||This article needs additional citations for verification. (August 2012)|
|Classification and external resources|
Example of Trisomy 21 detected via qPCR Short Tandem Repeat assay
Description and causes 
Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Human beings have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 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. (See non-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.
"Full trisomy" means that an entire extra chromosome has been copied. "Partial trisomy" means that there is an extra copy of part of a chromosome. 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. Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.
Human trisomy 
Trisomies can occur with any chromosome, but often result in miscarriage. For example, Trisomy 16 is the most common trisomy in human pregnancies, occurring in more than 1% of pregnancies; only those where there are a number of cells with a normal complement of chromosomes called mosaic trisomy 16 survive. This condition, however, usually results in spontaneous miscarriage in the first trimester.
The most common types of autosomal trisomy that survive to birth in humans are:
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Trisomy 9
- Trisomy 8 (Warkany syndrome 2)
- Trisomy 22
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, mental retardation and shortened life expectancy.
See also 
- "CRC - Glossary T". Retrieved 2007-12-23.
- Hassold, T; Merrill, M; Adkins, K; Freeman, S; Sherman, S (1995). "Recombination and maternal age-dependent nondisjunction: molecular studies of trisomy 16". American Journal of Human Genetics 57 (4): 867–74. PMC 1801507. PMID 7573048.
- Disputable term: https://en.wikipedia.org/wiki/Talk:Trisomy#Trisomy_of_.2Asex.2A_chromosomes.3F