Down syndrome
chromosomal condition
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.
The public source identifies “Down syndrome” as chromosomal condition. This brief keeps that definition visible, then builds a research path around Down, syndrome and chromosomal.
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Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated Sep 24, 2026. The linked authority identifier is Q47715. The Library of Congress control number is sh85039232. 1 of 1 selected statements include explicit references; 0 carry qualifiers and 0 use preferred rank.
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This entry incorporates text from “Down syndrome” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.