土豆日報

Chromosome 21 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 21 is the smallest human chromosome, spanning almost 47 million base pairs (the building material of DNA) and representing about 1.5 percent of the total DNA in cells.

I don't think it is possible to answer, in general, what does Human Chromosome 21 do. Despite its small size (it is the smallest human chromosome), it is still estimated to have 200-400 genes, and each gene can possibly serve a function.

I can tell you some conditions are caused by changes in the structure or number of copies of chromosome 21 though:

cancers
Rearrangements (translocations) of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer. For example, acute lymphoblastic leukemia (a type of blood cancer most often diagnosed in childhood) has been associated with a translocation between chromosomes 12 and 21. Another form of leukemia, acute myeloid leukemia, has been associated with a translocation between chromosomes 8 and 21.

Down syndrome
Down syndrome is most often caused by trisomy 21. Trisomy 21 means that each cell in the body has three copies of chromosome 21 instead of the usual two copies. Down syndrome can also result from an extra copy of chromosome 21 in only some of the body's cells (mosaic Down syndrome).

other chromosomal conditions
Other changes in the number or structure of chromosome 21 can have a variety of effects, including mental retardation, delayed development, and characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome. Changes to chromosome 21 include a missing segment of the chromosome in each cell (partial monosomy 21) and a circular structure called ring chromosome 21. A ring chromosome occurs when both ends of a broken chromosome are reunited.

參考資料
http://ghr.nlm.nih.gov/chromosome=21