THE MUTABLE UNIT OF HEREDITY 45 



but a single mutant. It is clear that early occurring mutations give 

 rise to more mutants than late occurring ones. In each case of mutation 

 represented in Figure 2.3, only one event has occurred, yet there is a 

 maximallv broad distribution, from zero to four, of the numbers of 

 mutants present at the end of growth. You may be struck by the fact 

 that the number of clones with four mutants is represented as half of 

 that with two mutants, which, in turn, is half of that with one mutant. 

 This is because, if the chance for mutation is constant per bacterium 

 per generation, the event is likely to occur one quarter and one half 

 as frequently when there are one quarter or one half as many bacteria 

 present. As a consequence, a series results in which the number of 

 clones with two or more mutants is about half, but not quite half, of 



TABLE 2.1 



Agreement between the Observed Number of Clones with DifFerent Numbers 



of Mutants in Phage and Bacteria and the Number Expected when Mutation 



is a Very Rare Event Occurring at Random in Small Populations 



Phage Mutants (T2, r, and w)° 



Mutant Particles in the Phage Observed Number Expected Number 



Yield from Single Bacteria of Clones of Clones 



1 or more 



2 or more 

 4 or more 

 8 or more 



16 or more 

 32 or more 

 64 or more 



Bacterial Mutants (£. coli, str-r, and str-d)\ 

 Divisions Prior to Number of Descendants Observed Number Expected Number 



"From Luria, 1951, C.S.H. Symp. Quant. Biol., 16:463. 

 f From Newcombe and Hawirko, 1949, /. Bact., 57:565. 



