Trisomys

Trisomy, an additional copy of a chromosome, is caused by nondisjunction; two homologous chromosomes migrate to the same pole rather than to the opposite pole.  This can occur in Meiosis I and Meiosis II, althought it is much more common in Meiosis I (70%) compared to Meiosis II (30%).  Depending on what the additional chromosome is, the conditions will vary.  For example, Trisomy 21 will result in Down Syndrome whereas Trisomy 18 will cause Edwards Syndrome.  Trisomys generally occur prior to fertilastation (prezygotic event), however they can happen after fertilisation (postzygotic event) during mitosis.  If the nondisjunction happens as a posyzygotic event, not all of the somatic cells will contain the additional chromosome and this is condition is called a chromosomal mosiacism.  

Miller, O & Therman, E (2001) Human Chromosomes. 4th Edn. Springer - Verlag: New York.  Page 157

Passarge, E (2007) Color Atlas of Genetics. 3rd Edn.  Thieme: New York. Page 412