UNITED STATES DEPARTMENT OF AGRICULTURE 
JJfe BULLETIN No. 905 
<^Mk^S^iM^» Contribution from the Bureau of Animal Industry *i 
S^P'^^SU JOHN R. MOHLER, Chief S^fS^^Su 
Washington, D. C. 
December 8, 1920 
PRINCIPLES OF LIVESTOCK BREEDING. 
By Sewall Wright, 
Senior Animal Husbandman, Animal Husbandry Division. 
CONTENTS. 
Evolution of animal breeding 
Reproduction 
The celltheory 
The reproductive cells 
Sexual maturity 
Frequency of service 
The breeding season and oestrous cycle 
The gestation period 
Fertility 
Hybrids 
The reproductive cells in relation to heredity- 
General considerations 
Modification of heredity 
Inheritance of acquired characteristics 
Telegony 
Maternal impressions 
Details of hereditary transmission 
Blending and alternative inheritance 
Hereditary units 
Color and albinism in guinea pigs 
Mendelian inheritance 
The chromosomes and heredity 
Linkage 
The determination of sex 
The normal method 
Sex-linked inheritance 
The sex ratio 
The freemartin 
Mendelian heredity in livestock 
Polled cattle 
Colors of cattle 
Colors of horses 
Colors of hogs 
Page 
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. 90.5 1 
