46 LABORATORY MOUSE 



mating hairless males to heterozygous females. The resulting 

 hairless males are crossed to their heterozygous sisters, and 

 so on. 



A certain amount of sterility is encountered in short-eared 

 mice. This may or may not be associated in some fashion with 

 the short-ear character, but is complicated in its inheritance. 



The sterility often found among waltzers is usually due to 

 the fact that these animals are very delicate and require the 

 best of care in a very special environment. Under ideal con- 

 ditions they breed well. They are, however, poor mothers 

 and their young should when possible be given to a foster 

 mother of some non-waltzing variety to nurse. 



