46 



CELL HEREDITY 



3-4 5-8 9-16 17-32 33-64 >64 

 Number of Mutants 



FIGURE 2.4. Actual distribution of the numbers of lactose fermenting mutants of 

 E. col. in different cultures of nonfermenters compared witfi the theoretical distribu- 

 tion calculated according to Lea and Coulson. The probability of drawing a sample 

 like that observed from a population of cultures in which mutant numbers were dis- 

 tributed according to the theory is 0.2 (after Ryan, 1952, Nature, 169:882). 



the number with one or more mutants, and so on. In Figure 2.3 the 

 number with one or more is seven; with two or more, three; and with 

 four or more, one. As the maximum size of the mutant clone is in- 

 creased, there is a closer approach to one half. This pattern has been 

 observed for both bacteria and bacterial viruses (Table 2.1 ). 



In these considerations, the assumption has been made that mutation 

 is a very rare event so that the overwhelming majority of cultures have 

 none. Only when they are rare is it likely that two or more mutations 

 will not occur in the same clone. In case they did, one could not be 

 sure, by observing only the final number of four mutants, whether they 

 arose by four mutations in the last generation, two mutations one genera- 

 tion back, or one mutation two generations back. Clearly, as the new 

 mutants grow, mutations and mutants no longer equal one another in 

 number. In usual practice, although the chance of mutation is small, 

 the number of bacteria is so large that the occurrence of several muta- 

 tions in a single large clone is a realizable event. Luria and Delbriick 

 predicted and observed the great variation in numbers of mutants among 

 different cultures (Table 2.2), and Lea and Coulson in England gave a 



