336 BIOLOGY OF THE LABORATORY MOUSE 



found that the standard deviation of a random bred stock was decreased 

 only 23% by inbreeding. 



Increased variation following inbreeding. — The most careful studies of 

 this, as of many other effects of inbreeding, have been made by Wright. 

 To give one example, Wright (19) obtained several inbred strains of guinea 

 pigs that showed more variation than the random bred control stock in 

 respect to number of digits. In one strain, 69% of the animals showed 

 various grades of development of an extra toe on the hind foot ; whereas the 

 random bred control stock showed less than 1% with any development of 

 an extra toe. Wright showed that, within each strain, inbreeding had 

 eliminated genetic variation influencing this character. The increased 

 phenotypic variation could be attributed only to fixation of genotypes that 

 resulted in strains more susceptible to environmental variation affecting 

 development of toes. 



The Jackson Laboratory C57 black strain of mice shows more variation 

 in development of eyes than that recorded for random bred stocks. In 

 some sublines more than 20% of the females exhibit eye abnormalities rang- 

 ing from slight cataract to an eyeless condition, although it has been found 

 (unpublished data of the author) that within sublines there is no genetic 

 variation affecting this character. 



Green (3) has shown that the Bagg albino strain of mice exhibits a 

 variation in number of presacral vertebrae that is probably greater than that 

 of the original stock prior to inbreeding. 



In several strains of mice variation in development of tumors is greater 

 than that characteristic for random bred stocks. 



Other cases can be found in the literature. Many more have 

 undoubtedly occurred. The fact that they have not been reported may 

 be due to two causes. First, workers have not been on the look-out for 

 this effect, because most geneticists have stressed the decrease in genetic 

 variation, but have not pointed out how increased phenotypic variation 

 might occur. Second, early work by geneticists was naturally focussed 

 on such characters as intensity of coat color and tissue specificity, that is 

 characters which are not much aft'ected by environmental variation, and 

 which are, therefore, likely to exhibit decreased variation in inbred strains. 



Different Effects in the Two Sexes 



It should be remembered that even when sex-linked genes have become 

 fixed in an inbred line the sexes will still differ genetically in their sex 



