110 MODES OF RESEARCH IN GENETICS 



matings of the n + 1 generation, and q n+l the ac- 

 tual number of different individuals involved in 

 these matings. Z n may be called a coefficient of 

 inbreeding. If the value of Z for successive gen- 

 erations in the ancestral series be plotted to 

 the generation numbers as a base, the points so 

 obtained will form a curve which may be des- 

 ignated as the curve of inbreeding. 



It will be noted that the coefficient of inbreeding 

 Z is the percentage of the difference between the 

 maximum possible number of ancestors in a given 

 generation, and the actual number realized, in the 

 former. The coefficient may have any value 

 between and 100. When there is no breeding 

 of relatives whatever (that is, in the entire absence 

 of inbreeding), its value for each generation is 0. 

 As the intensity of the inbreeding increases, the 

 value of the coefficient rises. 



4. The above measure of inbreeding has to do 

 primarily with the relationship aspect of the 

 problem. 



5. Since the only possible infallible criterion of 

 relationship between individuals is common an- 

 cestry in some earlier generation, we are led to the 

 practical rule, in measuring the degree of inbreed- 

 ing in a pedigree, to regard all different individuals 

 as entirely unrelated until the contrary is proved 

 by the finding of a common ancestor. This no 

 doubt appears at this stage of the discussion as an 

 exceedingly obvious truism. The reader is urged 



