54 



Outline of Genetics 



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

 single dose produces pink while a double dose pro- 

 duces 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 heredi- 

 tary characters. In such a case, just as a given quantity of sol- 

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



Red Parent 



Gamete Gamete 



White Parent 



(R)p,nkQ 



^ggs 



wm"^ 



0P.® 



R) ( r 

 Pink 



r ) ( r 

 White 



Fig 8. — Diagram illustrating blending inheritance, discovered by 

 CoRRENS in Mirabilis Jalapa. 



can express hereditary characters only to a definite limited 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 a complete dominance, how- 

 ever, a single dose completely saturates the cells and a double dose 

 can do nothing more. This analogy assists in visualizing, on the 

 one hand, the necessary mechanism of blends (apparent failure 

 of dominance), and, on the other hand, that for cases of complete 

 dominance. 



