204 W. Hayes 



120 minutes after plating and thereafter multiply with a 

 generation time of about 60 minutes (Fig. 1 A). In nutrient 

 broth at 37° (Fig. 1 B), the first division of recombinant 

 segregants is initiated at 100 minutes after diluting the 

 zygote suspension into fresh broth, and the generation time is 

 20 minutes. 



When the kinetics of phenotypic expression of either 

 sodium azide or phage Tl resistance are plotted in terms of the 

 generation time of the recombinants, closely similar results 

 are obtained on synthetic minimal agar, nutrient agar and in 

 nutrient broth. The patterns of expression of these two 

 characters, however, are very different. Expression of the 

 character Az'^ (Fig. 1 B) begins at the time of dilution, or 

 of plating, of the zygotes and then rises exponentially to 

 become complete just before the recombinants which inherit 

 it start to divide. There is good evidence that during the 

 greater part of this period, at least, the genes Az^ from the 

 donor and Az^ from the recipient parent must be present 

 together in the partially diploid zygote. In fact, it is likely 

 that the early stages of expression occur before the process 

 of recombination proper begins (Wollman, Jacob and Hayes, 

 1956). From this it follows that the gene Az^ is dominant 

 to Az^ 



In contrast, resistance to phage Tl does not begin to be 

 expressed until after segregation, while full expression is 

 delayed until the fourth recombinant generation (Fig. 1 A). 

 This is in conformity with the finding of Lederberg (1949) that 

 phage Tl resistance is recessive to sensitivity in Esch, coli 

 diploids. The duration of the phenotypic lag also accords well 

 with that estimated by indirect methods by Newcombe (1948) 

 for the expression of phage resistance in spontaneous mutants. 

 The coincidence of the commencement of expression of this 

 character with initiation of the first segregant division 

 strongly suggests that at this time the recombinant cells make 

 their first appearance as independent units. 



The exponential nature of the rise in phenotypic expression 

 of both characters is reminiscent of Dr. Pollock's curves for 



