SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 2 



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there existed another race of men who never died of the malady, 

 and suitable matings were made between the two races. 



Suppose again that all old men died of pneumonia. Could we 

 say that susceptibility to pneumonia, after eighty, is inherited? 

 Again, yes! But again we could get no information as to the 

 way in which this susceptibility is inherited without crossing to 

 an immune race. 



Xow suppose there are strains of mice all of which die of can- 

 cer after their first year. Could we say that in them cancer is in- 

 herited? The answer would depend in part on what connota- 

 tions the word inherit carries with it, for, either susceptibility 

 might be meant, or the "spontaneous" development of cancer 

 might be meant. The latter interpretation is, I think, generally 

 implied, which carries with it two further implications. First 

 implication, viz.. that when a certain age is reached, a certain in- 

 herited complex leads to the development of cancer in one or 

 more regions of the body. Here some such process as that of 

 the hardening of the arteries seems to be vaguely implied. 

 Second implication, viz., that a change in method of growth (a 

 release from the ordinary restraining influences ) suddenly oc- 

 curs, beginning in a single cell of some particular tissue. Stated 

 in this second way, the appearance of spontaneous cancer suggests 

 at once a comparison with the mutation process that is known to 

 occur in somatic cells as well as in germ cells. 



Xow if the first interpretation is to be placed on the word 

 heredity, when applied to cancer, there is nothing more to be said, 

 except that the only way such a situation can be studied as a ge- 

 netic problem is to out-cross the strain of cancer mice in question 

 to another that never develops spontaneous cancer. But if the 

 second interpretation is implied, then the whole situation is put in 

 a very different light. Let us examine this a little more closely. 



Suppose, as a theoretical possibility, that spontaneous cancer 

 is due to a recurrent somatic mutation of a specific gene to a domi- 

 nant one that leads to cancer. Then the proportion of individ- 

 uals that develop spontaneous cancer in such a strain will depend 

 on the frequency of mutation of this specific gene. Conse- 



