24 SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 



criminal has been painted as the ancestral brute from which the 

 more docile human animal has arisen through loss of " wild-type " 

 genes. I need not state, perhaps, that no one takes such specula- 

 tions seriously today from a genetic standpoint. 



Immunity and resistance to disease are subjects of great in- 

 terest to geneticists as well as to pathologists. 



Setting aside, of course, cases where the immunity is due to 

 some temporary physiological state (little understood at present, 

 I believe), and also setting aside immunity acquired by recovery 

 from attack or inoculation, there still remain races that have, as 

 we say, a constitutional resistance. 



The best ascertained cases in this field are those worked out by 

 Tyzzer and Tyzzer and Little. A carcinoma that originated in 

 Japanese waltzing mice grew in practically every individual ot 

 the race when implanted. It failed to grow in " common " mice. 

 The hybrid mice from these two races were also susceptible in 

 nearly every case. 



When the F/s were back-crossed to "common mice" the off- 

 spring were not susceptible. When the F x 's were backcrossed 

 to the Japanese waltzer all were susceptible. When the F/s were 

 inbred only about 2.j per cent, of the offspring were susceptible, 



Fig- 13. 



These results show at least that there must be more than one, 



two or three factor differences between the two races that are 

 concerned with tumor susceptibility. 



Tyzzer and Little suggest in fact that 12 to 14 independently 

 inherited factors are involved. Larger numbers of tests will be 

 necessary before it is possible to state how many factors are 

 needed. A curious feature of the case should not pass unnoticed. 

 A I any or all of the factors for susceptibility must be assumed to 

 be dominant. It is not generally known, but there is some evi- 

 dence that the so-called Japanese waltzer originated from Asiatic 

 house mice, which according to some writers belong to a distinct 

 species or at least a distinct variety. The results suggest that 

 we may be dealing here with species or varietal differences, hence 

 the large number of factor differences involved. It may be nee- 



