Definition:
A trisomy is a chromosome abnormality in which a person has three copies of a particular chromosome rather than the expected two.
Trisomies can have varying effects on a person's health, depending on which chromosome is affected by the trisomy. Some trisomies, such as trisomy X, have only mild effects on a person, whereas others such as trisomy 21 (also known as Down syndrome) usually cause health problems and developmental delays. Many trisomies can be fatal, such as trisomy 16 or trisomy 18, and frequently cause a miscarriage or loss of the baby shortly after birth.
Doctors believe that trisomies are caused by random errors in cell division.
