CHAPTER XIX 

 THE FACTOR HYPOTHESIS AS APPLIED TO PLANTS 



JOHN M. COULTER AND MERLE C. COULTER 



Thus far we have been considering Mendel's law in its simple form 

 and have enlarged but little upon Mendel's original statement. The 

 value of the law is apparent. Upon its republication in 1900 it was 

 taken up by biologists and numerous breeders set to work to test it. 

 As a consequence data for and against it began to accumulate. As 

 might be expected, there was much apparent evidence against the law, 

 but as geneticists developed a better conception of the mechanism the 

 contradictory evidence was explained away. Almost every type of 

 inheritance has now been explained according to Mendel's law. Some 

 of the explanations are veiy complicated and cannot be in eluded in 

 this presentation. A few of the more important cases, however, will 

 be presented. 



I. PRESENCE AND ABSENCE HYPOTHESIS 



This may be regarded as a new method of Mendelian thought. It 

 was first suggested by Correns, but later was worked out in detail by 

 other geneticists, especially Hurst, Bateson, Shull, and East. It is 

 merely a modification of the mechanism involved. For example, in 

 the case of a hybrid obtained by crossing tall and dwarf parents the 

 result had been explained as due to the fact that one chromosome bears 

 a determiner for tallness and the other one of the pair carries the deter- 

 miner for dwarfness. In other words, each one of a pair of allelo- 

 morphs is represented by a determiner, two determiners thus being 

 present. Dwarfness in this case would be the result of the interaction 

 of that determiner and its environment during the development of the 

 body; and the same for tallness. When both were present, however, 

 the conception of the situation was as follows. The determiner for 

 dwarfness, setting up its usual series of reactions, early became para- 

 lyzed by the determiner for tallness or its products. This result was 

 called the dominance of the character for tallness. It was as if the 

 determiner for tallness completely prevented the activity of the deter- 

 miner for dwarfness. This conception was apparently borne out 



1 From Coulter and Coulter, Plant Genetics (The University of Chicago Press, 

 copyright 191 8). 



2 53 



