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Classical genetics (genes on chromosomes)
1900 (Mendel rediscovered); 1910s (Morgan's flies)· Idea, religion or movement· Germs, drugs & genetics
Well documentedSolid records, physical evidence or scientific consensus.
The early-1900s science showing heredity is carried by discrete genes lined up on chromosomes inside the cell nucleus.
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In 1900 Hugo de Vries, Carl Correns and Erich von Tschermak each rediscovered Mendel's laws. Walter Sutton and Theodor Boveri noticed that chromosomes pair and separate just as Mendel's factors do. From 1910 Thomas Hunt Morgan's 'fly room' at Columbia University bred fruit flies and tied a white-eye mutation to the X chromosome. His student Alfred Sturtevant used how often traits were inherited together to draw the first gene map in 1913. William Bateson coined the word 'genetics' in 1905.
Why it mattered
It gave heredity a physical address in the cell, set off the hunt for what genes are made of, and guided crop breeding. It was also misused to justify eugenics laws.
Think of it like
Like a plat map: chromosomes are streets, genes are lots at fixed addresses, and the distance between lots can be measured.
How we know
The 1900 rediscovery papers; Morgan's fruit-fly breeding records and his 1933 Nobel Prize; Sturtevant's 1913 map; Sutton's and Boveri's microscope studies of chromosomes.
Still unknown or debated
Whether each 1900 rediscoverer reached Mendel's laws independently, or only after reading him, is debated.
Inside this (1)
Connections (3)
▸Gregor Mendel and the laws of heredity was rediscovered in 1900, founding Classical genetics (genes on chromosomes)Well documented
The connection
Mendel's 1866 paper sat largely unread for 34 years. In 1900 three botanists, Hugo de Vries, Carl Correns and Erich von Tschermak, reached similar results and found his paper. His ratios and 'factors' became the framework of the new science, championed in English by William Bateson.
When and how
The Netherlands, Germany and Austria, spring 1900; in Britain, Bateson's 1902 book 'Mendel's Principles of Heredity: A Defence'.
How we know
The 1900 papers cite Mendel, while his work drew only a handful of citations before then, showing how little it had been noticed.
Debates and open questions
Why it was ignored is debated: an obscure journal, math unfamiliar to biologists, and Nägeli's lukewarm response are all blamed.
▸Classical genetics (genes on chromosomes) led to the DNA double helixWell documented
The connection
Classical genetics showed genes are real units on chromosomes but not what they are made of. Chromosomes contain both protein and DNA. Experiments by Oswald Avery's team in 1944 and by Alfred Hershey and Martha Chase in 1952 pointed to DNA. Watson and Crick then found its structure in 1953, showing how genes store information and copy themselves.
When and how
Rockefeller Institute, New York, 1944 (Avery, MacLeod, McCarty); Cold Spring Harbor, 1952 (Hershey, Chase); Cambridge and King's College London, 1953.
How we know
The 1944, 1952 and 1953 papers build on classical genetics; the double helix showed physically how genes copy and mutate, fitting the rules Mendel and Morgan found.
Debates and open questions
Many scientists assumed into the 1940s that genes were proteins; why DNA was underrated so long, and whether Avery deserved a Nobel, are debated.
▸Classical genetics (genes on chromosomes) guided the breeding of the Green Revolution (Borlaug)Mostly agreed
The connection
Breeders used Mendel's and Morgan's rules of inheritance to combine traits on purpose. Borlaug crossed thousands of wheat lines, adding dwarfing genes from the Japanese variety Norin 10 to make short stems that could hold heavy grain. IRRI likewise crossed Indonesian and Taiwanese rice to create IR8.
When and how
Mexico, 1950s (Borlaug, using Norin 10 crosses from Orville Vogel at Washington State); Los Baños, Philippines, 1962–66 (IRRI's IR8).
How sure are we?
Pedigree records trace each variety's gene sources, and the main wheat dwarfing genes (Rht1 and Rht2) were later identified at the DNA level.
Debates and open questions
How much credit belongs to Borlaug versus the Japanese, American and Mexican breeders whose genes and work he built on is still debated.