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Key dates in the understanding of human genetics and trisomy 21.

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).1665: Discovery of the cell by Robert Hooke.
1866: Publication of Mendel's laws on inheritance .
1866: Identification of "mongolism" by John Langdon Down, now called trisomy 21 or Down syndrome.
1880: first observation of chromosomes by Flemming.
1911: Theory of heredity demonstrated by Thomas Hunt Morgan.
1944: Identification of the chemical nature of DNA by Oswald Avery, Colin MacLeod, and McCarty.
1950: Observation of the helical shape (corkscrew) DNA by Rosalind Franklin and Maurice Wilkins.
1950: Discovery of bases constituting the DNA by Erwin Chargaff.
1952: Detection of DNA as the molecular carrier of genetic information by Alfred Hershey and Martha Chase.
1953: Determination of the structure of the DNA molecule by James Watson and Francis Crick.
1956: Determination that the number of chromosomes in humans is 46 (23 from the father and 23 from the mother) by JH Tijo and A. Levan.

1956: First use of amniocentesis in the diagnosis of genetic disease by Fuchs and Riis.
1957: Discovery of the reproductive mechanism of DNA (replication), by Matthew Meselson and Franklin Stahl.
1959: Publication of the discovery of trisomy 21 by Jerome Lejeune, Marthe Gautier, Raymond Turpin.

1968: First reported use of amniocentesis to prenatally diagnose trisomy 21 by Harold Nadler.

2003: End of the sequencing of the human genome (Human Genome Project) launched in 1993 involving 18 countries.

2012: First major pharmaceutical company begins a Phase I clinical trial on a drug to improve cognition in Down syndrome

2012: Jérôme Lejeune Institute begins clinical trial on the use of folinic acid and thyroxine to improve psychomotor development of young children with Down syndrome.

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