Humans have 23 pairs of chromosomes (including sex chromosomes). The twenty first pair of chromosomes is described as chromosome 21, this is the smallest human chromosome with 48 million nucleotides and representing 1.5 to 2 percent of the total DNA in cell. During DNA replication, if the spindle fibres fail to separate the chromosomes apart equally, daughter will result in either monosomic or trisomic. Trisomy 21, a condition in which the karyotype includes three copies of chromosome 21, is the cause of Down Syndrome.
It is reported that 95% of people with Down Syndrome have a trisomy of Chromosome 21 as reported on the Centres for Disease and Control Website - http://www.cdc.gov/ncbddd/birthdefects/DownSyndrome.html