m. 645] COEFFICIENTS OF INBREEDING 331 



out, that it may come out the same for systems of breed- 

 ing which we know are radically different as far as the 

 effects of inbreeding are concerned. For example, in 

 the continuous mating of double first cousins, an indi- 

 vidual has two parents, four grandparents, four great 

 grandparents and four in every generation, back to the 

 beginning of the system. Exactly the same is true of 

 an individual produced by crossing different lines, in 

 each of which brother-sister mating has been followed. 

 Yet in the first the individual will be homozygous in all 

 factors if the system has been in progress sufficiently 

 long; in the second he will be heterozygous in a maxi- 

 mum number of respects. 



In order to overcome this objection Pearl has devised 

 a partial inbreeding index which is intended to express 

 the percentage of the inbreeding which is due to relation- 

 ship between the sire and dam, inbreeding being meas- 

 ured as above described. A coefficient of relationship 

 is used in this connection. These coefficients have been 

 discussed by Ellinger^ who suggests certain alterations 

 and extensions by means of which the total inbreeding 

 coefficient, a total relationship coefficient and a total re- 

 lationship-inbreeding index for a given pedigree can be 

 compared on the same scale. 



An inbreeding coefficient to be of most value should 

 measure as directly as possible the effects to be expected 

 on the average from the system of mating in the given 

 pedigree. 



There are two classes of effects which are ascribed to 

 inbreeding: First, a decline in all elements of vigor, as 

 weight, fertility, vitality, etc., and second, an increase in 

 uniformity within the inbred stock, correlated with 

 which is an increase in prepotency in outside crosses. 

 Both of these kinds of effects have ample experimental 

 support as average (not necessarily unavoidable) conse- 

 quences of inbreeding. The best explanation of the de- 

 crease in vigor is dependent on the view that Mendelian 



2 American Naturalist, 1920, 54: 540-545. 



