It may appeal to lazy minds (mine included) to think that progress in science takes place along gradual and predictable lines. Somehow, this seems simpler and therefore more satisfying than the alternative. However, nothing could be more wrong. Nature may not make jumps but human comprehension certainly proceeds unevenly and often capriciously. So it was with the development of genetics and molecular biology. This took several wrong turns, the most egregious being that for quite a while, genes were believed to consist of proteins alone. Of much lesser consequence was another belief, summed up in the dictum "one gene-one enzyme." We know now that instances of "one gene-several enzymes" and "several genes-one enzyme" are not uncommon.
These and other historical twists and turns are beautifully described in an article by Bernard Strauss entitled "Biochemical Genetics and Molecular Biology: The Contributions of George Beadle and Edward Tatum." Here you will find the connection between genetics and biochemistry that emerged in the early 1940’s. Until then, the two had pretty much marched to their own tunes. Turning to the mold Neurospora, which was simple to handle in the lab partly because of its few nutritional requirements, these two investigators used X-ray mutagenesis to make a large number of auxotrophic mutants. By 1945, they had about 500 mutants that required amino acids or vitamins for growth.
Reference: Strauss BS. (2016). Biochemical Genetics and Molecular Biology: The Contributions of George Beadle and Edward Tatum. Genetics 203:13-20.