People with Down's syndrome probably do not all look alike to those who know them personally. There are, however, physical characteristics that they have in common. Down's syndrome is a genetic disorder that was established at the moment of conception. Normally, there are 23 pairs of chromosomes that make up the fertilized egg. One of each pair is from the sperm and the other from the egg. In Down's there is an extra chromosome 21. The medical term is trisomy 21. This is the usual reason for Down's syndrome. \n\nThe most obvious consequenses of having an extra chromosome 21 are an epicanthal fold around the eyes, short stature, dental deformaties, broad chest (that's why they seem to look alike) and mental retardation. The epicanthal fold about the eyes give them an Asiatic or Mongoloian appearance, hence, the older term mongoloid. \n\nAs in any group of people, those with Down's exhibit different personalities and varied abilities.