Aneuploidy

Aneuploidy is a condition where the nucleus of a cell has one or a few chromosomes more or less than the usual number in the species. It is a common cause of genetic disorders. Aneuploidy occurs during cell division when the chromosomes do not separate properly between the two cells. 
Aneuploidy often causes birth defects and miscarriages. Birth defects make up about 3% of births in the United States. Among those who survive birth, Down syndrome is the most common form of aneuploidy. Many with Down syndrome do survive to adulthood. Some cancer cells also have abnormal numbers of chromosomes. The aneuploid theory of cancer suggests that aneuploidy is a cause of cancer rather than a result of it.