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In discussing so extensive and somewhat vague a topic as heredity of disease it is necessary, first of all, to define terms. It is obvious, in view of the diversity of attitude of medical men toward heredity, that there is a corresponding diversity of connotations connected with the word. In the present article, I will use heredity as covering the internal factors that direct development. This is not to say that there are any internal factors which, alone of themselves, determine the course of development but that internal factors play a very important rôle, such that through them are determined the specific form and most of the details of development, given an adequate and appropriate environment in which the development can take place.
The end-result of development is the reaction between the internal factors and the external factors. The internal factors may be called the genotype and the end-result may be spoken of as the phenotype.
DAVENPORT CB. HEREDITY OF DISEASE. JAMA. 1926;87(9):664–667. doi:10.1001/jama.1926.92680090006014
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