Even though Down syndrome is the most common chromosomal condition in the United States, most people don’t know that there are actually three different types of Down Syndrome. All 3 types of Down syndrome are caused by an extra copy of the 21st chromosome. The most common type is Trisomy 21 which accounts for 95% of all cases. In Trisomy 21, every cell has 3 copies of the 21st chromosome. So instead of 46 chromosomes (23 pairs), every cell has 47 chromosomes. Mosaic Down syndrome is the least co

Chromosomes are... - National Human Genome Research Institute