| Image = Trisomy Detection in GeneMarker.jpg
| Caption = Example of Trisomy 21 detected via qPCR Short Tandem Repeat assay
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}}A trisomy is a form of aneuploidy with the presence of three copies, instead of the normal two, of a particular chromosome. 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 chr