|A service of the U.S. National Library of Medicine®|
Reviewed November 2013
What is chromosome 21?
Humans normally have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 21, one copy inherited from each parent, form one of the pairs. Chromosome 21 is the smallest human chromosome, spanning about 48 million base pairs (the building blocks of DNA) and representing 1.5 to 2 percent of the total DNA in cells.
In 2000, researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced.
Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. Chromosome 21 likely contains 200 to 300 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.
Genes on chromosome 21 are among the estimated 20,000 to 25,000 total genes in the human genome.
How are changes in chromosome 21 related to health conditions?
Many genetic conditions are related to changes in particular genes on chromosome 21. This list of disorders associated with genes on chromosome 21 provides links to additional information.
Changes in the structure or number of copies of a chromosome can also cause problems with health and development. The following chromosomal conditions are associated with such changes in chromosome 21.
Is there a standard way to diagram chromosome 21?
Geneticists use diagrams called ideograms as a standard representation for chromosomes. Ideograms show a chromosome's relative size and its banding pattern. A banding pattern is the characteristic pattern of dark and light bands that appears when a chromosome is stained with a chemical solution and then viewed under a microscope. These bands are used to describe the location of genes on each chromosome.
See How do geneticists indicate the location of a gene? in the Handbook.
Where can I find additional information about chromosome 21?
You may find the following resources about chromosome 21 helpful. These materials are written for the general public.
You may also be interested in these resources, which are designed for genetics professionals and researchers.
Where can I find general information about chromosomes?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding chromosome 21?
acute ; acute lymphoblastic leukemia ; acute myeloid leukemia ; AML ; cancer ; cell ; chromosome ; differentiation ; DNA ; gene ; genome ; human genome ; human genome project ; hypotonia ; leukemia ; monosomy ; mosaic ; muscle tone ; mutation ; myeloid ; protein ; rearrangement ; reproductive cells ; somatic mutation ; sperm ; syndrome ; translocation ; trisomy ; white blood cells
You may find definitions for these and many other terms in the Genetics Home Reference Glossary.
See also Understanding Medical Terminology.
References (16 links)
The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.