332 THE AMERICAN NATURALIST [Vol.LVI 



factors unfavorable to vigor in any respect are more 

 frequently recessive than dominant, a situation which is 

 the logical consequence of the two propositions that 

 mutations are more likely to injure than improve the 

 complex adjustments within an organism and that injuri- 

 ous dominant mutations will be relatively promptly 

 weeded out, leaving the recessive ones to accumulate, 

 especially if they happen to be linked with favorable 

 dominant factors. On this view it may readily be shown 

 that the decrease in vigor on starting inbreeding in a 

 previously random-bred stock should be directly pro- 

 portional to the increase in the percentage of homozygo- 

 sis. Numerous experiments with plants and lower 

 animals are in harmony with this view. Extensive ex- 

 periments with guinea-pigs conducted by the Bureau of 

 Animal Industry are in close quantitative agreement. 

 As for the other effects of inbreeding, fixation of char- 

 acters and increased prepotency, these are of course in 

 direct proportion to the percentage of homozygosis. 

 Thus, if we can calculate the percentage of homozygosis 

 which would follow on the average from a given system 

 of mating, we can at once form the most natural coeffi- 

 eioTit of inbreeding.. The writer^ has recently pointed out 

 ;i method of calculating this percentage of homozygosis 

 which is applicable to the irregular systems of mating 

 found in actual pedigrees as well as to regular systems. 

 This method, it may be said, gives results widely different 

 from Pearl's coefficient, in many cases even as regards 

 the relative degree of inbreeding of two animals. 



Taking the typical case in which there are an equal 

 number of dominant and recessive genes (^4 and a) in 

 the population, the random-bred stock will be composed 

 of 25 per cent. AA, 50 per cent. Aa and 25 per cent, aa. 

 Close inbreeding will tend to convert the proportions to 

 50 per cent. A A, 50 per cent, aa, a change from 50 per 

 cent, homozygosis to 100 per cent, homozygosis. For a 

 natural coefficient of inbreeding, we want a scale which 



s Genetics, 1921, 6: 111-178. 



