The discovery by Jerome Lejeune that Down syndrome is caused by an error in cell division leading to one parent providing two copies of the 21st chromosome during fertilization did not happen in isolation. Scientific knowledge is cumulative. Below we list a few of the more significant events in the development of genetic understanding that allowed the discovery of the genetic cause of Down syndrome to happen. Of course, technology is critical to scientific development. Just as the microscope was essential to the discovery of the cell, Marthe Gautier's knowledge of how to prepare a karyotype was essential to Jerome Lejeune being able to identify 3 copies of the 21st chromosome in an individual with Down syndrome.

1590: Invention of the microscope Janssen (as Attributed to Galileo in 1609).