The passage discusses the genetic control of biochemical reactions in *Neurospora*, a fungus used in physiological genetics studies. The authors, G. W. Beadle and E. L. Tatum, describe a method to induce mutations in genes responsible for specific biochemical reactions using X-ray treatment. They establish that if an organism cannot perform a particular biochemical reaction to survive on a specific medium, it will be lethal on that medium. However, if the essential product of the blocked reaction is added to the medium, the mutant can grow. This approach allows them to identify and maintain mutant strains that are unable to perform specific biochemical processes. Three mutant strains were identified: 1. A strain unable to synthesize vitamin B6 (pyridoxine). 2. A strain unable to synthesize vitamin B1 (thiamine), specifically the thiazole half of the molecule. 3. A strain unable to synthesize para-aminobenzoic acid. The growth of the pyridoxinless mutant is dependent on the concentration of vitamin B6 in the medium, and the inability to synthesize it is inherited as a single-gene trait. The authors conclude that this method offers promise for understanding how genes regulate development and function and for discovering new essential substances for the growth of *Neurospora*.The passage discusses the genetic control of biochemical reactions in *Neurospora*, a fungus used in physiological genetics studies. The authors, G. W. Beadle and E. L. Tatum, describe a method to induce mutations in genes responsible for specific biochemical reactions using X-ray treatment. They establish that if an organism cannot perform a particular biochemical reaction to survive on a specific medium, it will be lethal on that medium. However, if the essential product of the blocked reaction is added to the medium, the mutant can grow. This approach allows them to identify and maintain mutant strains that are unable to perform specific biochemical processes. Three mutant strains were identified: 1. A strain unable to synthesize vitamin B6 (pyridoxine). 2. A strain unable to synthesize vitamin B1 (thiamine), specifically the thiazole half of the molecule. 3. A strain unable to synthesize para-aminobenzoic acid. The growth of the pyridoxinless mutant is dependent on the concentration of vitamin B6 in the medium, and the inability to synthesize it is inherited as a single-gene trait. The authors conclude that this method offers promise for understanding how genes regulate development and function and for discovering new essential substances for the growth of *Neurospora*.