THE GENETICS OF SPONTANEOUS TUMOR FORMATION 259 



Evidence that genetic factors also influence the incidence of breast 

 tumors. — Perhaps the most interesting experiments to test this point are 

 those recently conducted by Bittner (25, 26). These are based upon the fact 

 that reciprocal crosses between "high" (A) and "low" (C57 black) breast 

 tumor strains produce very different degrees of tumor incidence in Fi. 

 These were compared with the pure "high" and "low" tumor stocks by 

 fostering young in both series.' 



The results are summarized in Table 7. 



According to Bittner 's theory all Fi hybrids whether produced from 

 H 9 X L cf or from L 9 X H d^ should carry one group of "high" tumor 

 genes from their high parent. When to this genetic tendency the extra- 

 chromosomal influence is added, the results are very different from the 

 parallel fostering between the pure stocks, one of which lacks the "high" 

 tumor genes. Thus pure "low" stock females fostered by "high" stock 

 produce only 9% breast tumors, while Fi females, themselves ''low," produce 

 93% breast tumors when fostered by high tumor females. 



While further experiments are necessary, the evidence at present favors 

 Bittner's theory that some influence of genes is active. 



Evidence that hormonal influences also affect the incidence of breast 

 tumors. — Primary evidence for this theory is to be found in the comparative 

 behavior of breeding and virgin females in three high tumor strains of mice. 

 These strains are designated respectively as C3H, dba and A. The approxi- 

 mate incidence of breast tumors is shown in Table 8. 



Table 8 



It is very evident that the absence of pregnancy and lactation has a 

 markedly different effect in the three strains. 



Further evidence -of hormonalinfiuence has been derived from the experi- 

 ments of Bagg and others (5-7.) who have shown that forced breeding with- 

 out opportunity for nursing- increases the incidence of breast cancer in 

 animals where some genetic tendency to form such cancer exists. 



There is also a series of experifnents involving the artificial prolongation 

 of lactation and nursing in mice reported by Fekete (40). In this case the 



