GENETICS OF INFECTIOUS DISEASES 399 



The really big decrease in survival came when all the cells of the body were exposed ; 

 but 20 per cent of this decrease was due to direct effects on the mid, head, or rear 

 regions. The 80 per cent was accounted for by the absence of any unexposed cells. 

 It mattered little which of the three body regions was left unexposed. Each third 

 contributed much more than its share to typhoid resistance if it escaped exposure 

 to the irradiation. These results are taken to mean that, when exposed to a disease 

 of this type, all cells of the body retain much of their initial genetically controlled ability 

 to resist disease invasion even when they differentiate into highly specialized tissues. 

 From the methodologic viewpoint, high-energy irradiation offers a valuable means of 

 specifically depressing the functions of individual organs so far as their properties of 

 resistance to disease are concerned. Greater changes may be accomplished by whole- 

 body irradiation when all cells are irradiated. The effects are genotype specific to 

 some degree but are of similar kind for all genotypes. 



Methodologic problems pertaining to the basis of genetic resistance to disease 

 such as differences in leucocytes and serum-proteins ; growth of heart, kidney, spleen, 

 and other organs; active- and passive-immunity phenomena; mutation and other 

 similar changes in hosts or pathogen; and diverse environmental effects are all part 

 of this complex pattern of disease. Techniques for discriminating between various 

 parameters are available, and introductory references to this material may be found in 

 the bibliography. 



SUMMARY 



The expression of infectious disease is dependent on complex multivariate forces 

 of host and pathogen genotypes as well as on a wide variety of environmental influences. 

 Quantitative studies of disease have largely depended on restricting the range of varia- 

 tion exhibited by two of these three variables. The methodology by which these 

 restrictions have been imposed and some of their consequences have been discussed 

 for host genotypes and breeding behavior. These include the numbers of pathogens 

 and their route of invasion during initiation of the disease as well as radiation as a means 

 of altering qualities of disease resistance of the host. 



DISCUSSION 



Dr. Burdette: Dr. J. A. Weir of the University of Kansas will open the discussion 

 of Dr. Gowen's paper. 



Dr. Weir: Mice are expensive to maintain and require considerable space; 

 consequently, workers employing mice exclusively are committed in a way that leads 

 to specialization and narrowness in research interests. The role of the discussant, 

 as I see it, is to relate the material presented to the larger picture. I find this a difficult 

 task because Dr. Gowen paints with a broad brush, and he follows the axiom of genetics 

 that problems are important and materials only secondary. Although he has limited 



