down syndrome is an abnormality of chromosomes
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