Cnapter 10 



INBRED AND HYBRID ANIMALS AND THEIR 

 VALUE IN RESEARCH 



By W. Lawson Russell, Roscoe B. Jackson Memorial Laboratory. 



Introduction, 325. Genetic effects of inbreeding, 327. Random mating, 327, 

 Inbreeding, 327. Phenotypic effects of inbreeding, 330. General causes of pheno- 

 typic variation, 331. Measurement of phenotypic variation, 333. Effect of inbreed- 

 ing on the "average," t,s3- Effect of inbreeding on the variation, 334. Causes of 

 a change in variation, 334. Decreased variation following inbreeding, 335. Increased 

 variation following inbreeding, 336. Different effects in the two sexes, 336. The 

 value of inbred lines in research, 337. The value of the genetic effects of inbreeding, 



337. Discovering major gene differences in cases obscured by variation in modifiers 

 or environment, 337. Estimating the relative importance of heredity and environ- 

 ment, 337. Estimating the relative importance of various environmental factors, 338. 

 Recognizing new mutations, 338. The value of the phenotypic effects of inbreeding, 



338. Change in "average," 338. Reduced variation, 338. Increased variation, 338. 

 Uniformity in time, 339. Combination of effects, 339. Differences between inbred 

 lines, 339. Hybrids, 340. Genetic characteristics, 340. Phenotypic characteristics, 

 340. Average, 340. Variation, 341. Differences between reciprocal hybrids, 341. 

 The value of hybrids in research, 341. Compared with inbred hnes, 341. Hybrid 

 vigor, 342. The great variety available, 342. Special uses, 342. The building and 

 maintenance of inbred lines, 343, Selection, 343. Preserving vigor, 343. Tests of 

 genetic uniformity, 343. Sublines, 344. Risk of contamination, 344. Effect of relax- 

 ing inbreeding, 345. Fallacies, 345. Misinterpretation of variation within strains, 

 345. Misinterpretation of differences between strains, 346. Bibliography, 347. 



Introduction 



During the past few years there has been a tremendous increase in the 

 number of inbred animals, particularly mice, used in research. A large part 

 of this increase can be attributed to the efforts of Dr. C. C. Little, who has 

 not only repeatedly advocated the use of inbred material, e.g. (7), but has, 

 with the aid of students and colleagues, established many inbred strains 

 of mice and made them available in large quantity to other research workers. 

 Thus, the number of mice, mostly from inbred lines, supplied by the Jack- 

 son Laboratory to other laboratories has increased from 12,000 in 1933 to 

 120,000 in 1939. 



325 



