50 



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RGUSE 2_5. Actual distnbution of nL~;- : zrtose-utilizing mutations in different 

 c o loni e s ol E. coB compared with tfie Pci:-- z i-.oution. The probability of drowing 

 a sample fike that observed from a population of colonies in which tfie mutations 

 were detributed according to ffieory is 0.2 Ryan, unpublished). 



mutates to conditions where it is either only methionineless (hii~ met~ ^ 

 or histidineless hvf met' ' but it is never obsen ed to back-mutate in 

 Mie step to the original nonrequiring. prototrophic state his' met~\ 

 This suggests that mutation in the two genetic units is not a Unked 

 process but that one mutable unit can mutate independently of the other: 

 the chance that the two will mutate in the same cell is the ver\' small 

 product of the chances of mutation in the independent genes (e.g.. 

 lO"** X 10""* = 10"^*^). Furthermore, when the number of mutants of 

 the two sorts is determined in many different cultures, no correlation is 

 found (Figure 2.6 >. We may conclude, therefore, that mutation in the 

 gene determining the histidineless condition is independent of mutation 

 in the gene for the methionineless state, and. once again, that most of the 

 cells are equally mutable. This free behavior in mutation has been 

 found for most genes studied: the status of some apparent exceptions is 

 uncertain because of alternative structural and physiological {possibilities. 

 Independence in mutation demonstrates that stability is an independent 



