386 



HOST-PARASITE RELATIONSHIPS 



patterns of morbidity which are apparent at different stages in the disease cycle. The 

 pattern of morbidity for each strain is characteristic, indicating the specificity of ex- 

 pression of the disease. 



TRANSMISSION OF DISEASE RESISTANCE IN CROSSES 



Convincing evidence for the dependence of resistance and suceptibility to infectious 

 disease on the strain genotypes comes in repetition of the test results for the different 

 strains generation after generation despite varying environments characteristic of 

 changing seasons and years. However, this is not all the evidence desired. To 

 collect the evidence properly would require an experiment comprising 1 ,600 + cells 

 and possibly 100,000 mice. The time, energy, and space required for this analysis 

 together with the three generations of breeding required have prevented us from doing 

 the experiment up to this time. However, valid data covering critical crosses are avail- 

 able through the work of Hetzer, 573 Zelle, 1467 Weir, 1376 and Gowen and Stadler 

 (unpublished). The combined data for the first generation are given in table 62. 

 The S strain was under intense selection for resistance at that time. 



Table 62 



Resistance of parents and crosses of inbred strains of mice to murine typhoid 

 Salmonella typhimurium 11C, 200,000 organisms 



Res = resistant. Sus = susceptible. P L = parental purebreds. Fi = hybrids. 



Table 62 gives data on 3 resistant inbred lines and 2 that are susceptible. Com- 

 parison with table 61 shows that the S mice of this period in the genetic development 

 of the strain were 5 per cent more resistant than in the 1944-1956 period. The same 

 was true for the mice of the RI strain, 12 per cent, and for the Ba mice, 1.8 per cent. 



