COEFFICIENTS OF INBREEDINa AND 

 RELATIONSHIP 



DR. SEWALL WEIGHT 

 OF Agriculture 



In the breeding of domestic animals consanguineous 

 matings are frequently made. Occasionally matings are 

 made between very close relatives— sire and daughter, 

 brother and sister, etc.— but as a rule such close inbreed- 

 ing is avoided and there is instead an attempt to concen- 

 trate the blood of some noteworthy individual by what 

 is known as line breeding. No regular system of mating 

 such as might be followed with laboratory animals is 

 practicable as a rule. 



The importance of having a coefficient by means of 

 which the degree of inbreeding may be expressed has 

 been brought out by PearP in a number of papers pub- 

 lished between 1913 and 1917. His coefficient is based on 

 the smaller number of ancestors in each generation back 

 of an inbred individual, as compared with the maximum 

 possible number. A separate coefficient is obtained for 

 each generation by the formula 



Z„ = 100 (1 - ) = 100 (1 - ) 



where qn^JT"'^ is the ratio of actual to maximum pos- 

 sible ancestors in the n -f 1st generation. By finding the 

 ratio of a summation of these coefficients to a similar 

 summation for the maximum possible inbreeding in 

 higher animals, viz., brother-sister mating, he obtains a 

 single coefficient for the whole pedigree. 



This coefficient has the defect, as Pearl himself pointed 



1 American Naturalist, 1917, 51: 545-559; 51: 63&-639. 



330 



