THE GEXETICS OE SPOXTANEOUS TUMOR EORMATION 251 



Mammary Epithelial Tumors 



Evidence that the tendency to form such tumors is not due to the action 

 of a single recessive gene. — Preliminaty evidence was provided by the work 

 of Lynch (61) who, in a series of crosses between various strains of mice, 

 showed that mammary tumors occurred in Fi animals. The strains used 

 were the best then available but left much to be desired as regards the extent 

 of genetic analysis previous to crossing. 



Statistical analysis based on tabulations of Slye's extensive pedigree data 

 by Little (52) showed that her results could be as well explained on the basis 

 of dominance of the tendency to form breast tumors as on its recessive 

 nature. 



Data published by the Staff of the Roscoe B. Jackson Memorial Labora- 

 tory (44) and confirmed independently by Korteweg (46) who published at 

 almost the same time showed, 



1. That Fi hybrids between ''high" breast tumor and "low" breast 

 tumor strains formed large numbers of mammary tumors. 



2. That this result was more in agreement with a theory of dominance 

 than of recessive nature of the tendency to form these tumors. 



Evidence of an extra-chromosomal influence. — These same experiments 

 showed that a most interesting and unexpected difference exists between the 

 reciprocal crosses which produced such Fi generation mice. 



Where the cross was made between "high tumor" strain female and "low 

 tumor" strain male, the rate of breast tumor incidence in Fi generation 

 females approached that of the "high tumor" parent strain. When, how- 

 ever, the cross was made between "low tumor" strain females and "high 

 tumor" strain males, the tumor rate in Fi females was little if any above that 

 of the "low tumor" parent strain. The F2 generations followed closely the 

 rate of mammary tumor incidence shown by the type of Fi generation from 

 which they were derived. These results have now been confirmed and 

 established by the work of a number of investigators. 



Tables taken from ISIurray and Little (77) show the incidence and age 

 distribution of mammary' tumors in the Fi hybrids from reciprocal crosses 

 between the "high tumor" dba strain and the "low tumor" C57 black 

 strain. In Table i is the hybrid generation derived from the cross dba 

 9 X C57 black d" . 



It will be noted that the rate of tumor incidence usually falls between 

 30 and 45 per cent. This may be contrasted with the reciprocal generation 

 (Table 2) produced by crossing C57 black females with dba males. 



