MENDEL'S LAW 95 



to obtain it in a condition which will breed true. He must, 

 therefore, obtain homozygous individuals. If he is dealing 

 with a combination which contains only recessive characters, 

 this will be easy enough, for such combinations are invari- 

 ably homozygous. His task will become increasingly diffi- 

 cult, the more dominant characters there are included in the 

 combination which he desires to fix. 



The most direct method for him to follow is to test by 

 suitable matings the unit-character constitution of each in- 

 dividual 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 individuals, 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 

 gradually purified, but a large stock can be built up much 

 more quickly. 



We may next discuss a cross in which three unit-character 

 differences exist between the parents, instead of two. If 

 guinea-pigs are crossed which differ simultaneously in three 

 unit-characters, color, length, and direction of the hair, a 

 still larger number of phenotypes is obtained in F2, namely, 

 eight. A cross between a short-haired, colored, smooth 

 guinea-pig (Fig. 40) and one which was long-haired, albino, 

 and rough (Fig. 41) produced offspring in Fi which were 

 short-haired, colored, and rough (Fig. 42), these being the 

 three dominant characters, two derived from one parent, one 

 from the other. The F2 offspring were of eight distinct t>T)es, 

 two like the respective grandparents, one like the Fi indi- 

 viduals (parents), and the other five new, shown in Figs. 43- 

 47. The largest of the eight apparent classes {phenotypes) 

 was the one which manifested the three dominant charac- 



