INBRED AND HYBRID ANIMALS AND THEIR VALUE 337 



chromosome balance. Therefore, the average and variation of a char- 

 acter may, and usually do, differ in the two sexes. 



The Value or Inbred Lines in Research 



The variety of ways in which inbred lines have already been used in 

 research is extensive enough to warrant an attempt at classification. In 

 the space available here the classification can be illustrated only by a few 

 examples. The two main headings (value of genetic effects and value of 

 phenotypic eft'ects) given below are based on a division of experiments into 

 those in which the emphasis is on the genetic constitution of the stock and 

 those which are concerned with the phenotypic nature of the stock irrespec- 

 tive of its genetic explanation. To date, most of the experiments in the 

 former group have been made by geneticists. 



The Value of the Genetic Effects of Inbreeding 



Discovering major gene differences in cases obscured by variation in 

 modifiers or environment. — The value of inbreeding, here, lies in the pos- 

 sibility it aftords of obtaining at least one of a pair of alleles in homozygous 

 condition along with a uniform set of modifiers. Wright's analysis, reviewed 

 m 1936 (22), of white spotting in the guinea pig provides an excellent 

 example. By crossing self animals with inbred spotted strains and repeat- 

 edly backcrossing to the spotted, he was able to show that this character 

 is determined by a major pair of alleles even though it is greatly affected 

 by modifiers and environment. 



Estimating the relative importance of heredity and environment. — The 

 importance of environment can be judged by the amount of phenotypic 

 variation remaining in an isogenic inbred line. The importance of heredity 

 in the foundation stock can be estimated from the amount by which varia- 

 tion is reduced by inbreeding. This has already been discussed on p. 335, 

 where it was pointed out that, by this criterion, tissue specificity is deter- 

 mined largely, if not entirely, by genetic factors; while variation in amount 

 of white spotting in piebald guinea pigs is determined largely by environ- 

 mental factors. 



Unless a suitable transformation of scale can be made (see footnote, 

 p. 334) the method is not applicable for an estimate of the importance of 

 heredity when inbreeding results in increased variation. Its use in any 

 case should be guarded by a consideration of the type of scale upon which 

 the character is measured. See Wright and Chase (22) for an example of 

 correct usage. 



