52 M. Demerec 



genetic markers with which it has been tested ; in other words, 

 in this respect also streptomycin resistance behaves as a gene- 

 determined character. 



Thus the most reasonable conclusion to be reached from 

 quantitative studies of resistance to streptomycin is that a 

 genie mechanism is responsible for its origin and nature. It 

 would be difficult indeed to visualize an adaptive mechanism 

 that would account for the complex and at the same time 

 precise patterns of action revealed by the genetical studies. 

 The predictability with which changes from dependence to 

 non-dependence occur, and the ease with which they can be 

 observed experimentally, have made it possible to utilize this 

 property in developing one of the most efficient methods now 

 available for the study of induced mutability, and particularly 

 for the detection of mutagenicity among various chemicals 

 (Demerec, Bertani and Flint, 1951). 



Mendelian analysis 



Although there is very little doubt at present that the basic 

 genetic mechanism in bacteria is similar to that operating in 

 higher organisms, bacteria as a group are still notoriously 

 unsuited to simple Mendelian analysis — which continues to 

 be regarded by many as the only source of indisputable 

 evidence of genie inheritance of characters. It seems to me 

 that at the present stage of our knowledge about genes and 

 their action we can afford, in cases where segregation tests 

 cannot be carried out because of a lack of suitable techniques, 

 to accept the results of analysis by other, more recently 

 developed methods. 



Evidence about drug resistance obtained by the recom- 

 bination method, which closely approaches standard Men- 

 delian tests, supports the conclusion that in the bacteria 

 studied by this method a genetic mechanism is responsible for 

 resistance. The recombination method has been used to 

 analyse high resistance to and dependence on streptomycin 

 in strain K 12 of Esch. coli. The results indicate that a single 

 gene locus is involved in the transmission of both resistance 



