INBRED AXD HYBRID ANIMALS AND THEIR VALUE 343 



difference in reciprocal hybrids will lead to increased understanding of 

 variation in the character under investigation. 



Another use of hybrids lies in their common ability to grow tumors of 

 both parental strains. This is of value in research and also provides an 

 economical method for maintaining transplantable tumors. 



The Building and ^Maintenance of Inbred Lines 



Some of the concepts of value to those workers who wish to start or 

 maintain their own inbred lines are apparent in earlier sections. Others 

 are presented below. 



Selection 



Inbreeding is often combined with selection for desired characteristics, 

 for example high or low tumor incidence. The eft'ectiveness of various 

 methods of selection on various types of characters is discussed by Wright 

 (21). It may be pointed out here that, when there is a lot of variation which 

 is not genetic, selection of individuals within a single inbred line (e.g.. 

 among the offspring of a single brother-sister mating) is of little value. 

 Individuals which are good by accidents of environment, and not by 

 heredity, may be chosen and undesirable genes fixed. In these cases selec- 

 tion is most effective when applied to a number of separate inbred lines; 

 for only between them can real hereditary dift'erences be easily recognized. 



Phenotypic variation may remain after a line has become isogenic, but 

 selection cannot change it. In a highly inbred strain selection is, there- 

 fore, of value only for its possible control in fixing desirable, or eliminating 

 undesirable, new mutations. 



Preserving Vigor 



Selection of lines, rather than individuals, applies particularh- to vigor 

 and fertility; for Wright (13) has shown that variation in these characters 

 is determined largely by environment. A satisfactory inbred strain can 

 usually be obtained only by starting a large number of strains. In fact, 

 if only one brother-sister line is started from a heterogeneous stock there 

 is a fair chance that it will die out in spite of selection of the two most 

 vigorous animals in each generation. 



Tests of Genetic Uniformity 



It is often desirable to know whether the variation remaining in a char- 

 acter after considerable inbreeding is due solely to environment or partly 

 to unfixed genes. This can be tested. If there are unfixed genes affecting 



