Neo-Mendelism 41 



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

 of allelomorphs is represented by a determiner, two 

 determiners thus being present. Dwarfness in this 

 case would be the result of the interaction of that deter- 

 miner 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 paralyzed 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 pre- 

 vented the activity of the determiner for dwarfness. 

 This conception was apparently borne out 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 dif- 

 ferent point of view. Tallness is the result of a deter- 

 miner, but dwarfness is merely the result of the absence^ 

 cj the determiner for tallness. The dominant character 

 is produced by an inheritable determiner, but the reces- 

 sive character appears only when the dominant deter- 

 miner is lacking. This conception has some evident 

 advantages and may modify the previous Mendelian 

 diagram, as shown in fig. 6. This appears to be a simpler 

 mechanism to account for the phenomenon called domi- 

 nance. 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 stimulates cell 

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

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



