Down's Syndrome is caused by an extra chromosome at position 21. In most cases trisomy in Down's syndrome occurs during cell division.

Down's Syndrome is caused by an extra chromosome at position 21.  In most cases trisomy in Down's syndrome occurs during cell division.  What  is that phenomena called and at which stage of division does it occur?  Less frequently it call also happen by a rearrangement called a translocation.  

What are some symptoms of trisomy 21?