UNITED STATES DEPARTMENT OF AGRICULTURE 



BULLETIN No. 905 



Contribution from the Bureau of Animal Industry 

 JOHN R. MOHLER, Chief 



JTL^'^Lf^ 





Washington, D. C. 



December 8, 1920 



PRINCIPLES OF LIVESTOCK BREEDING. 



By Sewall Wright, 

 Senior Animal Husbandman, Animal Husbandry Division. 



CONTENTS. 



Evolution of animal breeding 1 



Reproduction 2 



The cell theory 2 



The reproductive cells 3 



Sexual maturity 4 



Frequency of service 5 



The breeding season and oestrous cycle 6 



The gestation period 7 



Fertility 8 



Hybrids 10 



The reproductive cells in relation to heredity. . 11 



General considerations 11 



Modification of heredity 12 



Inheritance of acquired characteristics 13 



Telegony 14 



Maternal impressions 15 



Details of hereditary transmission 15 



Blending and alternative inheritance 15 



Hereditary units 15 



Color and albinism in guinea pigs 16 



Mendehan inheritance 18 



The chromosomes and heredity. 20 



Linkage 20 



The determination of sex 23 



The normal method 23 



Sex- linked inheritance 25 



The sex ratio 27 



The freemartin 29 



Mendelian heredity in livestock 30 



Polled cattle 30 



Colors of cattle 30 



Colors of horses 31 



Colors of hogs 32 



Mendelian heredity in livestock — Continued. 



Colors of sheep 33 



Colors and comb shape of poultry 33 



Heredity of form and function in livestock 34 



Relations of theory to practice 34 



Equality of inheritance from the sexes 34 



Prepotency 34 



Variation 36 



Fixation of heredity by selection 37 



Fixation of heredity by inbreeding "... 37 



Isolation of genetic differences by inbreeding. 39 



The effect of inbreeding on vigor 40 



Crossbreeding 42 



The system of breeding 42 



The purposes of livestock breeding 42 



Uniformity of type- 43 



Crossbreeding for the market 44 



Improvement 45 



Grading up 46 



Methods of selection 47 



General considerations 47 



Individual performance and livestock judg- 

 ing 49 



The breeding record 50 



Pedigrees 50 



The value of purebreds 54 



Dairy cattle 54 



Quality in meat 57 



Breeding and soundness in horses 61 



Poultry 62 



Satisfaction from pleasing appearance 63 



Summary 65 



EVOLUTION OF ANIMAL BREEDING. 



The breeding of domestic animals dates back to remote antiquity, 

 when the most advanced races of the Old World were still on the 

 border line between savagery and barbarism. It far antedates any 



5254°— 20— Bull. 905 1 



