Children with this genetic disease have 3 no. 21 chromosomes instead of the usual 2
When an 
individual has more than two chromosomes instead of a pair, the condition is ... 
has three copies of chromosome 21 rather than two; for that reason,... [DOC] [TLE] 2p deletions FTNW - Unique The Rare Chromosome Disorder ... [PAR] contain just the right amount of genetic material (DNA)  not ... than usual.