abnormalities-aneuploidies-290 | title=Chromosomal Abnormalities: Aneuploidies | author=O'Connor, Clare | journal=Nature Education | year=2008 | volume=1 | issue=1 | pages=172}}</ref>

*XXX ([[Triple X syndrome]])
*XXY ([[Klinefelter syndrome]])
*[[XYY syndrome|XYY]] (Jacobs syndrome)
Compared to trisomy of the autosomal chromosomes, trisomy of the sex chromosomes normally has less severe consequences. Individuals may show few or no symptoms and have a normal life expectancy.<ref name="O'Connor"/>

==See also==
*[[Chromosome abnormalities]]
*[[Aneuploidy]]
*[[Karyotype]]
*[[Sexual reproduction]]
*[[Monosomy]]

==References==
{{Reflist}}

{{Medical resources
| DiseasesDB      = 
| ICD10           = {{ICD10|Q|90||q|90}}-{{ICD10|Q|92||q|90}}<BR>{{ICD10|Q|97||q|90}}-{{ICD10|Q|98||q|90}}
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| MedlinePlus     = 
| eMedicineSubj   = 
| eMedicineTopic  = 
| MeshID          = D014314
| Orphanet        = 3376
| SNOMED CT       = 78989007
}}
{{Chromosomal abnormalities}}
{{Authority control}}

[[Category:Cytogenetics]]
[[Category:Chromosomal abnormalities]]