INBRED AND HYBRID ANIMALS AND THEIR VALUE 345 



separate parts of the laboratory or cage rack, certainly not in adjacent 

 sections of wooden boxes. When several inbred lines are to be started it is 

 desirable to mark them with different coat colors or other genetic char- 

 acteristics contamination of which will be readily recognized. 



Effect of Relaxing Inbreeding 



If a strain is to be maintained with maximum homozygosis, there should 

 be no relaxation of inbreeding. Relaxation of inbreeding in a population 

 containing different sublines would, of course, introduce heterozygosis 

 immediately. It should be avoided even in an isogenic line; for it would 

 tend to preserve heterozygosis introduced by mutation. Haldane (5) has 

 discussed this effect. 



There is one case, however, in which relaxation of inbreeding might be 

 of advantage. If a large group of animals is to be set aside for experimental 

 purposes, it is preferable to set aside a single pair and breed their descendants 

 at random to obtain the experimental animals. The effect of this is to 

 distribute at random throughout the population any genes which are unfixed 

 in the pair set aside. In practice, however, it is usually adequate to take 

 all the animals from the inbred strain provided they have a recent common 

 ancestry. 



Fallacies 



Misinterpretation of Variation within Strains 



Publicity on the genetic uniformity to be obtained by inbreeding has 

 apparently led some experimentalists to expect complete phenotypic uni- 

 formity; although their own observations on variation in such characters 

 as litter size, and weight at weaning, must belie this conclusion. Possibly 

 the extreme uniformity obtained in a few characters like tissue specificity, 

 and coat color, is responsible for this view. At any rate, surprise is some- 

 times ex-pressed when a character is found to show variation in an inbred 

 strain, and attempts have been made to explain the variation away, particu- 

 larly when it shows itself in an all-or-none effect, such as tumor or no tumor. 

 Thus, the fact that some tumors occur in a "low" tumor strain has been 

 attributed to residual heterozygosis, and the occurrence of non-tumorous 

 animals in a "high" tumor strain has been "explained" by stating that 

 these animals would have had tumors if they had lived long enough. These 

 explanations may be true in special cases, but the former cannot apply 

 when the homozygosis has been tested, and the latter will not account for 

 variation in time of appearance, rate of growth, region affected, etc. 



