536 GENERAL CONSIDERATIONS 



604. Breeding for Certain Characteristics. — In both animal and plant 

 breeding the end sought is the bringing together of desirable characters 

 and the development of stock which will breed true for those characters. 

 This involves the careful selection of breeding animals and the elimina- 

 tion of all progeny which are not homozygous for each pair of characters 

 desired. In this way, and by persistent inbreeding, a line can gradually 

 be developed which not only possesses the desired characters but also 

 has them in pure form or, in other words, is homozygous for them. The 

 final result is an artificial variety or strain which can be maintained by 

 the exercise of sufficient care. If only one character is sought which is 

 a pure mendelian character, this is not a long process, but since characters 

 may be in view which are the result of interaction between genes, consider- 

 able experimentation and judicious selection are often required to arrive 

 at the desired result. 



605. Inbreeding and Crossbreeding. — Inbreeding is the breeding of 

 closely related individuals; crossbreeding, the breeding of those not so 

 related. There are genes in man which correspond to certain defects such 

 as deafness, certain types of insanity, feeble-mindedness, and a tendency 

 to excessive bleeding known as hemophilia. These defects are usually 

 recessive. Since they are usually recessive, when present and paired with 

 a normal character they do not appear; if not so paired the defect becomes 

 apparent. For this reason inbreeding is likely to bring out these defects 

 to a much greater degree than crossbreeding, in which the chances are 

 great that the other individual will be normal with respect to the defect 

 which the one may have. If there are no defects in the inheritance, 

 inbreeding has no ill effects; on the contrary if the inheritance contains 

 many valuable qualities the result may be to produce superior individuals. 



Crossbreeding has been found in some cases to result in increased 

 vigor, called heterosis. That is true in the breeding of mules. This 

 is, however, often accompanied by steriUty. 



606. Inheritance of Acquired Characters. — From what has been said 

 it is evident that since the characters of an animal are determined by 

 its inheritance, any somatic character acquired during the lifetime of the 

 individual cannot be passed on unless it is accompanied by a germinal 

 modification. Since zoologists define acquired characters as purely 

 somatic modifications, it is not possible for them as such to be passed on. 

 Whether or not these somatic modifications can so affect the germ cells 

 as to produce a germinal modification which can be transmitted is not 

 known. No convincing proof, however, has ever been presented of this 

 having occurred, and so at the present time the possibility of the inher- 

 itance of acquired characters in any fashion is not generally accepted, 

 though it is recognized by many as conceivable. 



607. Inheritance of Disease and Abnormalities. — In Chap. LXVIII 

 reference was made to the inheritance of disease. Abnormalities, such 



