!56 



BIOLOGY OF THE LABORATORY MOUSE 



In sharp contrast to this is the low incidence (5.96 per cent) among the 

 BdF2 mice produced by inbreeding the BdFi animals (Table 4). 



The eventual weakening and disappearance of the high tumor producing 

 tendency has been shown by Murray and Little (80) (Table 5) in a series of 

 backcross generations which were intended to provide a test of the relative 

 importance of genes and of other influences. 



The data obtained from first generation animals backcrossed with parent 

 strains showed only a slight decrease in the incidence of mammary tumors in 

 those animals with extra-chromosomal influences (E) derived from "high" 

 tumor female ancestors. These backcross generations are shown in the 



Table 5 

 Incidence of Mammary Cancer 



columns marked B and D in Table 5. The animals descended from "low" 

 cancer females with extra-chromosomal influences (e) showed a greater 

 proportional decrease in incidence of mammary tumors (columns A and C, 

 Table 5). The relative independence of all from chromosomal influences 

 (C = high tumor, c = low tumor) is also shown in this table for A and B had 

 three representatives of C while C and D had only one. 



The system of matings used, in further studies of advanced backcross 

 generations, to concentrate the chromosomes from respective parent strains 

 is shown in Table 6. 



Starting with individuals of the 8th backcross generations (Table 6), 

 which were virtually homozygous, a variety of crosses were made. Indi- 

 viduals from these crosses were identified as follows: S, T, U and V were 

 animals originally derived from maternal ancestors with the "high" tumor 

 extra-chromosomal influence (E). They were, however, eight generations 

 removed from the pure strain originally employed. Had the extra-chro- 

 mosomal influence remained unchanged, there should have been approxi- 

 mately 196 mammary tumors among the 372 mice recorded. Actually there 

 were 6. This is only 3 per cent of the former tumor rate. The extra- 



