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Genetic and Rare Diseases Information Center (GARD)

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Down syndrome

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* Not a rare disease
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Other Names for this Disease
  • Down's syndrome
  • Trisomy 21
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Overview



What is Down syndrome?

What are the signs and symptoms of Down syndrome?

What causes Down syndrome?

How might Down syndrome be treated?


What is Down syndrome?

Down syndrome is a chromosome condition that is associated with intellectual disability, a characteristic facial appearance, and low muscle tone in infancy. The degree of intellectual disability varies from mild to moderate. People with Down syndrome may be born with a variety of health concerns, including heart defects or digestive abnormalities. In addition, they have an increased risk of developing gastroesophageal reflux, celiac disease, hypothyroidism, hearing and vision problems, leukemia, and Alzheimer disease. Down syndrome is caused by having three copies of chromosome 21, instead of the usual two copies.[1] This condition occurs in about 1 in 800 newborns. Researchers have identified genes that cause some of the features of Down syndrome and are working to learn more in order to improve treatment options.[2]

Last updated: 6/8/2012

What are the signs and symptoms of Down syndrome?

People with Down syndrome may develop the following medical problems:[3] 

Children with Down syndrome are also more likely to develop chronic respiratory infections, middle ear infections, and recurrent tonsillitis. In addition, there is a higher incidence of pneumonia in children with Down syndrome than in the general population.[3]

Children with Down syndrome have developmental delay. They are often slow to turn over, sit, and stand. Developmental delay may be related to the child's weak muscle tone. Development of speech and language may also take longer than expected. Children with Down syndrome may take longer than other children to reach their developmental milestones, but many of these milestones will eventually be met.[3]

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.[4]

Last updated: 6/8/2012

What causes Down syndrome?

Three genetic variations can cause Down syndrome. 

1) Trisomy 21. Most often, Down syndrome is caused by an extra chromosome 21 in all cells of the individual.[1][5]  In these cases, a pair of number 21 chromosomes fails to separate during the formation of an egg (or sperm); this is referred to as "nondisjunction". When that egg unites with a normal sperm to form an embryo, the embryo ends up with three copies of chromosome 21 instead of the normal two.[1][5] The extra chromosome is then copied in every cell of the baby's body.[6]

The cause of nondisjunction is unknown, but research has shown that it increases in frequency as a woman ages.  However, due to higher birth rates of younger women, 80% of children with Down syndrome are born to women under 35.  Nondisjunction is not known to be caused by environmental factors or anything the parents did or did not do before or during the pregnancy.[5]
 
2) Mosaic Trisomy 21. For some individuals, approximately 1-2% of cases, only some of the cells have an extra chromosome 21; this is called "mosaic trisomy 21".[1][2][5]  In this situation, the fertilized egg may have the right number of chromosomes, but due to a cell division error early in the development of the embryo, some cells acquire an extra chromosome 21.  Thus, an individual with mosaic trisomy 21 will typically have 46 chromosomes in some cells and 47 chromosomes (including an extra chromosome 21) in others.  The signs and symptoms for individuals with mosaic trisomy 21 may have a wider range of variability.[5]

3) Translocation Trisomy 21. Approximately 3-4% of individuals with Down syndrome have cells that contain 46 chromosomes; however, there is extra chromosome 21 material attached  (translocated ) onto another chromosome  - this is called "translocation trisomy 21".[1][5]  For parents of a child with Down syndrome due to a translocation, there may be an increased chance of Down syndrome in future pregnancies. This is because one of the two parents may be a balanced carrier of the translocated chromosome. However, it important to realize that not all parents of individuals with translocation trisomy 21 are balanced carriers.[1]

Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. This extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome.[1][5]  
Last updated: 12/22/2011

How might Down syndrome be treated?

Early intervention services, quality educational programs, a stimulating home environment, good health care, and positive support from family and friends can help individuals with Down syndrome develop to their full potential.[7] The overall objective of treatment is to boost cognition by improving learning, memory, and speech. Scientific advances have made it possible to understand how specific genes are linked to specific abnormalities in the structure and function of the brain. Although there are hundreds of genes on chromosome 21, researchers believe it likely that only a handful significantly impact cognition. Many researchers now believe that it will be possible to isolate the effects of these specific genes and determine how their expression in the brain can cause problems with cognition. As researchers gain a better understanding of these mechanisms, they can begin the process of discovering treatments that enhance brain function.[8]   

Visit the GARD Services tab above to find resources that provide a list of specialty centers located across the U.S and internationally for individuals with Down syndrome.
Last updated: 6/8/2012

References
  1. Down syndrome. Genetics Home Reference (GHR). 2008; http://ghr.nlm.nih.gov/condition/down-syndrome. Accessed 3/9/2012.
  2. Down syndrome. March of Dimes. 2009; http://www.marchofdimes.com/baby/birthdefects_downsyndrome.html. Accessed 3/9/2012.
  3. Down syndrome. Genetics Home Reference Web site. 2005; http://ghr.nlm.nih.gov/condition=downsyndrome . Accessed 10/9/2007.
  4. Down syndrome. Genetics Home Reference. June 2008; http://ghr.nlm.nih.gov/condition/down-syndrome. Accessed 6/1/2010.
  5. What Causes Down Syndrome?. National Down Syndrome Society. 2011; http://www.ndss.org/index.php?option=com_content&view=article&id=60:what-causes-ds&catid=35:about-down-syndrome&Itemid=77. Accessed 12/22/2011.
  6. Down Syndrome. Genetic Science Learning Center. 2011; http://learn.genetics.utah.edu/content/disorders/whataregd/down/index.html. Accessed 12/22/2011.
  7. Down Syndrome Fact Sheet. National Down Syndrome Society. 2012; http://www.ndss.org/index.php?option=com_content&view=article&id=54&Itemid=74. Accessed 3/9/2012.
  8. DS 101. Down Syndrome Research and Treatment Foundation. 2012; http://www.dsrtf.org/page.aspx?pid=352. Accessed 3/9/2012.