338 BIOLOGY OF THE LABORATORY MOUSE 



Estimating the relative importance of various environmental factors. — 



To gauge the importance of different environmental factors it is naturally 

 desirable to have genetic variation eliminated, or controlled as much as 

 possible. This can be achieved by using inbred strains. Some of the char- 

 acters studied in this way are: Polydactyly (19), and white spotting (22), 

 in the guinea pig; harelip (9), and skeletal variation (3), in the mouse. 

 Recognizing new mutations. — In mammals particularly, geneticists are 

 anxious to find new major mutations and observe the frequency with which 

 mutations occur. The appearance of a mutant animal is usually recog- 

 nized more easily in an inbred strain than in a stock in which there is con- 

 siderable genetic variation. 



The Value of the Phenotypic Effects of Inbreeding 



Change in average. This is one of the most used results of inbreeding. 

 An inbred line frequently provides in quantity a type of animal that is rare, 

 or perhaps never observed, in random bred stocks. Thus, inbred lines of 

 mice are available in which the incidence of certain types of tumors is very 

 high. To mention only two uses, these lines are of value both to experimen- 

 talists who want spontaneous tumor tissue in quantity and to those who 

 want animals known to be susceptible to tumor growths. Other examples 

 of valuable strains are: those with low resistance to carcinogens and those 

 with high susceptibility to bacterial infection. Thus, the susceptibility of 

 C57 black mice proved of use in testing the protective value of typhoid 

 vaccine (10). 



Reduced variation. — Although the preceding effect probably has been 

 used as much as, if not more than, the reduced variation following inbreed- 

 ing, the reduced variation has been by far the most publicized effect. Its 

 value is obvious when, as frequently occurs, inbred lines are superior to 

 random bred animals in their uniformity of response to such experimental 

 treatments as: hormone injections, feeding deficient diets, administering 

 drugs, excision of organs, exposure to carcinogens, and immunity tests. 

 Results are more clear-cut, and a significant difference between experi- 

 mental and control can be demonstrated with fewer animals. 



We have already mentioned the uniformity of tissue specificity found in 

 inbred lines. This has been of great value in work on transplanted tumors, 

 transplanted normal tissues, and parabionts. 



Increased variation. — Increased variation in Polydactyly in the guinea 

 pig, and in eye defects and skeletal characters in the mouse, facilitated 



