Reviewed June 2008
What is Down syndrome?
Down syndrome is a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance, and poor muscle tone (hypotonia) in infancy. The degree of intellectual disability varies, but it is usually mild to moderate.
People with Down syndrome may be born with a variety of birth defects. About half of all affected children have a heart defect. Digestive abnormalities, such as a blockage of the intestine, are less common.
Individuals with Down syndrome have an increased risk of developing several medical conditions. These include gastroesophageal reflux, which is a backflow of acidic stomach contents into the esophagus, and celiac disease, which is an intolerance of a wheat protein called gluten. About 15 percent of people with Down syndrome have an underactive thyroid gland (hypothyroidism). The thyroid gland is a butterfly-shaped organ in the lower neck that produces hormones. Individuals with Down syndrome also have an increased risk of hearing and vision problems. Additionally, about 1 percent of children with Down syndrome develop cancer of blood-forming cells (leukemia).
Adults with Down syndrome have an increased risk of developing Alzheimer disease, a brain disorder that results in a gradual loss of memory, judgment, and ability to function. Although Alzheimer disease is usually a disorder that occurs in older adults, about half of adults with Down syndrome develop this condition by age 50.
How common is Down syndrome?
Down syndrome occurs in about 1 in 740 newborns. Although women of all ages can have a child with Down syndrome, the chance of having a child with this condition increases as a woman gets older.
What are the genetic changes related to Down syndrome?
Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome.
A small percentage of people with Down syndrome have an extra copy of chromosome 21 in only some of the body's cells. In these people, the condition is called mosaic Down syndrome.
Down syndrome can also occur 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. Affected individuals with this genetic change are said to have translocation Down syndrome.
Can Down syndrome be inherited?
Most cases of Down syndrome are not inherited. When the condition is caused by trisomy 21, the chromosomal abnormality occurs as a random event during the formation of reproductive cells. The abnormality usually occurs in egg cells, but it occasionally occurs in sperm cells. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 21. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 21 in each of the body's cells.
Mosaic Down syndrome is also not inherited. It occurs as a random event during cell division early in fetal development. As a result, some of the body's cells have the usual two copies of chromosome 21, and other cells have three copies of this chromosome.
Translocation Down syndrome can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 21. Although they do not have signs of Down syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
Where can I find information about treatment for Down syndrome?
You may find information on treatment or management of Down syndrome or some of its symptoms in the links below, particularly the links for
MedlinePlus Encyclopedia, Educational resources, and Patient support.
Where can I find additional information about Down syndrome?
You may find the following resources about Down syndrome helpful. These materials are written for the general public.
- MedlinePlus - Health information
- Encyclopedia: Down syndrome (http://www.nlm.nih.gov/medlineplus/ency/article/000997.htm)
- Health Topic: Congenital Heart Defects (http://www.nlm.nih.gov/medlineplus/congenitalheartdefects.html)
- Health Topic: Down Syndrome (http://www.nlm.nih.gov/medlineplus/downsyndrome.html)
- Additional NIH Resources - National Institutes of Health
- National Human Genome Research Institute (http://www.genome.gov/19517824)
- National Institute of Child Health and Human Development (http://www.nichd.nih.gov/health/topics/Down_Syndrome.cfm)
- National Institute of Diabetes and Digestive and Kidney Diseases: Hypothyroidism (http://www.endocrine.niddk.nih.gov/pubs/Hypothyroidism/)
- Educational resources - Information pages
- California Department of Developmental Services (http://www.ddhealthinfo.org/coursebuilder/?id=19)
- Centre for Genetics Education (http://www.genetics.com.au/pdf/factsheets/fs28.pdf)
- Children's Hospital Boston (http://www.childrenshospital.org/az/Site803/mainpageS803P0.html)
- Cincinnati Children's Hospital Medical Center (http://www.cincinnatichildrens.org/health/heart-encyclopedia/disease/syndrome/down.htm)
- Cleveland Clinic: Alzheimer's Disease and Down Syndrome (http://my.clevelandclinic.org/disorders/alzheimers_disease/hic_alzheimers_disease_and_down_syndrome.aspx)
- Down Syndrome: Health Issues (Len Leshin, M.D., F.A.A.P.) (http://www.ds-health.com/)
- Genetic Science Learning Center, University of Utah (http://learn.genetics.utah.edu/content/disorders/whataregd/down/index.html)
- JAMA Patient Page (http://www.hmc.psu.edu/childrens/healthinfo/articles/downsyndrome.pdf)
- Kennedy Krieger Institute (http://www.kennedykrieger.org/kki_diag.jsp?pid=1081)
- KidsHealth from the Nemours Foundation (http://kidshealth.org/kid/health_problems/birth_defect/down_syndrome.html)
- Madisons Foundation (http://www.madisonsfoundation.org/index.php/component/option,com_mpower/diseaseID,210/)
- NetWellness: Down syndrome and Alzheimer disease (http://www.netwellness.org/question.cfm/8478.htm)
- NetWellness: Risk of having a child with Down syndrome (http://www.netwellness.org/question.cfm/30137.htm)
- NetWellness: Testing for Down syndrome (http://www.netwellness.org/question.cfm/46568.htm)
- New York Online Access to Health (http://www.noah-health.org/en/genetic/conditions/downs/)
- Orphanet (http://www.orpha.net/consor/cgi-bin/OC_Exp.php?Lng=EN&Expert=870)
- The Merck Manual of Medical Information, Second Home Edition (http://www.merck.com/mmhe/sec23/ch266/ch266b.html)
- The Wellcome Trust (http://genome.wellcome.ac.uk/doc_WTD020859.html)
- University of British Columbia: Mosaic Down Syndrome (http://www.medgen.ubc.ca/wrobinson/mosaic/specific/trisomy21.htm)
- Patient support - For patients and families
- Chromosome Disorder Outreach (http://www.chromodisorder.org/)
- Down Syndrome Research and Treatment Foundation (http://www.dsrtf.org)
- National Down Syndrome Congress (http://www.ndsccenter.org)
- National Down Syndrome Society (http://www.ndss.org)
- National Organization for Rare Disorders (http://www.rarediseases.org/search/rdbdetail_abstract.html?disname=Down+Syndrome)
- Resource list from the University of Kansas Medical Center (http://www.kumc.edu/gec/support/down_syn.html)
You may also be interested in these resources, which are designed for healthcare professionals and researchers.
What other names do people use for Down syndrome?
- Down's Syndrome
- Trisomy 21
- 47,XX,+21
- 47,XY,+21
See How are genetic conditions and genes named? (http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/naming) in the Handbook.
What if I still have specific questions about Down syndrome?
- See How can I find a genetics professional in my area? (http://ghr.nlm.nih.gov/handbook/consult/findingprofessional) in the Handbook.
- Ask the Genetic and Rare Diseases Information Center (http://rarediseases.info.nih.gov/GARD/).
- Submit your question to Ask the Geneticist (http://www.askthegen.org/).
What glossary definitions help with understanding Down syndrome?
atypical ;
cancer ;
celiac disease ;
cell ;
cell division ;
chromosome ;
digestive ;
egg ;
esophagus ;
gastroesophageal reflux ;
hormone ;
hypothyroidism ;
hypotonia ;
intestine ;
leukemia ;
mosaic ;
muscle tone ;
nondisjunction ;
protein ;
rearrangement ;
reproductive cells ;
sign ;
sperm ;
stomach ;
syndrome ;
thyroid ;
translocation ;
trisomy
You may find definitions for these and many other terms in the Genetics Home Reference
Glossary (http://ghr.nlm.nih.gov/glossary).
References
- 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/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=15510164)
- Cohen WI. Current dilemmas in Down syndrome clinical care: celiac disease, thyroid disorders, and atlanto-axial instability. Am J Med Genet C Semin Med Genet. 2006 Aug 15;142(3):141-8. Review. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=16838307)
- Lott IT, Head E. Alzheimer disease and Down syndrome: factors in pathogenesis. Neurobiol Aging. 2005 Mar;26(3):383-9. Review. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=15639317)
- 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/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=12382149)
- Roizen NJ, Patterson D. Down's syndrome. Lancet. 2003 Apr 12;361(9365):1281-9. Review. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=12699967)
- Shapiro BL. Down syndrome and associated congenital malformations. J Neural Transm Suppl. 2003;(67):207-14. Review. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=15068252)
- Sherman SL, Allen EG, Bean LH, Freeman SB. Epidemiology of Down syndrome. Ment Retard Dev Disabil Res Rev. 2007;13(3):221-7. Review. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=17910090)
- Zigman WB, Lott IT. Alzheimer's disease in Down syndrome: neurobiology and risk. Ment Retard Dev Disabil Res Rev. 2007;13(3):237-46. Review. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=17910085)
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? (http://ghr.nlm.nih.gov/handbook/consult/findingprofessional) in the Handbook.
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