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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 have 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 chromosme has been copied. "Partial trisomy" means that there is an extra copy of part of a chromosome.[How to reference and link to summary or text]
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.[How to reference and link to summary or text] Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.