Reviewed June 2012
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.
Genetics Home Reference includes these genes on chromosome 21:
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.
Genetics Home Reference includes these conditions related to genes on chromosome 21:
- Alzheimer disease
- amyotrophic lateral sclerosis
- autoimmune polyglandular syndrome, type 1
- Bethlem myopathy
- familial atrial fibrillation
- glutamate formiminotransferase deficiency
- hereditary cerebral amyloid angiopathy
- holocarboxylase synthetase deficiency
- Jervell and Lange-Nielsen syndrome
- Knobloch syndrome
- microcephalic osteodysplastic primordial dwarfism type II
- nonsyndromic deafness
- Romano-Ward syndrome
- Ullrich congenital muscular dystrophy
- Unverricht-Lundborg disease
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.
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 a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance, and weak muscle tone (hypotonia) in infancy. This condition 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.
Less commonly, Down syndrome occurs when part of chromosome 21 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in fetal development. Affected people have two copies of chromosome 21 plus extra material from chromosome 21 attached to another chromosome, resulting in three copies of genetic material from chromosome 21. Affected individuals with this genetic change are said to have translocation Down syndrome.
In a very small percentage of cases, Down syndrome results from an extra copy of chromosome 21 in only some of the body's cells. In these people, the condition is called mosaic Down syndrome.
Researchers believe that having extra copies of genes on chromosome 21 disrupts the course of normal development, causing the characteristic features of Down syndrome and the increased risk of health problems associated with this condition.
- other chromosomal conditions
Other changes in the number or structure of chromosome 21 have a variety of effects on health and development. Chromosome 21 abnormalities can cause intellectual disability, delayed development, and characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome.
Changes involving chromosome 21 can 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 a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure.
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? (http://ghr.nlm.nih.gov/handbook/howgeneswork/genelocation) 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.
Research Resources - Tools for researchers
- Cancer Genetics Web (http://www.cancerindex.org/geneweb/clinkc21.htm)
- Database of Genomic Variants: Chromosome 21 (http://projects.tcag.ca/variation/cgi-bin/tbrowse/tbrowse?source=hg17&table=Locus&show=table&keyword=&flop=AND&fcol=_C19&fcomp==&fkwd=chr21&cols=)
- Ensembl Human Map View: Chromosome 21 (http://www.ensembl.org/Homo_sapiens/Location/Chromosome?chr=21)
- The DNA sequence of human chromosome 21. Nature. 2000 May 18;405(6784):311-9. (http://www.nature.com/nature/journal/v405/n6784/full/405311a0.html)
- U.S. Department of Energy: Human Chromosome Launchpad (http://www.ornl.gov/sci/techresources/Human_Genome/launchpad/chrom21.shtml)
- University of Geneva and the Swiss Institute of Bioinformatics: Human Chromosome 21 cSNP Database and Map (http://csnp.unige.ch/)
- PubMed - Recent literature (http://www.ncbi.nlm.nih.gov/pubmed?term=(Chromosomes,%20Human,%20Pair%2021%5BMAJR%5D)%20AND%20(Chromosome%2021%5BTI%5D)%20AND%20english%5Bla%5D%20AND%20human%5Bmh%5D%20AND%20%22last%20720%20days%22%5Bdp%5D)
- Map Viewer - Genetic maps (http://www.ncbi.nlm.nih.gov/mapview/maps.cgi?org=human&maps=ideogr,morbid,pheno&zoom=100&chr=21)
What glossary definitions help with understanding chromosome 21?
acute lymphoblastic leukemia ;
acute myeloid leukemia ;
human genome ;
human genome project ;
muscle tone ;
reproductive cells ;
You may find definitions for these and many other terms in the Genetics Home Reference
- Aït Yahya-Graison E, Aubert J, Dauphinot L, Rivals I, Prieur M, Golfier G, Rossier J, Personnaz L, Creau N, Bléhaut H, Robin S, Delabar JM, Potier MC. Classification of human chromosome 21 gene-expression variations in Down syndrome: impact on disease phenotypes. Am J Hum Genet. 2007 Sep;81(3):475-91. Epub 2007 Jul 19. (http://www.ncbi.nlm.nih.gov/pubmed/17701894?dopt=Abstract)
- Antonarakis SE, Lyle R, Dermitzakis ET, Reymond A, Deutsch S. Chromosome 21 and down syndrome: from genomics to pathophysiology. Nat Rev Genet. 2004 Oct;5(10):725-38. Review. (http://www.ncbi.nlm.nih.gov/pubmed/15510164?dopt=Abstract)
- Antonarakis SE, Lyle R, Deutsch S, Reymond A. Chromosome 21: a small land of fascinating disorders with unknown pathophysiology. Int J Dev Biol. 2002 Jan;46(1):89-96. Review. (http://www.ncbi.nlm.nih.gov/pubmed/11902692?dopt=Abstract)
- Antonarakis SE. Chromosome 21: from sequence to applications. Curr Opin Genet Dev. 2001 Jun;11(3):241-6. Review. (http://www.ncbi.nlm.nih.gov/pubmed/11377958?dopt=Abstract)
- Ensembl Human Map View: Chromosome 21 (http://www.ensembl.org/Homo_sapiens/Location/Chromosome?chr=21)
- Gardiner K, Costa AC. The proteins of human chromosome 21. Am J Med Genet C Semin Med Genet. 2006 Aug 15;142C(3):196-205. (http://www.ncbi.nlm.nih.gov/pubmed/17048356?dopt=Abstract)
- Gardiner K, Davisson M. The sequence of human chromosome 21 and implications for research into Down syndrome. Genome Biol. 2000;1(2):REVIEWS0002. Epub 2000 Aug 4. Review. (http://www.ncbi.nlm.nih.gov/pubmed/11178230?dopt=Abstract)
- Gilbert F. Disease genes and chromosomes: disease maps of the human genome. Chromosome 21. Genet Test. 1997-1998;1(4):301-6. (http://www.ncbi.nlm.nih.gov/pubmed/10464663?dopt=Abstract)
- Hattori M, Fujiyama A, Taylor TD, Watanabe H, Yada T, Park HS, Toyoda A, Ishii K, Totoki Y, Choi DK, Groner Y, Soeda E, Ohki M, Takagi T, Sakaki Y, Taudien S, Blechschmidt K, Polley A, Menzel U, Delabar J, Kumpf K, Lehmann R, Patterson D, Reichwald K, Rump A, Schillhabel M, Schudy A, Zimmermann W, Rosenthal A, Kudoh J, Schibuya K, Kawasaki K, Asakawa S, Shintani A, Sasaki T, Nagamine K, Mitsuyama S, Antonarakis SE, Minoshima S, Shimizu N, Nordsiek G, Hornischer K, Brant P, Scharfe M, Schon O, Desario A, Reichelt J, Kauer G, Blocker H, Ramser J, Beck A, Klages S, Hennig S, Riesselmann L, Dagand E, Haaf T, Wehrmeyer S, Borzym K, Gardiner K, Nizetic D, Francis F, Lehrach H, Reinhardt R, Yaspo ML; Chromosome 21 mapping and sequencing consortium. The DNA sequence of human chromosome 21. Nature. 2000 May 18;405(6784):311-9. Erratum in: Nature 2000 Sep 7;407(6800):110. (http://www.ncbi.nlm.nih.gov/pubmed/10830953?dopt=Abstract)
- Licht JD. AML1 and the AML1-ETO fusion protein in the pathogenesis of t(8;21) AML. Oncogene. 2001 Sep 10;20(40):5660-79. Review. (http://www.ncbi.nlm.nih.gov/pubmed/11607817?dopt=Abstract)
- Lubec G, Engidawork E. The brain in Down syndrome (TRISOMY 21). J Neurol. 2002 Oct;249(10):1347-56. Review. (http://www.ncbi.nlm.nih.gov/pubmed/12382149?dopt=Abstract)
- Map Viewer: Genes on Sequence (http://www.ncbi.nlm.nih.gov/mapview/maps.cgi?ORG=human&MAPS=ideogr,ugHs,genes&CHR=21)
- Sawińska M, Ładoń D. Mechanism, detection and clinical significance of the reciprocal translocation t(12;21)(p12;q22) in the children suffering from acute lymphoblastic leukaemia. Leuk Res. 2004 Jan;28(1):35-42. Review. (http://www.ncbi.nlm.nih.gov/pubmed/14630078?dopt=Abstract)
- UCSC Genome Browser: Statistics (http://genome.cse.ucsc.edu/goldenPath/stats.html)
- Valero R, Marfany G, Gil-Benso R, Ibáñez MA, López-Pajares I, Prieto F, Rullan G, Sarret E, Gonzàlez-Duarte R. Molecular characterisation of partial chromosome 21 aneuploidies by fluorescent PCR. J Med Genet. 1999 Sep;36(9):694-9. (http://www.ncbi.nlm.nih.gov/pubmed/10507727?dopt=Abstract)
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
See How can I find a genetics professional in my area? (http://ghr.nlm.nih.gov/handbook/consult/findingprofessional) in the Handbook.