HEREDITY 



1900 of Mendel's thirty-five-year-old paper, for biologists 

 to realize that the Mendelian type of inheritance derived 

 from some fundamental mechanism. This conclusion v^as 

 justified because Mendelian inheritance had been found in 

 various orders of flow^ering plants and, among animals, in 

 mollusks, insects, fishes, birds, and mammals. Man himself 

 was no exception. Carefully collected genealogies had 

 already show^n that a dozen or more contrasting conditions 

 such as sound mentality and feeble-mindedness, ordinary 

 stature and dw^arfness, and normal vision and color-blind- 

 ness were due to differences in a single gene. 



In spite of the facts available at this time, the criticism 

 was often raised that this interpretation could not serve for 

 all forms of inheritance, even in a given species, because 

 quantitative characters — that is to say, size characters — 

 could not possibly be inherited in this manner. These 

 objections stimulated work upon size inheritance; and 

 shortly the Theory of Multiple Factors, a theory which 

 brought the inheritance of all classes of characters under 

 one interpretation, was announced. It consisted merely 

 in the assumption that genes Aa, Bb, Cc, etc., can exert cu- 

 mulative effects directed either toward the development 

 of the whole body or toward that of a single organ. Because 

 of the complexity of such inheritance, and more especially 

 because of the extraordinary effects which slight differences 

 in environmental factors have upon size characters, this 

 theory was difficult to prove. But by analyzing statistically 

 the results from ordinary Mendelian inheritance, and 

 then using the statistical procedure to analyze experimental 

 data, satisfactory proofs were offered. 



The Mendelian interpretation of heredity having been 

 shown to be satisfactory for all characteristics in widely 

 separated groups of animals and plants, a fundamental 

 underlying mechanism, common to these groups, was 

 sought which would provide the gene distribution required 



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