Post Mendelian Era ~1920-1930

The next major development in Genetics happened mid way through the 1920’s. It was 1924 when J.B.S. Haldane began his work on the mathematical basis for evolutionary biology. Titled ‘A mathematical theory of natural and artificial selection’, Haldane worked on this paper for another decade. Although all this work was highly theoretical, it still turned out to be one of the groundworks for the ‘modern synthesis’ of Darwin’s theory of natural selection and Mendel’s laws of inheritance, The work of R.A. Fisher and Sewall Wright also contributed to laying the groundwork for this also. Once Haldane was done writing the paper, he published it into a book in 1932, it was named ‘The Causes of Evolution’.

Three years later the results of an experiment by Hermann Muller. Muller was seeking to map chromosome specific mutations of fruit flies. He then came up and tested the idea of exposing the flies to radiation would increase the frequency of mutations. What he did to test it was he irradiated male fruit flies and mated them with female fruit flies. He won a Nobel Prize in 1946, and was quoted saying “over a hundred times as many mutations… as would have occurred..spontaneously in the course of a whole generation.” Mullen demonstrated that most mutations are detrimental in health, and the ones that are favorable, the ones that evolution by natural selection depends on, must be extremely rare.

In 1928 Fred Griffith was working with colonies of the bacterium Streptococcus pneumoniae, when he noticed that after some time in culture, they changed from smooth to rough. He discovered that when the rough cells were injected into mice they were completely harmless, but the when the smooth cells were injected, they were fatal. To make it more interesting, the other part of the discovery made was that when live rough cells of one strain and dead smooth cells of another were injected simultaneously, the mice again died, and were filled with live smooth bacteria of the second strain. His conclusion was that true-breeding strains could be transformed by some force that could be transferred from one strain to another.

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