of Heredity 11 



numerical law being followed so far as has yet been 

 observed. 



Mendel made further experiments with Pisum sativum, 

 crossing pairs of varieties which differed from each other 

 in two characters, and the results, though necessarily much 

 more complex, showed that the law exhibited in the simpler 

 case of pairs differing in respect of one character operated 

 here also. 



In the case of the union of varieties AB and ab 

 differing in two distinct pairs of characters, A and a, 

 B and b, of which A and B are dominant, a and b 

 recessive, Mendel found that in the first cross-bred gene- 

 ration there was only one class of offspring, really AaBb. 



But by reason of the dominance of one character of 

 each pair these first crosses were hardly if at all distin- 

 guishable from AB. 



By letting these AaBb's fertilise themselves, only four 

 classes of offspring seemed to be produced, namely, 



AB showing both dominant characters. 

 Ab dominant A and recessive 6. 

 aB ,, recessive a and dominant B. ' 

 ab both recessive characters a and b. 



The numerical ratio in which these classes appeared 

 were also regular and approached the ratio 



; 3Ab : 3aB : lab. 



But on cultivating these plants and allowing them to 

 fertilise themselves it was found that the members of the 



RATIOS 



1 ab class produce only 6's. 



fl aB class may produce either all aB's, 

 [2 or both aB's and ab's. 



