NCI Dictionary of Genetics Terms

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Welcome to the NCI Dictionary of Genetics Terms, which contains technical definitions for more than 150 terms related to genetics. These definitions were developed by the PDQ® Cancer Genetics Editorial Board to support the evidence-based, peer-reviewed PDQ cancer genetics information summaries.

trisomy
(TRY-soh-mee)
The presence of an extra chromosome, resulting in a total of three copies of that chromosome instead of the normal 2 copies (e.g., trisomy 21, or Down syndrome).