Down syndrome involves an abnormality in chromosome 21. Each person has 23 pairs of chromosomes in each body cell -- 22 regular pairs and one pair of sex chromosomes. The abnormality associated with Down syndrome and chromosome 21 can occur three ways:

• Standard trisomy 21. Three entire copies of chromosome 21 occur in all the nonreproductive cells in a child's body. This form results from a problem that occurs during egg or sperm cell development. The vast majority of Down syndrome cases are of this type.
  
  
• Translocation 21. Due to an abnormality in the genes of a parent or an error in egg or sperm cell development, an extra piece of chromosome 21 is attached to a different chromosome. The normal pair of chromosome 21 is still present. This is an uncommon form of Down syndrome.
  
  
• Mosaic trisomy 21. Three entire copies of chromosome 21 occur in some, but not all, cells in the child's body. This is caused by a problem in embryo development after the sperm and egg have joined. It is a rare form of Down syndrome.