Trisomy 13 (also called Patau syndrome) is a genetic disorder in which a person has three copies of genetic material from chromosome 13, instead of the usual two copies. Rarely, the extra material may be attached to another chromosome (translocation).

Trisomy 13 occurs when extra DNA from chromosome 13 appears in some or all of the body's cells.

• Trisomy 13 -- the presence of an extra (third) chromosome 13 in all of the cells.
• Trisomy 13 mosaicism -- the presence of an extra chromosome 13 in some of the cells.
• Partial trisomy -- the presence of a part of an extra chromosome 13 in the cells.

The extra material interferes with normal development.

Trisomy 13 occurs in about 1 out of every 10,000 newborns. Most cases are not passed down through families (inherited). Instead, the events that lead to Trisomy 13 occur in either the sperm or the egg that forms the fetus.