MENDEL'S LAW OF HEREDITY 



The most direct method for him to follow is 

 to test by suitable matings the unit-character 

 constitution of each individual which shows the 

 desired combination of characters, and to reject 

 all which are not homozygous. In this way a 

 pure race may be built up from individuals 

 proved to be pure. Such a method, however, 

 though sure, is slow in cases where the desired 

 combination includes two or more dominant 

 unit-characters, for it involves the application 

 of a breeding test to many dominant individ- 

 uals, most of which must then be rejected. It 

 is therefore often better in practice to breed 

 from all individuals which show the desired 

 combination, and eliminate from their offspring 

 merely such individuals as do not show that 

 combination. The race will thus be only grad- 

 ually purified, but a large stock of it can be 

 built up much more quickly. 



We may next discuss a cross in which three 

 unit-character differences exist between the par- 

 ents, instead of two. If guinea-pigs are crossed 

 which differ simultaneously in three unit-char- 

 acters, color, length, and direction of the hair, 

 a still larger number of phenotypes is obtained 



47 



