332 



BIOLOGY OF THE LABORATORY MOUSE 



stock. There are, then, two mam paths by which a character may be 

 influenced by genes: the direct path within the individual, and the path 

 through the ''biological environment" from the genetic constitution of the 

 stock (including, conceivably, the genes of the individual), (Fig. 133). 



GENETIC 



CONSTITUTION 



OF STOCK 



BIOLOGICAL 

 ENVIRONMENT 



CHARACTER 



GENETIC 

 CONSTITUTION 

 OF INDIVIDUAL 



Fig. 133- 



To the genetic factors we can now add the remaining cause determining 

 the phenotype, namely the "physical environment," using this term to 

 denote environmental influences which are not aft'ected by the genetic con- 

 stitution of the stock (Fig. 134). 



Fig. 134. 



The distinction between two major paths of gene action is, perhaps, not 

 essential for an understanding of the rest of this chapter, but it is hoped 

 that it will be of use in emphasizing two points: first, that a character 

 may be influenced by genes other than those of the individual bearing the 

 character; and second, a corollary of the first, that some so-called "environ- 

 mental" factors may be under genetic control. Since the genetic effects of 

 inbreeding can act on the phenotype through the "biological environment," 

 as wefl as through the individual, their potential results are greater than is 

 often supposed. 



