Chromosome 21

Humans have 23 pairs of chromosomes (including sex chromosomes). The twenty first pair of chromosomes is described as chromosome 21. 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.