Trisomy is the presence of three chromosomes in the cells of a living organism where two are normal. Trisomies result from the incorrect duplication of egg and sperm cells, where the donor cell contains two of the same chromosome.

In humans, trisomies are the cause of several genetic diseases, most notably Trisomy-21 or Down syndrome. Three trisomies of the sex chromosomes are known, including Klinefelter syndrome.

Trisomy at Wikipedia