EFFECTS OF INBREEDING AND CROSSBREEDING. 49" 



zygosis, are more likely to be deleterious than are their dominant 

 allelomorphs. The differentiation among the families is due to th& 

 chance fixation of different combinations of the factors present in 

 the original heterozygous stock. Crossing results in improvement 

 because each family in general supplies some dominant factors 

 lacking in the others. Dominance or even imperfect dominance in 

 each unit character is built up into a pronounced improvement over 

 both parent stocks in the complex characters actually observed. 



A certain portion of the increase in vigor of the first cross between 

 inbred families is maintained on resuming random mating. One- 

 half of this increase is maintained in stock founded on 2 inbred 

 lines, two-thirds in the case of 3 lines, three-fourths in the case of 

 4 lines, four-fifths in the case of 5 lines and so on. 



It is believed that the results point the way to an important 

 application of inbreeding in the improvement of livestock. Nearly 

 all of the characteristics dealt with here, like most of those of economic 

 importance with livestock, are of a kind which is determined only to 

 a slight extent by heredity in the individual. About 70 per cent of 

 the individual variation in resistance to tuberculosis and over 90 

 per cent of that in the rate of gain, and size of litter is determined 

 by external conditions. Progress by ordinary selection of individuals 

 would thus be very -slow or nil. A single unfortunate selection of a 

 sire, good as an individual, but inferior in heredity, is likely at any 

 time to undo all past progress. On the other hand, by starting a 

 large number of inbred lines, important hereditary differences in these 

 respects are brought clearly to light and fixed. Crosses among these 

 lines ought to give a full recovery of whatever vigor has been lost 

 by inbreeding, and particular crosses may safely be expected to show 

 a combination of desired characters distinctly superior to the original 

 stock. Thus a crossbred stock can be developed which can be main- 

 tained at a higher level than the original stock, a level which could 

 not have been reached by selection alone. Further improvement 

 is to be sought in a repetition of the process — the isolation of new 

 inbred strains from the improved crossbred stock, followed ulti- 

 mately by crossing and selection of the best crosses for the founda- 

 tion of the new stock. 



This method of improvement has not been unknown in the past. 

 In fact, most of the recognized breeds of livestock were developed, 

 more or less unconsciously, in this way. Close inbreeding was prac- 

 ticed by the pioneer breeders — Bakewell, the Collings, Bates, Cruick- 

 shank, Hewer, etc. The relatively few promising families and the 

 successful nicks between them were the foundation stock of the 

 breeds. Further development may be expected by the intelligent 

 application of the same principles. 



