Tisomy

A Trisomy  is a type of polysomy inwhich instead of the usual two chromosomes, there are three copies of a particular chromosome. A trisomy is a type of aneuploidy (an abnormal number of chromosomes). 

The majority of human trisomies result in miscarriage, however there are a number of types that survive to birth. These include; Trisomy 13 (Patau Syndrome), Trisomy 18 (Edward's syndrome) and Trisomy 21 (Down Syndrome). Autosomal trisomy shows various levels of mental retardation and is associated with physical traits.

Trisomy of sex chromosomes also occurs. XXX (Triple X syndrome) and XXY (Klinefelter syndrome).