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Mosaic trisomy 2
- ... form of Down syndrome is called trisomy 21. "Trisomy" means having three copies of a chromosome in your cells instead of the usual two. With less common types of Down syndrome, a person may have: An extra piece of chromosome 21 attached to another chromosome (translocation Down syndrome) An extra chromosome 21 in some, but not all of their cells (mosaic Down syndrome) The extra chromosome affects the way ...