Menders Law 49 



PRESENCE AND ABSENCE HYPOTHESIS 



Tliis may be regarded as a new method of Mendelian 

 thought. It was first suggested by Correxs (i), but 

 later was worked out in detail by other geneticists. It 

 is merely a different way of regarding the MendeHan 

 mechanism. For example, in the case of a hybrid 

 obtained by crossing tall and dwarf parents, the result 

 had been explained by Mendel as due to the fact that 

 one chromosome bears a determiner for tallness and the 

 other one of the pair carries the detemiiner for dwarfness. 

 In other words, each one of a pair of allelomoq)lis is 

 represented by a determiner, two detemiiners thus being 

 present. Dwarfness in this case would be the result of 

 the interaction of that determiner and its enviromuent 

 during the development of the body; and the same for 

 tallness. When both were present, howe\'er, the con- 

 ception of the situation w^as 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 detemiiner 

 for tallness completely prevented the acti\'ity of the 

 determiner for dwarfness. This conception was apjxir- 

 ently borne out by the facts and was the ex]3lanation of 

 the mechanism generally accepted. 



According to the presence and absence h\'])othesis, 

 however, the situation is looked at from a different 

 point of view. Tallness is the result of a detemiiner, 

 but dwarfness is merely the result of the absence of the 

 detemiiner for tallness. The dominant character is 

 produced by an inheritable detemiiner, but the recessive 

 character appears only when the dominant detemiiner 



