Down syndrome is an abnormality of chromosomes.
Chromosome 21 (human) . People without Down 's syndrome have two copies of chromosome 21 , while those with three copies of chromosome 21 have Down syndrome , also called `` trisomy 21 '' . Down syndrome ( DS or DNS ) , also known as trisomy 21 , is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21 . In 1957 , the genetic cause of Down syndrome , an extra copy of chromosome 21 , was discovered . A genetic disorder is a genetic problem caused by one or more abnormalities in the genome , especially a condition that is present from birth ( congenital ) . Whether , when and to what extent a person with the genetic defect or abnormality will actually suffer from the disease is almost always affected by the environmental factors and events in the person 's development .