Children with this genetic disease have 3 no. 21 chromosomes instead of the usual 2
In the news: 
Sequenom Validates MaterniT21 for Trisomy 18 and 13 Detection ... individual 
has three copies of chromosome 16 instead of the usual two and... [DOC] [TLE] Genetic Disorders | Children's Hospital of Wisconsin [PAR] A person may carry the nonfunctioning gene but have no symptoms.