UNITED STATES DEPARTMENT OF AGRICULTURE 



Washington, D. C. 



PROFESSIONAL PAPER 



December, 1922 



THE EFFECTS OF INBREEDING AND CROSS- 

 BREEDING ON GUINEA PIGS.^ 



m. CROSSES BETWEEN HIGHLY INBRED FAMILIES. 



By Sewah Wright, Senior Animal Husbandman in Animal Genetics, Animal Hus- 

 bandry Division, Bureau of Animal Industry. 



CONTENTS. 



Page. 



1 

 2 

 2 

 3 

 5 

 10 



Introduction 



The inbred families 



Crossbreeding experiments 



Description of tables 



Allowance for seasonal fluctuations 



Allowance for heredity 



Allowance for age of dam 12 



Allowance for sex 12 



Significance of differences 13 



Kesults 15 



The percentage born aUve 16 



The percentage raised among the yoimg bom 



alive 18 



The percentage raised of all young born 20 



Birth weight 21 



Daily gains 22 



Weight at weaning 23 



Adult weight , 



Frequency of litters 



Age of maturity , 



Complete sterility , 



Size of litter , 



Total fertility 



Coat color 



Resistance to tuberculosis 



Early views on inbreeding 



Pre-Mendehan experiments 



Mendelian heredity and the problem of in 



breeding 



Mathematical considerations 



Comparison of results with theory 



Summary and general conclusions , 



Literature cited 



Page. 



24 

 25 

 27 

 28 

 28 

 30 

 32 

 32 

 33 

 34 



35 

 39 

 46 

 48 

 59 



INTRODUCTION. 



It has been shown in Part I of this series of papers'- that con- 

 tinuous mating of brothers with sisters in a stock of guinea pigs has 

 been accompanied by a dechne in all of the elements of vigor which 

 have been studied. These include the percentage born alive, the 

 percentage raised to weaning (33 days) , birth weight, rate of gain to 

 weaning, adult weight, size of litter, and number of litters produced 

 per year. Just how much of the decline in these respects was due to 

 inbreeding and how much to environmental conditions was not 

 whoUy certain. It was shown, however, that the inbred stock had 



1 The two preceding parts of these studies have been published as Bulletin 1090, U. S. Department of 

 Agriculture. 



6448— 22— Bull. 1121 1 1 



