Kinetics of Phenotypic Expression 205 



induced enzyme formation (this symposium, p. 78) and prob- 

 ably reflects the fact that both are the end-result of enzyme 

 synthesis. 



The curve of expression of valine resistance closely follows 

 that of segregation, so that the gene determining this character 

 is dominant and, like the selected genes T+L-j-, is fully 

 expressed very shortly after entry into the recipient cells. 



REFERENCES 



Cavalli, L. L. (1950). Boll. 1st. sieroter. Milano, 29, 1. 



Demerec, M. (1946). Proc. nat. Acad. Sci., Wash., 32, 36. 



Hayes, W. (1953). ColdSpr. Harb. Symp. quant. Biol., 18, 75. 



Hayes, W. (1957). J. gen. Microbiol., 16, 97. 



Kelner, a. (1953). J. Bad., 65, 252. 



Lederberg, J. (1949). Proc. nat. Acad. Sci., Wash., 35, 178. 



Lederberg, J., Lederberg, E. M., Zinder, N. D., and Lively, E. R. 



(1951). ColdSpr. Harb. Symp. quant. Biol., 16, 413. 

 Lederberg, J., and Tatum, E. L. (1946). ColdSpr. Harb. Symp. quant. 



Biol., 11, 113. 

 Newcombe, H. B. (1948). Genetics, 33, 447. 

 Ryan, F. J. (1954). Proc. nat. Acad. Sci., Wash., 40, 178. 

 Ryan, F. J. (1955). Amer. Nat., 89, 159. 



WiTKiN, E. M. (1956). ColdSpr. Harb. Symp. quant. Biol., 21, 123. 

 WoLLMAN, E., Jacob, F., and Hayes, W. (1956). Cold Spr. Harb. 



Symp. quant. Biol., 21, 141. 

 Zinder, N. D. (1953). ColdSpr. Harb. Symp. quant. Biol., 18, 261. 

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DISCUSSION 



Lederberg: There is one assumption that has to be considered further 

 in this type of analysis, and that is whether the fact that the inhibitor 

 prevents a cell from undergoing a further div^ision likewise prevents it 

 from completing that process of phenotypic development which can lead 

 to the resistant phenotype. In other words, I very strongly suspect that a 

 cell which has both an azide-resistant and an azide-sensitive gene may 

 still be at least partly sensitive to azide in terms of division, but never- 

 theless be capable of completing those particular synthetic processes 

 which are required for the development of azide resistance. In our tests 

 on the dominance of azide resistance in diploid, we ran into some trouble 

 because the markers could not be scored on synthetic medium and had 

 to be scored only on a complete medium. My impression was that there 

 was an intermediate degree of sensitivity. 



Hayes: I have been worried about this very point regarding the azide. 



