286 PHYSIOLOGIC GENETICS 



aa genotype is so resistant that it will not convulse under ordinary circumstances, since 

 compensatory reactions occur within the nervous system before a convulsive level of 

 activity is reached. It is now further assumed that the Aa genotype is poised upon a 

 genetic threshold such that the occurrence or nonoccurrence of a convulsion is dependent 

 upon environmental circumstances. This particular model has been employed to 

 account for the pattern of inheritance of convulsions in crosses between DBA/2 and 

 G57BL/6 mice 409 with the added postulation that the threshold was dependent upon 

 multiple loci rather than a single locus. The evidence for multiple factors was derived 

 from breeding experiments. There are good reasons to believe that threshold situations 

 of this sort may be extremely frequent in the field of behavior. 1187 Certain features 

 of this model should be carefully noted. In the first place, a heterozygote theoretically 

 might be expected to be particularly adaptable since its behavior is a function of the 

 immediate environment; but this very fact implies greater variation in behavior, hence 

 lower reliability of measurement. A second feature of the model is that the herit- 

 ability of a threshold character decreases as the incidence of the character approaches 

 50 per cent. 245 This fact creates problems in selection for such characters and in the 

 interpretation of Mendelian experiments. 



GENERAL METHODS 



The developmental method. — Learning itself may be considered as a process of func- 

 tional differentiation of behavior. 1186 This implies that the nervous system of a 

 mammal is differentiating throughout life in the way that most other organ systems 

 differentiate during the embryonic period. Consequently, the developmental method 

 becomes extremely important in studying the effect of heredity upon behavior. 



It is almost a truism to state that any phenotypic character is not inherited but 

 developed. The importance of this concept in the genetic study of behavior rests on 

 the fact that a very large proportion of the behavioral development of a mammal 

 takes place after birth and proceeds far into adult life. A given type of behavior may 

 be completely absent early in development, may appear in one form a little later, 

 and in still another form at a later age. This may enormously complicate the usual 

 method of analyzing genie mechanisms from a single phenotype taken at one age. 



An example of changes in a trait with age is our study of the inheritance of barkless- 

 ness versus barking in dogs (figure 38) . At 5 weeks of age, barking was extremely un- 

 common in the test situation, and there was little difference between the breeds. At 1 1 

 weeks of age, barking had risen to a maximum, and there was a wide gap between the 

 barkless basenji and the other breeds. By 15 weeks the rate of barking was declining, 

 so that it was again difficult to separate breeds on the basis of this character. In such 

 cases, the necessity of the developmental method is obvious, as is the problem of 

 determining just what is a barkless dog. We are still not sure whether barking should 

 be treated as a unit character or as a quantitative character. 



Another example of the change of behavioral phenotype with age is the syndrome 



