Neo-Mendelism 45 



The question this introduces, therefore, is that of a 

 mechanism which could account for such a result. The 

 easiest explanation offered is that the red parent was a 

 homozygote for redness (double dose) and the hybrid a 

 heterozygote (single dose) ; the inference is that a single 

 dose produces pink while a double dose produces red. 

 A theoretical explanation of this occasional difference 

 in the result of double and single doses is as follows. 

 Imagine that the body cells of a plant have a certain 

 capacity for expressing hereditary characters. In such a 

 case, just as a given quantity of solvent can dissolve 

 only a given amount of solute, so the body cells can 

 express hereditary characters only to a definite Umited 

 extent. In the four-o'clock a single dose of redness 

 may be thought of as half saturating the body cells, 

 while a double dose completely saturates them. In 

 cases showing complete dominance, however, a single 

 dose completely saturates the cells and a double dose 

 can do nothing more. This analogy assists in visuaUzing 

 on the one hand the necessary mechanism of blends 

 (apparent failure of dominance) and on the other hand 

 that for cases of complete dominance. 



Another example of simple blending inheritance is 

 the case of Adzuki beans, described by Blakeslee 

 (i). In this bean the mottling of the seed coat is 

 dominant to the lack of mottUng. In the hybrid 

 condition, however, the mottling is lighter than in 

 the pure or homozygous condition. Heterozygous 

 plants, therefore, can be easily distinguished from 

 homozygous plants, so that the 1:2:1 ratio is evi- 

 dent on external inspection rather than the usual 3 : i 

 ratio. 



