Claim: Down syndrome is an abnormality of chromosomes.
Evidence: Down syndrome -LRB- DS or DNS -RRB- , also known as trisomy 21 , is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21.
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 ''.
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.