failure of meiosis to seperate cell division (nondisiunction) this is called aneuploidy and either more or fewer than 46 chromosomes is present. there are types
- 1. (trisomy) a disorder in which a normally diploid organism has an extra copy of one of the chromosomes
- 2. (monosomy) in which only one member of a pair of chromosomes is present
- 3. (polysomy), in which one chromosomes is represented four or more times