50 



Outline of Genetics 



is lacking. This conception has some evident advan- 

 tages and may modify the previous Mendelian diagram, 

 as shown in fig. 6. This appears to be a simpler mech- 

 anism 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 stimulates cell growth, 

 or cell division, or both. 



This hypothesis introduces some additional termin- 

 ology suggested by Bateson. In our illustration, the 



Dwarf Parent 



Gametes 



Fig. 6. — Diagram showing how the original scheme must be modi- 

 fied to satisfy the presence and absence hypothesis. 



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 nulliplex 

 with respect to its character of tallness. 



Additional advantages of the presence and absence 

 hypothesis will appear later in connection with a con- 

 sideration of blending inheritance and of cumulative fac- 

 tors in inheritance. Attention, however, should be called 

 to the fact that those who accept the presence and 



