1921 Warren Harding becomes twenty-ninth president of the United States.
  L. V. Morgan discovers attached-X chromosomes in Drosophila.
  C. B. Bridges discovers chromosomal translocations in Drosophila.

A. E. Boycott and C. Diver describe "delayed" Mendelian inheritance controlling the direction of the coiling of the shell in the snail Limnea peregra. A. H. Sturtevant suggests that the direction of coiling of the Limnea shell is determined by the character of the ooplasm, which is in turn controlled by the mother's genotype.

1923 Harding dies in office.

Calvin Coolidge becomes thirtieth president of the United States.

  A. H. Sturtevant analyzes the Bar-eye phenomenon in Drosophila and discovers position effect.
  A. H. Sturtevant finds the first inversion in Drosophila.
  J. B. S. Haldane suggests that the genes known to control certain coat colors in various rodents and carnivores may be evolutionarily homologous.

B. O. Dodge initiates genetic studies on Neurospora.

H. J. Muller reports the artificial induction of mutations in Drosophila by x-rays.

  L. J. Stadler reports the artificial induction of mutations in maize, and demonstrates that the dose-frequency curve is linear.

F. Griffith discovers type-transformation of pneurnococci. This lays the foundation for the work of Avery, MacLeod, and McCarthy (1944).

1929 Herbert Hoover becomes thirty-first president of the United States.