28 SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 



quently, if such a strain is out-crossed to another race (that in- 

 troduces the allelomorph of the postulated gene), the number of 

 Fi offspring- that develop the specific cancer would be half as nu- 

 merous as in the original cancer strain (since the gene in question 

 occurs only half as many times as in the original complex). In 

 the F 2 generation the frequency for the extracted double domi- 

 nant will be that of the original strain, that of the F 2 heterozy- 

 gotes will be the same as that of the F 1} and the extracted double 

 recessive class will not develop cancer at all. Now, if it is not 

 possible to distinguish between these different F 2 classes by in- 

 spection, the difficulty of finding out how cancer is " inherited " 

 would be very great. In such an imaginary situation, the ratio of 

 cancer-developing mice may not appear to correspond to any of 

 the known Mendelian ratios, because superimposed on the genetic 

 situation there would be added results depending on the fre- 

 quency of mutation when a specific gene is present. 



Other complicating conditions will also suggest themselves to 

 any one familiar with genetic and mutation processes ; for, the pos- 

 sibility that the mutation itself is more or less likely to occur in 

 one or another genetic complex must be reckoned with, as well as 

 the likelihood of the mutation showing itself or developing in 

 any tissue or only in cells of specific tissues, etc. 



I am far from wishing to suggest that spontaneous cancer is 

 a mutational process, despite certain rather obvious resemblances 

 to mutational effects in plants and animals, but I should like to 

 insist that the appearance of spontaneous cancer is in its nature 

 so peculiar that one can not afford to ignore such a possibility in 

 any discussion as to whether spontaneous cancer is or is not " in- 

 herited." 



There are several cases of inheritance of tumors in our Dro- 

 sophila material. Here I am on safer ground. One of them, 

 discovered by Dr. Bridges, worked out by Dr. Stark, I should 

 like to speak about, because it shows how linkage of characters 

 can be used in the study of heredity of a character and conversely 

 in its elimination. In a certain culture one fourth of the mag- 

 gots develop one or more black masses of pigment in the body, 



