142 THE BREEDING OF ANIMALS 



character when it has been formed by two gametes each 

 bearing the character, and all the gametes of a homozygote 

 bear the character in respect of which it is homozygous. 

 When, however, the zygote is formed by two gametes of 

 which one bears the given character while the other does 

 not, it is said to be heterozygous for the character in 

 question, and only half the gametes produced by such a 

 heterozygote bear the character. An individual may be 

 homozygous for one or more characters, and at the same 

 time may be heterozygous for others." 1 



The conception of " gametic purity " as originally 

 stated requires some modification. It is no longer held 

 that characters are transmitted as units, but rather the 

 factors which combine to form characters. The factors 

 are so far purely imaginary. 



When mendelism was first seriously considered, there 

 was no doubt among its most enthusiastic exponents 

 that the characters existed in a pure unmixed state in 

 the gamete. 2 



136. Application of Mendel's law. Can the practical 

 breeder apply the principles of heredity embodied in 

 Mendel's law to the improvement of 'the domestic ani- 

 mals ? The domestic animals are valued by man because 

 of certain desirable characteristics which they possess. 

 These characteristics are clearly recognized by the breeder. 

 The meat animal is produced because it is endowed with 

 certain qualities which give it a special value either to 

 the consumer or the producer. The dairy cow is highly 

 prized because she has the ability of producing large 

 quantities of milk, cream or cheese. The horse is the 

 burden-bearer. Its value depends on the amount 



1 Punnett, " Mendelism," 1913, p. 28. 



*See Castle, " General Heredity," vol. V, No. 3, p. 93. 



