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:

13- Patau Syndrome

18- Edward's Syndrome

21- Down'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