38 L. L. Cavalli-Sforza 



differences in growth rate between mutant and parent in drug- 

 free medium. Corrected figures for the numbers of spon- 

 taneous mutants are given in the last Hne of Table II. When 

 this was done for 18 cultures, no significant deviation was 

 found from the hypothesis that all resistants are the con- 

 sequence of spontaneous mutations. 



Some precautions 



The following precautions should be taken in applying this 

 test: 



(1) The growth rate of mutants is usually lower than that 

 of the normal sensitive. In 18 independent mutants, the dis- 

 tribution of relative growth rates k given in Table III was 



Table III 



Distribution of the Growth Rates of Streptomycin-resistant Mutants 

 k = Growth rate of resistant relative to growth rate of sensi tive 



k Number of strains 



Total 18 



obtained, where k is the ratio between the growth rate of the 

 resistant and that of the sensitive parent in mixed culture. 



If A^T^l, the expected numbers of resistants per culture 

 differ from those expected on the basis of the ratio of increase 

 in total cell numbers (the "multiplication factors" in Tables I, 

 II and IV). Table IV gives the expected numbers of resistants 

 for some k values. When k is small the "enriched" mutants 

 are not easily sorted out from the background of new mutants. 



(2) In some circumstances, e.g. chloramphenicol resistance, 

 first-step mutation does not determine a high level of resist- 

 ance, and resistants may be incompletely recovered in tests 



