254 



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 dwarfness 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 

 Mendelian diagram, as shown in Figure 58. 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 



F, Hybrid 



Dwarf Parent 



Gamet< 



Fig. 58. — 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. 



[It should be emphasized here that the word "determiner" as used 

 in the foregoing paragraphs is not synonymous with the word "gene." 

 A recessive character is not due to the absence of a gene, but merely to 

 the absence of something that is present in the dominant gene. In 

 this form the presence and absence idea is acceptable.] 



Additional advantages of the presence and absence hypothesis 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 



