BEHAVIORAL DIFFERENCES 



287 



of audiogenic seizures. A number of investigators 407, 13ii have demonstrated that the 

 pattern of age-susceptibility to convulsions differs among strains. Frings and Frings 

 have pointed out that one could obtain almost any genetic ratio desired by conducting 

 tests at an appropriate age. Classification of individuals into reactors and nonreactors 

 is thus often a function of some underlying developmental process. One must be 

 cautious in interpreting such a classification in terms of a dichotomy of genotype. 



Fig. 38. Average number of barks given by five different breeds of dogs in competition 



FOR A BONE AT DIFFERENT AGES. 



From top to bottom, the breeds are the cocker spaniel, beagle, Shetland sheepdog, 

 wire-haired fox terrier, and "barkless" basenji. 



Controlling environmental contributions to variation. — In all genetic experiments it is 

 essential to separate those differences between individuals produced by biological 

 inheritance from those produced by environmental differences. In the case of behavior, 

 the range of environmental effects is much greater than for such characters as coat 

 color or blood type. Some characteristics of an organism such as body size are, of 

 course, determined in large measure by environmental factors. However, the in- 

 vestigator of physical characters need not usually pay as much attention to the previous 

 history and to the conditions of measurement as the behavioral geneticist. Maternal 

 effects upon behavior, for example, extend beyond the uterine period through lactation 

 and, in some species, far beyond weaning. Maternal effects may be transmitted by 

 means as diverse as chemicals diffusing through the placental barrier, composition 

 and amount of milk, amount of sheltering of the young, and age of weaning. Thus, in a 

 recent study of inheritance of behavior in the dog, F x males were backcrossed to the 

 parental stock females in order to compare backcross and F x generations reared by the 

 same mothers. Such a design helps to separate environmental and genetic sources 



