256 



EVOLUTION, GENETICS, AND EUGENICS 



individuals of the F x generation which showed the blend. By in- 

 breeding the pink hybrids Correns obtained the perfect 1:2:1 ratio, 

 that is, 1 red like one grandparent, 2 pink like the hybrid parent, and 

 1 white like the other grandparent. Segregation was evidently taking 

 place, the only unusual thing being the appearance of the F x indi- 

 viduals, and that was explained immediately as failure of dominance 

 (see Fig. 60). 



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 



® ®K®MQ) 







Red Parent 



Gamete Gamete 



White Parent 



Sperms 



®«® 



©«® 



®«0 







FlG. 60. — Diagram illustrating blending inheritance, discovered by Correns 



in Mirabilis jalapa. (From Coulter and Coulter.) 



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 charac- 

 ters 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 complete 

 dominance, however, 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 



