238 GENETICS AND EUGENICS 



that gametes as well as zygotes vary in vigor. Some can 

 exist as gametes alone, so great is their natural vigor. Here 

 there can be no heterozygosity. Examples are found both 

 in animals and in plants (honeybee drone, fern gameto- 

 phyte). Others can exist only as zygotes, so feeble are they 

 (the majority of the higher animals and plants). Still others 

 cannot exist as homozygotes, but only as heterozygotes, be- 

 cause they are still feebler (the yellow mouse, the aurea snap- 

 dragon). 



The experience of Miss King in inbreeding rats brother 

 with sister for twenty-five generations, shows that heterozy- 

 gosity is not indispensable to vigor even in bisexual repro- 

 duction, for she did not observe any evidences of decline in 

 vigor, size or fecundity, yet in all probability great increase 

 in homozygosity took place, since variability decreased. 



Pearl (1915) has attempted to devise a precise measure 

 of inbreeding based on the number of times that the same 

 individual or individuals appear in the pedigree of a particu- 

 lar animal. Thus, in bi-parental reproduction each individual 

 has two parents, each of these also had two parents, which 

 may or may not be the same pairs. If the parents were 

 brother and sister, then their parents were one pair, not two. 

 Thus the maximum number of different ancestors would be 

 two parents, four grandparents, eight great-grandparents, 

 etc. Such would be the condition when no inbreeding had 

 occurred. But occurrence of the same individual more than 

 once in a pedigree would show a certain amount of inbreed- 

 ing, and the extent of the inbreeding would increase with 

 every repetition of an individual in the pedigree. Pearl 

 makes this the basis of his "coefficient of inbreeding," which 

 is intended to express the relation between the possible (max- 

 imum) number of different ancestors and the actual number 

 of different ancestors, each individual being counted only 

 once, no matter how many times it is mentioned in the 

 pedigree. 



The chief utility of such a coefficient is to show what ap- 

 proach to homoz^^gosity of genetic factors has probably been 



