Did you know that March 21st is National Down Syndrome day? On this day you can wear crazy or mismatched socks! But you may also be wondering, what is Down syndrome? A more specific/ scientific name for Down syndrome is trisomy 21. Trisomy is having an extra chromosome, it is a type of aneuploidy. Aneuploidy is when you have too many (trisomy) or not enough (monosomy) chromosomes. What is a chromosome? a chromosome is stored inside the nucleus of a cell, it is where the DNA is stored. Your average person is born with 23 pairs of chromosomes, so an overall of 46 total. Aneuploidy is not from genetics; parents can have a perfect set of chromosomes but they get unevenly distributed between children.