UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN NO. 905 
Washington, D. C. 
Issued December 8, 1920 
Revised July, 1923 
PRINCIPLES OF LIVESTOCK BREEDING. 
By Sewall Wright, 
Senior Animal Husbandman, Animal Husbandry Division, Bureau of Animal Industry. 
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 cestrous 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 
Pedigree 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 
Books on the breeding of, animals 68 
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 
45957°— 23— Bull. 905 1 
