UNITED STATES DEPARTMENT OF AGRICULTURE 



H BULLETIN No. 1090 ASK 



Washington, D. C. 



PROFESSIONAL PAPER 



November 15, 1922 



THE 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. 



I. 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 mortality 



Changes in inbred and control stocks. 

 Sex ratio 



Page. 

 .. 1 



is 

 20 

 29 

 Tests for disease resistance 29 



Page. 

 .. 31 

 .. 32 



.. 37 



I. Decline in vigor— Continued. 



Summary 



Appendix— Tables 6 to 22 , 



II. Differentiation among inbred families: 



Differentiation in color 



Abnormalities 38 



Differentiation in vigor 42 



Detailed study of family characters 45 



Records of five families in 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 

 much discussion for centuries. The marriage customs of primitive 

 peoples indicate that definite views on them were entertained long 

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

 ently different among different peoples, as the customs of some 

 seem designed to prevent inbreeding, while the reverse seems true 

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



Livestock breeders have generally endeavored to prevent close 

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

 gressive degeneration, exhibited by reduction in size, constitutional 



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