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MeshID = D014314A trisomy is a form of aneuploidy with the presence of three copies, instead of the normal two, of a particular chromosome.cite web |url= |title=CRC - Glossary T |accessdate=2007-12-23 |format= |work=] The presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.



Full trisomy of an individual occurs due to non-disjunction when the cells are dividing (meiosis I or II) to form egg and sperm cells (gametogenesis). This can result in an extra or missing chromosome (either 24 or 22 chromosomes instead of the typical 23) in a sperm or egg cell. After fertilization, the resulting fetus has 47 chromosomes instead of the typical 46.


A partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes, or if one of the chromosomes has two copies of part of its chromosome.

A mosaic trisomy is a condition where extra chromosomal material exists in only some of the organism's cells.

Human trisomy

Trisomies can occur with any chromosome, but often result in miscarriage. For example, Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester. The most common types of trisomy that survive to birth in humans are:

Autosomal trisomies

*Trisomy 21 (Down syndrome)
*Trisomy 18 (Edwards syndrome)
*Trisomy 13 (Patau syndrome)
*Trisomy 12 (A prognostic indicator of Chronic Lymphocytic Leukemia)
*Trisomy 9
*Trisomy 8 (Warkany syndrome 2)

The most common forms of autosomal trisomy are trisomy of chromosome 21 which results in Down syndrome and trisomy of chromosome 18 which results in Edwards syndrome. In rare cases, a fetus with trisomy of chromosome 13 can survive, giving rise to Patau syndrome. Autosomal trisomy can be associated with birth defects, mental retardation and shortened life expectancy.

Note the chart below is from a book that is dated 1984.... live births for Trisomy 17 is certainly not 0%, as one healthy child born in 2002 in Georgia has his rare genetic disorder, and at least five infants in the US and UK have been since diagnosed with the same and are thriving.

ex-chromosome trisomies

Trisomy of sex chromosomes can also occur:

*XXX (Triple X syndrome)
*XXY (Klinefelter syndrome)
*XYY (XYY syndrome)


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Look at other dictionaries:

  • Trisomy 9 — is a chromosomal disorder caused by having three copies (trisomy) of chromosome number 9. It can appear with or without mosaicism. ymptomsSymptoms vary, but usually result in dysmorphisms in the skull, nervous system, and mental retardation.… …   Wikipedia

  • Trisomy 16 — is a chromosomal abnormality in which there are three copies of chromosome 16 rather than two. [cite web|author=Mary Kugler, R.N. |date=2005 08 20 |url= |title=Chromosome 16 Disorders …   Wikipedia

  • trisomy 13 — n. The condition of having three copies of chromosome 13 that results in a syndrome characterized by mental retardation and defects to the central nervous system and heart. * * * also called  Patau s Syndrome,        human chromosomal disorder… …   Universalium

  • trisomy 13 — thər(t) tēn n a congenital condition that is characterized esp. by usu. severe mental retardation and by craniofacial, cardiac, ocular, and cerebral abnormalities, is caused by trisomy of the human chromosome numbered 13, and is typically fatal… …   Medical dictionary

  • trisomy 18 — ā(t) tēn n a congenital condition that is characterized esp. by mental retardation and by craniofacial, cardiac, gastrointestinal, and genitourinary abnormalities, is caused by trisomy of the human chromosome numbered 18, and is typically fatal… …   Medical dictionary

  • trisomy — trisomy. См. трисомия. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • trisomy 21 — n. See Down syndrome. * * * …   Universalium

  • trisomy 21 — twent ē wən n DOWN SYNDROME …   Medical dictionary

  • trisomy — 1930, from trisomic (1921), from TRI (Cf. tri ) + ending from CHROMOSOME (Cf. chromosome) …   Etymology dictionary

  • trisomy-21 — Down syndrome Down syn drome, Down s syndrome Down s syn drome, n. (Med.) A congenital disorder caused by an extra chromosome 21 in some or all cells. It causes a variable number and degree of abnormalities, including retarded growth, mental… …   The Collaborative International Dictionary of English

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