Down Syndrome

Down syndrome is a chromosomal abnormality.  A person with Down syndrome has three copies of chromosome 21, instead of the usual two.   The extra copy of chromosome 21 is the direct cause of Down syndrome.  Down syndrome is not caused by something the mother does (or does not do) during pregnancy.  Down syndrome can occur in one of three forms:

• Meiotic nondysjunction - An error occurs in the separation of chromosome 21 during cell division in the process of forming the embryo.     
• Translocation - The extra copy of chromosome 21 is attached (translocated) to another chromosome.
• Mosaicism - An error occurs during cell division (mitosis) at some point after formation of the embryo.