DELAYED EXPRESSION OF MUTATIONS 

 Table 6 



463 



Mutation rateof B/r to resistance to phage Tl, estimated from the number of resistant clones appearing 

 during bacterial multiplication on agar (method 4 of this paper). 



* Each replicate mutation rate is calculated from an independent single-plate estimate of each 

 of the following four values: (1) number of bacteria in the inoculum, (2) end number of bacteria, 

 (3) number of resistant bacteria in the inoculum, and (4) end number of resistant clones, using 

 formula 4 of this paper. 



t This mutation rate is calculated from a tenfold increase onward using the information in 

 figure 1 and the method outlined in the section on method 4. It is a more accurate estimate of the 

 rate of formation of resistant clones during logarithmic growth, since the bias from the high early 

 rate of appearance of resistant clones is removed. 



resistant clones appears, and reduces the average rate obtained from these 

 experiments from .59X10" 8 to .51X10- 8 . 



RATE OF APPEARANCE OF RESISTANT MICROCOLONIES DURING THE EARLY 

 DIVISIONS ON A SOLID MEDIUM 



Where there is a delay between mutation and phenotypic expression such 

 that the first phenotypically resistant individual appears n generations after 

 the mutation has occurred, there will be 2 n bacteria in the mutant clone when 

 it first becomes detectable. When a mutation occurs, a delay of n generations 

 in the time of its appearance will correspond to a 2 n -fold increase in the popu- 

 lation. The apparent number of bacterial divisions in which the mutation has 

 occurred will then be 2 n times the true number, and when an estimate is made 

 from the number of detectable mutant clones, the apparent rate will be reduced 

 to 



6s 



