326 BIOLOGY OF THE LABORATORY MOUSE 



Nevertheless, any geneticist who samples the recent literature in such 

 fields as physiology, biochemistry, bacteriology, pathology, cancer research, 

 and experimental medicine in general, is struck by three points. First, 

 most of the workers who are still using animals of uncertain origin could 

 profit by the use of inbred stocks. Second, even when inbred animals are 

 used, they are frequently not utilized to their full value. Third, owing to a 

 lack of understanding of the consequences of inbreeding, erroneous conclu- 

 sions are sometimes drawn from the results obtained with inbred material. 



As a geneticist, the author of this chapter may perhaps be permitted to 

 blame geneticists for the above failings. They have provided an excellent 

 theoretical analysis of the Mendelian consequences of inbreeding, and an 

 extensive series of critical experiments that have verified theory and brought 

 new facts to light ; but they have expended singularly little effort to sort out 

 and explain those results and conclusions which are of importance to 

 research workers in general. General discussions of inbreeding have been 

 concerned, on the one hand, with the genetic consequences and, on the other, 

 with the relation of these to evolutionary theory, improvement of livestock 

 and domesticated plants, and interpretation of such special phenotypic 

 effects as decline in vigor. Furthermore, of the six recent and better known 

 text-books of genetics only two mention the value of inbred animals in 

 research, and each of these devotes only one paragraph to this topic. 



This chapter was planned to bring together and classify those effects 

 of inbreeding which are of general value to experimentalists who are using 

 mice or other laboratory mammals in their research. Much of what is 

 discussed applies, of course, to other organisms as well. 



For this purpose the most serious gap in the literature is the lack of an 

 adequate treatment of the phenotypic effects resulting from inbreeding. 

 For example, general discussions of inbreeding have implied, if not definitely 

 stated, that the decrease in genetic variation following inbreeding neces- 

 sarily results in decreased phenotypic variation. Yet several cases have 

 been reported in which a particular character shows more variation in a 

 certain inbred line than it does in random bred stocks, or did in the stock 

 from which the inbred line was derived. It has been this author's experience 

 that this effect is a seemingly inexplicable paradox to many students and 

 research workers. It has, therefore, seemed desirable to discuss the pheno- 

 typic effects of inbreeding in more detail than the title of this book would, 

 at first sight, warrant. 



The attempt has been made to present the material of this chapter in a 

 form that can be understood by those not specially trained in genetics. 



