Editing Trisomy (section)

==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.