The presence of an extra copy of a chromosome, giving a total of 47 chromosomes rather than the usual 46. In addition to the usual two copies of a particular chromosome, there is an extra copy.
The most common example of this is Trisomy 21 (Down syndrome), where there is an additional copy of chromosome 21.
Trisomies are the result of an error during cell division (non-disjunction) which causes the incorrect number of chromosomes to be passed into a cell.
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