What kind of genetic disease is Down syndrome considered?

Chromosomal abnormal. Down syndrom is considered a chromosomal abnormality due to the presence of three number 21 chromosomes instead of the normal 2.
Extra chromosome mtl. The traditional ds patient has an extra 21 chromosome(c) that resulted from an improper splitting of the c pairs prior to conception. There are a few which are a mosaic, extra cs in some cells and normal cs in others.Their are rare cases where an extra piece of a 21 is found & those rare cases have a high recurrence risk while regular ds is little more than that due to maternal age.