The most common cause of Down Syndrome (DS) is presence of an extra copy of chromosome 21, resulting in trisomy 21. Although a critical region within 21q22.1--q22.3 houses many genes that contribute to DS phenotypes, there is not a single critical region sufficient to cause all Down syndrome phenotypes. Several disease models using the fly orthologs of or transgenes of human genes thought to have a role in development of DS have been characterized; see the "Related diseases" section.
[updated July 2016 by FlyBase; FBrf0222196]
[DOWN SYNDROME](https://omim.org/entry/190685)
Down syndrome is characterized by a particular combination of phenotypic features that includes mental retardation and characteristic facial structure. Individuals with Down syndrome often have specific major congenital malformations such as those of the heart (30-40% in some studies) and of the gastrointestinal tract. 90% of Down syndrome patients have a significant hearing loss (Mazzoni et al., 1994; pubmed:7881226). A number of other diseases are observed at higher frequency among Down syndrome individuals, including leukemia (Robison, 1992; pubmed:1532221) and Alzheimer disease (Wisniewski et al., 1985; pubmed:3158266). [from MIM:190685; 2016.07.19]
Mapping of the chromosomal regions that, if triplicated, result in the phenotypic characteristics of Down syndrome has been facilitated by the use of DNA samples from individuals who have partial trisomy 21 due to translocation. A region of approximately 3 Mb at chromosomal region 21q22 was proposed as the minimal critical region (DCR or DSCR) for the Down syndrome phenotype; the first gene to be identified in this region was RCAN1, also known as DSCR1 (Fuentes et al., 1995; pubmed:8595418). However, some phenotypic characteristics map outside the minimum critical region. [from MIM:190685, MIM:602917; 2016.07.19]
Down syndrome is caused by trisomy of all or a critical portion of chromosome 21; it is one of the most common chromosomal abnormalities in liveborn children. The risk of having a child with trisomy 21 increases significantly with maternal age. [from MIM:190685; 2016.07.19]
