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Down syndrome is a genetic condition that occurs when a person is born with an extra copy of chromosome 21, which affects how their body and brain develop. It matters because it is one of the most common chromosomal conditions in humans and can impact physical health, learning abilities, and development throughout a person's life.
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Down syndrome or Down's syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with developmental delays, mild to moderate intellectual disability, and characteristic physical features.
The parents of the affected individual are usually genetically normal. The incidence of the syndrome increases with the age of the mother, from less than 0.1% for 20-year-old mothers to 3% for those of age 45. It is believed to occur by chance, with no known behavioral activity or environmental factor that changes the probability. Three different genetic forms have been identified. The most common, trisomy 21, involves an extra copy of chromosome 21 in all cells. The extra chromosome is provided at conception as the egg and sperm combine. Translocation Down syndrome involves attachment of extra chromosome 21 material. In 1–2% of cases, the additional chromosome is added in the embryo stage and only affects some of the cells in the body; this is known as mosaic Down syndrome.
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Autosomal trisomiesChromosomal abnormalitiesDiseases named after discoverersDown syndromeGenetic syndromesSyndromes affecting the gastrointestinal tractSyndromes affecting the heartSyndromes affecting the nervous systemSyndromes with craniofacial abnormalitiesSyndromes with intellectual disabilitiesSyndromic autism