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In page Trisomy:

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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] Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.