[ 



UNITED STATES DEPARTMENT OF AGRICULTURE 



^mm BULLETIN No. 1090 ^ 



Washington, D. C. 



PROFESSIONAL PAPER 



November 15, 1922 



IHE EFFECTS OF INBREEDING AND CROSSBREEDING ON 



GUINEA PIGS. 



I. DECLINE IN VIGOR. 

 II. DIFFERENTIATION AMONG INBRED FAMILIES. 



By Sewall Wright, 



Senior Animal Husbandman in Animal Genetics, Animal Husbandry Division, 



Bureau of Animal Industry. 



CONTENTS. 



Decline in vigor: 



Earlier views on inbreeding 



Plan of the experiments 



History of the guinea-pig stock 



System of mating, care, and feed 



The data recorded 



The characters studied 



Indexes for growth and mortaUty 



Changes in inbred and control stocks . 



Sex ratio , 



Tests for disease resistance 



Page. 

 .. 1 

 .. 3 

 .. 3 



I. Decline in vigor— Continued. 



Summary ; 31 



Appendix— Tables 6 to 22 32 



II. Differentiation among iabred families: 



Differentiation in color 37 



AbnormaUties 38 



Differentiation in vigor 42 



Detailed study of family characters 45 



Records of five families m recent years . . 55 



Summary 57 



Appendix— Tables 7 to 15 59 



I. DECLINE IN VIGOR. 



EARLIER VIEWS ON INBREEDING. 



Inbreeding and crossbreeding are subjects on which there has been 

 nuch discussion for centuries. The marriage customs of primitive 

 *eoples indicate that definite views on them were entertained long 

 'efore the beginning of history. These views, however, were appar- 

 ntly different among different peoples, as the customs of some 

 eem designed to prevent inbreeding, while the reverse seems true 



ii other cases. A diversity of views continues to exist. 



I Livestock breeders have generally endeavored to prevent close 

 ibreeding, holding that such breeding is likely to produce a pro- 



, ressive degeneration, exhibited by reduction in size, constitutional 



106851°— 22-Bull. 1090 1 



