Very rare. It is a very rare chromosomal disorder where there is an extra copy of chromosome 13 in some of the body's cells while other cells are normal.
Chromosomal anomaly. A small percentage of people with trisomy 13 have an extra copy of chromosome 13 in only some of the body's cells. In these people, the condition is called mosaic trisomy 13. The severity of mosaic trisomy 13 depends on the type and number of cells that have the extra chromosome. The physical features of mosaic trisomy 13 are often milder than those of full trisomy 13.