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 areinherited. Thesubject, 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. Tlustrations 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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