342 BIOLOGY OF THE LABORATORY MOUSE 



phenotypic characteristics. Therefore, most of the uses which we have 

 Hsted for inbred strains apply equally well to hybrids. It must be remem- 

 bered, however, that hybrids will not breed true and that they can be 

 obtained only by maintaining two inbred stocks. In the following respects 

 they are sometimes of more value than inbred strains. 



Hybrid Vigor 



Some workers who are anxious to eliminate genetic variables from an 

 experiment have, nevertheless, rejected the use of inbred strains because 

 they commonly show decreased vigor. Since hybrids usually combine a 

 high degree of vigor with their genetic uniformity it is surprising that they 

 have not been used more. They are admirably suited for assay tests of 

 hormones and vitamins and for most experiments in which a healthy, 

 vigorous animal is required. The fact that they are commonly highly 

 resistant to disease should be of value to the bacteriologist. For research 

 which requires genetic uniformity, but not the special characteristics of a 

 certain inbred line, hybrids are to be preferred because their vigor makes 

 them more economical to raise. 



The Great Variety Available 



As the number of inbred strains being maintained increases, the number 

 of hybrids made potentially available increases much more rapidly. Thus, 

 25 pure strains can produce 300 hybrids (600, if reciprocals are listed 

 separately). There are probably many more than 25 inbred strains of mice 

 available. While many of these have been thoroughly investigated, only 

 a few of their hybrids have been produced and examined. Here, then, is a 

 wealth of genetically uniform material which is almost untapped. 



Special Uses 



We have already discussed the factors which may cause reciprocal 

 hybrids to differ. The obtaining of reciprocal hybrids is of value when 

 information on the importance of these factors is required. The staff of 

 the Jackson Laboratory (8) reported differences between reciprocal hybrids 

 in mammary tumor incidence in mice. This led Bittner to a discovery of 

 the important post-natal maternal influence on this character (Chap. 9). 

 In collaboration with E. L. Green the author is investigating skeletal dif- 

 ferences in reciprocal hybrids obtained in three different crosses between 

 inbred strains of mice. Here, again, it is hoped that the obtaining of a 



