GENETICS OF INFECTIOUS DISEASES 391 



Fig. 50. Relationship between survivals and days survived for unselected and 



ELEVENTH GENERATION S MICE SELECTED FOR RESISTANCE. 



Unselected Mice 



\V2xl0 5 



7 vX 



i V i >— 



Mice Selected for 



Resistance 



Nth Generation 



J L 



5 10 15 20 

 DAYS SURVIVED 



5 10 15 20 

 DAYS SURVIVED 



The mice were treated with the pathogen, S. typhimurium, in three dosages, 2 x 10 s 

 2 x 10 6 , and 1 x 10 7 organisms. 



only 4 per cent of the unselected mice survived. The median dose definitely caused 

 median mortality. In the eleventh generation, 76 per cent of selected mice survived; 

 whereas only 2 per cent survived in the unselected population. The high dose of ten 

 million organisms was rapidly and completely lethal to the unselected mice, but 60 

 per cent survived during the eleventh generation of selection. 



Not only did the selected mice have fewer deaths than those of the unselected 

 group from which they came, but also the distributions of the deaths took different 

 forms. The frequency curves in per cent of total deaths for those mice which died in each 

 group are given in figure 51. At the highest dose (1 x 10 7 ), the deaths occurred early 

 in the unselected group, the modal value being between 4 and 5 days, whereas in the 

 selected group the modal deaths were one or two days later. The mode for the unselected 

 group was more than twice as high as that for the resistant cohort. The range over 

 which deaths in the unselected group occurred was reduced to half that for the mice 

 of the eleventh generation. The syndrome obviously changed. The intermediate 

 dose acted on the selected and unselected mice in a similar manner. The modal day 

 for death was 9 days after inoculation. The proportions of deaths on that day were 

 about equal. Deaths occurred later in the unselected mice. It is difficult to compare 

 the results for the lowest dose. The main effect observed was an increase in the 



