CHAPTER VI 



NEO-MENDELISM {Continued) 



III. THE FACTOR HYPOTHESIS 



Mendel concluded that each plant character de- 

 pends upon a single determiner. Inheritance, however, 

 has proved to be a much more complex phenomenon 

 than indicated by Mendel's peas. Ratios have ap- 

 peared that were puzzling, and geneticists were forced 

 to the conclusion that there may be a compound 

 determiner for a single character. This conception is 

 called the factor hypothesis, and the growing complexity 

 of genetics has developed in connection with this 

 hypothesis. With the consideration of factors instead 

 of determiners one passes from elementary to advanced 

 genetics. Previously we have used the word determiner, 

 implying Mendel's idea that a single determiner is 

 responsible for the development of a plant character, and 

 this has been true of the examples of inheritance previ- 

 ously considered. It is understood now, however, 

 that a character is frequently determined by the inter- 

 action of two or more separately heritable factors, and 

 hence the factor hypothesis. The distinction between 

 factors and determiners should be made clear. In case 

 only one factor is involved in determining a character, 

 there is no distinction between factor and determiner; 

 and in such a case the term factor should not be used. 



I. Complementary factors. — This is the simplest 

 expression of the factor hypothesis and it may be 

 illustrated by some of East's work. Crossing red- 



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