262 



BIOLOGY OF THE LABORATORY MOUSE 



tumors in their incidence and relationship to various genetic factors present 

 interesting contrasts with the mammary group. We may review briefly 

 certain of these differences. 



Absence of extra-chromosomal influence. — It will be remembered that 

 reciprocal crosses between strains that were "high" and those that were 

 "low" in breast tumors gave very different results. Such is not the case in 

 similar crosses between "high" and "low" lung tumor strains. 



Lynch (63, 64) gave the first data, describing in a preliminary way crosses 

 of this sort between two inbred strains. She mentioned no difference 

 between reciprocal crosses but also gave no figures to differentiate between 

 them. This was not surprising, for, at that period, no such distinction 

 between reciprocal crosses had been described for mammary tumors where 

 later it was found to exist. A later paper by the same writer (65) gave 

 further results of a similar nature. 



The most conclusive data on this point, however, are those of Bittner (23) 

 who, having corrected his figures by the elimination of the disturbing factor 

 of "breast tumor" incidence, found the results shown in Table 10. 



Table 10 



There is no evidence, therefore, of "extra-chromosomal" influence. 



"Dominant" nature. — Lynch (63) gave as one of her conclusions the 

 statement that the tendency to form lung tumors in mice appeared to be 

 "dominant" in heredity. 



She, however, quite properly recognized the fact that dominance was far 

 from being regular or complete. 



Again, her later work as well as that of Andervont and Bittner (i, 13) 

 provided further supporting evidence. 



It remained for Bittner (26), however, to give the most complete data 

 on this question. Using, from his data, comparable groups of mice in differ- 

 ent generations, we find the results shown in Table 11. 



