HOROWITZ AND LEUPOLD 209 



In many cases, there can be little qucs- sion of variegation in Drosophila. That 



tion about the similarities in expression a heterochromatic element likewise is 



of variegation in Drosophila and responsible for the origin and behavior 



maize. A heterochromatic element has of variegation in maize has not been 



repeatedly been found to be basically proved, although it is indicated, as the 



associated with the origin and expres- analysis of Dt has shown. 



Some Recent Studies Bearing on the One Gene 

 One Enzyme Hypothesis 



N. H. HOROWITZ and URS LEUPOLD 



Reprinted by authors' and publisher's permis- 

 sion from Cold Spring Harbor Sy?nposia on 

 Quantitative Biology, vol. 16, 1951, pp. 65-72. 



As Beadle and Tatinn pointed out (see p. 166) in their introduc- 

 tio?i of Neurospora to genetics, the concept of the direct one-to-one 

 relationship of genes and enzyvies had bee?! suggested and hijited at 

 by many authors prior to themselves. There can be little doubt, 

 honjcever, that the %vork on the bioche?nical syntheses of Neurospora 

 rapidly became the most solid and believable basis for the one gene- 

 one enzyme hypothesis. Careful reading of the Beadle and Tatum 

 paper, particidarly i?i the experimental procedures, will show it 

 was to be anticipated that these tests would residt in evide?ice of a 

 one gene-one enzyme relationship, because more complicated rela- 

 tionships would not be demonstrable by these procedures. This 

 difficulty was disturbing even to the supporters of the hypothesis. 

 hi the following paper, Horowitz and Leupold describe a series of 

 experiments which elimifiate the essentially negative aspect of pre- 

 vious studies, and which give conclusive and positive evidence as to 

 the frequency of occurrence of the ujiitary relatiojiships of genes 

 and enzymes. It is delightful to note the way their evidence supple- 

 ments earlier knowledge, and at the sauie time implements further 

 ifivestigation. Thus, indeed, does a science grow. 



The assumption that a given gene production of but a single enzyme has 

 is involved, in a primary way, in the been implicit in most speculations on 



