"Down syndrome, also known as Down's syndrome and trisomy 21, is a condition that results from having an extra copy of the 21st chromosome. When the sex cells of the sperm or egg duplicate themselves, an uneven number of the normal 23 chromosomes may be transferred to the daughter cells, a process called non-disjunction. This ultimately results in an extra copy of chromosome 21 in the sperm or egg, and when combined with the opposite sex cell, it results in three copies of the chromosome."