Epstein, Charles J. Department of Pediatrics, University of California, San Francisco, California.
Last reviewed:September 2016
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- Nervous system abnormalities
- Maternal age
- Prenatal diagnosis
- Trisomy 21 and phenotype
- Gene dosage effects
- Mouse models
- Links to Primary Literature
- Additional Readings
The set of physical, mental, and functional abnormalities that result from trisomy 21, the presence in the genome of three (rather than the normal two) chromosomes 21. Down syndrome is one of the most frequent genetic causes of intellectual disability (formerly known as mental retardation). The genetic aberration that causes Down syndrome is trisomy 21, in which an individual has a full or partial extra copy of chromosome 21 (Fig. 1). This additional genetic material modifies the normal progression of development and leads to the identifying characteristics of Down syndrome. The physical abnormalities that together give rise to the distinctive facial appearance associated with this condition include upslanting eyes with epicanthic folds (folds of skin over the inner corners), flatness of the bridge of the nose and of the middle portion of the face, and a tendency to protrude the tongue, especially when affected individuals are very young. Many other minor, functionally inconsequential abnormalities of the ears, hands, and feet may also be present in individuals having Down syndrome, and stature is generally reduced. See also: Chromosome; Chromosome aberration; Developmental genetics; Genetic mapping; Genetics; Human genetics; Intellectual disability
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