72 PHYSICAL BASIS OF HEEEDITY 



Each F2 class of the two-factor case (9:3:3:1) will con- 

 tain a three-to-one ratio for the third factor-pair. Thus, 

 in the 9 class there will be 3 dominants of the third factor 

 to one recessive (27:9). So for each 3 class: each con- 

 tains the third factor in the ratio of 3:1. So also for 

 the 1 class. The total result therefore is : 



In actual practice the three-factor cases are almost 

 never used. Other methods are employed to detect the 

 factors present, so that these three-factor ratios have a 

 theoretical rather than a practical value. In cases where 

 multiple factors are suspected, some of them may be only 

 modifiers of some one of the other more conspicuous char- 

 acters and in such cases special methods of procedure will 

 recommend themselves. 



