Chromosome 21 (human)

In the human genome, chromosome 21 is the smallest of the 23 chromosomes. It is an acrocentric chromosome with a satellite, found within group G of the human karyotype. As with all autosomes, somatic cells carry two copies of chromosome 21, while gametes carry only one copy. In individuals who have 3 copies of chromosome 21 (trisomy 21), Down's Syndrome results.

Chromosome 21 is approximately 47 Mbp (million base pairs) in length, comprising ~1.5% of the haploid genome
Chromosome 21 has approximately 300-400 genes

Significant genes (and some related genetic diseases) on chromosome 21 include: