trisomy 21

n. A genetic disorder caused by the presence of an extra copy of chromosome 21, which is present in about 85% of cases of Down syndrome. Most individuals who have this abnormality will be mentally retarded, usually in the mild to moderate range of retardation. Physical symptoms include almond-shaped eyes with an epicanthic fold, a single transverse crease instead of the normal double crease in the palm, short limbs, poor muscle tone, and a protruding tongue.