414 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



by the facts and was the explanation of the mechanism generally- 

 accepted. 



According to the presence and absence hypothesis, however, the 

 situation is looked at from an entirely different point of view. Tall- 

 ness is the result of a determiner, but dwarf ness is merely the result 

 of the absence of the determiner for tallness. The dominant character 

 is produced by an inheritable determiner, but the recessive character 

 appears only when the dominant determiner is lacking. This con- 

 ception has some evident advantages and may modify the previous 

 Mendehan diagram, as shown in Fig. 76. This appears to be a simpler 

 mechanism to account for the phenomenon called dominance. In the 

 case of the dwarf form there is a normal course of development; in the 

 case of the tall parent or hybrid, however, an additional determiner 



Dwarf Parent 



Gametes 



Fig. 76. — Diagram showing how the original scheme must be modified to 

 satisfy the presence and absence hypothesis. {From Coulter and Coulter.) 



stimulates cell growth, or cell division, or both. It is a simpler and 

 more useful conception, so long as it fits the facts. Some investigators, 

 however, claim that it cannot be applied to all the situations that have 

 been discovered. 



This hypothesis introduces some additional terminology suggested 

 by Bateson. In our illustration the tall parent has two determiners 

 for tallness and therefore Bateson calls it duplex, having a double dose. 

 For the same reason the Fi individuals, having only one determiner for 

 tallness, he calls simplex. According to the same terminology the 

 dwarf parent is nuUiplex with respect to its character of tallness. 



Additional advantages of the presence and absence hj'pothesis will 

 appear in connection with a consideration of blending inheritance and 

 of cumulative factors in inheritance. Attention, however, should be 

 called to the fact that those who accept the presence and absence 



