CHAPTER IV 



THE FACTOR H\TOTHESIS 



Mendel concluded that each plant character depends 

 upon a single detenniner. Inheritance, however, has 

 proved to be a much more complex phenomenon than 

 was indicated by Mendel's peas. Ratios have appeared 

 that were puzzling, and geneticists have been forced 

 to the conclusion that there may be a complex of deter- 

 miners for a single character. This conception is known 

 as the factor h\^pothesis, and much of the growing com- 

 plexity of genetics has developed around this hypothesis. 

 Previously we have used the word ''determiner," imply- 

 ing 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 pre\iously 

 considered. It is understood, now, however, that a 

 character is frequently determined by the interaction 

 of two or more separately heritable factors, and hence 

 the factor hypothesis. The distinction between factors 

 and determiners should be clear. In case only one heredi- 

 tary unit is involved in the production of a character, 

 this unit should be referred to as a determiner; in case 

 two or more units interact in the production of a char- 

 acter, these 3i,xe factors .^ 



^ This distinction of terms has pedagogical value, but is frequently 

 violated in the literature, where "factor" is frequently used in the 

 sense of "determiner." A less restricted term, gene, refers to the heredi- 

 tary unit without implying whether it acts as a factor or simple 

 determiner. 



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