CHAPTER XXXIII 

 DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 



In the present chapter not only the subject of disease, but the related 

 subjects of defects and immunity will be dealt with from the genetic 

 standpoint. According to the strict definition of the term, disease is an 

 abnormal process exhibited in some part of the body and dependent for 

 its initial impetus upon an external cause. Obviously if a definition such 

 as this be applied, there could be no such thing as the inheritance of disease, 

 but genetic research has demonstrated beyond the shadow of a doubt 

 that conditions in the body which more or less directly predispose it to 

 disease are inherited. The subject, therefore, deserves treatment adequate 

 to the need of outlining clearly the relation of problems of disease to 

 inheritance. 



The Inheritance of Disease. The problem of the inheritance of disease 

 is one very much like the problem of the inheritance of acquired characters, 

 for it is hedged around with confusion of every day thought and the same 

 type of misconception that characterizes this latter problem. Thomson 

 has recognized these elements of difficulty and has given the subject, 

 particularly as it relates to human inheritance, an adequate, extended 

 treatment. 



Many of the misunderstandings .which have arisen have been derived 

 not only from common folk lore, but from loose thinking on the part of 

 those who practice the medical profession as well. It is not always an 

 easy matter to distinguish between inheritance of disease and inheritance 

 of predisposition to disease, although the distinction is one readily 

 conceivable from a theoretical standpoint, and necessary for clarity of 

 thought. It is necessary, moreover, to emphasize the fact that reappear- 

 ance of disease in successive generations does not constitute inheritance. 

 It is particularly true in the human race that successive generations are 

 often subjected to the same conditions of life. If these be unfavorable, 

 any predisposition to disease of a specific kind may result in the reappear- 

 ance of the disease. Illustrations of this sort occur in every day observa- 

 tion; they include such things as the tendency to tuberculosis in certain 

 families, the persistent reappearance of gout in successive generations, 

 nervous disorders which are expressed in various forms in a given line of 

 descent. Finally it is not common in every day thought to draw dis- 

 tinctions as finely as is necessary in scientific treatment. It is possible, 



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