DELAYED EXPRESSION OF MUTATIONS 455 



Where the mutation rate per bacterium per division cycle is a, and the rate 

 per bacterial division is a/(ln 2), the proportion of mutant bacteria in a cul- 

 ture will rise during growth by a fixed increment of a/2(ln2) per generation, 

 provided the inoculum is of sufficient size so that there are no appreciable 

 statistical fluctuations in the numbers of mutations occurring in the first divi- 

 sion. Thus mutation rate can be obtained from the formula 



a = 2(ln 2)(r 2 /N 2 - r 1 /N 1 )/g. 



(3) 



Table 2 



Mutation rates of B/r to resistance to phage Tl, calculated from the increase in the proportion of 

 resistant bacteria in cultures grown from large inocula {method 3 of this paper). Short grouth period. 



TEST CULTURE A B C D E INOCULUM 



10 



7.0 

 1.3 



Table 3 



Mutation rates of B/r to resistance to phage Tl, calculated from the increase in the proportion of 

 resistant bacteria in cultures grown from large inocula {method 3 of this paper). Long growth period. 



TEST CULTURE F G H I J INOCULUM 



The data from two sets of experiments (involving 6 generations and 11 genera- 

 tions of growth) are given in tables 2 and 3, together with the mutation rates 

 obtained. These rates average 2.8X10~ 8 and 3.4X10~ 8 , respectively— values 

 which are not appreciably different from each other, or from those obtained 

 by method 2, using the average number of resistant bacteria per culture in 

 cultures started with small inocula. 



Since there is no question of a statistical bias in calculating mutation rate 

 by method 3, the agreement may be interpreted as confirming the higher 



57 



