In 1959 the French geneticist Jérôme Lejeune discovered that DS babies are born with an extra chromosome in their cells, for a total of 47 rather than 46.
Prantsuse geneetik Jérôme Lejeune avastas aastal 1959, et Downi sündroomiga beebide rakkudes on üleliigne kromosoom, nii et 46 asemel on neid 47.