Trisomy
Trisomy is where there is an extra of a particular chromosome due to nondisjunction during meiosis. Trisomy often leads to miscarriage although there are a number of chromosomal abnormalities in which the foetus survives, mainly 13, 18, 21, X and Y. The chance of trisomy occuring increases with the age of the mother.
Main non-lethal trisomies in humans:
18- Edward's Syndrome
X- Triplo x or Trisomy X
Y- Klinefelter
Occassionally you can also get a tetrasomy in which there are four copies of a chromosome insteasd of two.
http://downsyndrome.about.com/od/whatcausesdownsyndrome/a/othtrisomies_ro.htm