03/21/2026
March 21st is celebrated as World Down Syndrome Day. Do you know why 3/21? The most common type of Down Syndrome is Trisomy 21 (3-21) in which an error in cell division results in the embryo having three copies of the 21st chromosome rather than two. While this type of Down Syndrome accounts for about 95% of cases, there are two other types as well.
Translocation Down Syndrome (3-4% of cases) occurs when the total number of chromosomes remains at 46; however, a full or partial piece of chromosome 21 breaks off and attaches to another chromosome.
Lastly, Mosaic Down Syndrome (1-2% of cases) occurs when some cells in the body have three copies of chromosome 21 while others only have the typical two copies.
Happy World Down Syndrome Day!
