Wentworth: Prepotency 



19 



long lived and redoubtable Baron's 

 Pride, the $47,500 Baron o' Buchlyvie, 

 and the sensational five-year-old sire, 

 Duntire Footprint; each a son of the 

 animal preceding and, with the exception 

 of Topgallant, each a distinct step in 

 advance of his progenitor, as far as 

 siring prize winners is concerned. Here 

 is an exceptionally prepotent line, 

 because all but Topgallant were the 

 leading sires of Scotland during their 

 tenure in stud. Yet even these great 

 horses begot a percentage of failures that 

 is startlingly large, even though smaller 

 than that of any other line of sires in 

 any breed. Prepotency is never a 

 property of the individual, but belongs 

 to a certain few characters that are part 

 of the hereditary makeup of the indi- 

 vidual, and their condition as to homo- 

 zygosis or heterozygosis is the entire 

 determining factor. The degree by 

 which one animal is more "strongly 

 bred" for a character than another 

 animal is this wide degree of purity or 

 hybridity. 



EXISTS IN BOTH SEXES. 



Many breeders deny the existence of 

 prepotency in the female, and consider 

 it entirely a property of the male sex. 

 This belief has come about as a result of 

 two conditions. Since in all domestic 

 breeds polygamy is practiced, it is 

 obvious that a smaller niimber of males 

 is required than females. This admits 

 of a more stringent selection in one sex 

 than in the other, and increases the 

 chances of the male's having both 

 greater numbers of homozygous char- 

 acters and also more desirable combina- 

 tions of characters. Furthermore, in 

 uniparous races, such as the horse and 

 ox (the species in which the art of animal 

 breeding has largely been developed), 

 there is only one individual in a season 

 that may be compared to a female, 

 while nimierous individuals occur that 

 may resemble the male. Where there 

 is much diversity among the females, 

 the fact that only one out of the season's 

 progeny may show resemblance to any 

 particular female, while numbers may 

 partake of the characters borne by the 

 sire, is bound to over-emphasize the 



importance of the male as far as heredi- 

 tary influence is concerned. 



Thus far the discussion has applied 

 only to simple qualitative characters 

 that depend on relatively few and easily 

 recognizable factors. Wheti one ap- 

 proaches quantitative characters such 

 as size, vigor, etc., from which domi- 

 nance is probably absent, the problem 

 becomes more complex. It seems very 

 doubtfiil if the principle differs here, 

 but the presence of larger numbers of 

 factors or of factors for greater develop- 

 ment must be assumed to take the place 

 of the dominant factors already dis- 

 cussed. At least the writer does not 

 believe that there are two schemes of 

 heredity involved in inheritance, and 

 since one has been found to hold in the 

 qualitative characters, and since the 

 data assembled on quantitative char- 

 acters seem to follow the same system 

 as far as they have been investigated, 

 it is no more than logical to make the 

 preceding assumption. 



Another cause of prepotency in quan- 

 titative characters may arise where the 

 male bears one factor necessary to link 

 up the factors in the female to produce 

 the desired character. Thus, in the 

 ordinary white mouse, the base for 

 pigment production, a factor denoted by 

 C, is lacking. In one variety of 

 Japanese waltzing mouse, white with 

 faint yellow marks, Darbishire found the 

 factors for agouti, black and chocolate 

 missing and the yellow diminished 

 quantitatively. Yet the progeny of 

 the cross were agouti, because the 

 Japanese variety supplied the color 

 base which the white mice lacked. 

 While the prepotent animal usually 

 breeds true for its character, this 

 extreme case is interesting because it 

 shows how one individual may supply 

 the one factor necessary to a relatively 

 uniform somatic expression. Breed his- 

 tory records many prepotent sires that 

 bred better than themselv^es. Perhaps 

 the trotter George Wilkes and the 

 Shorthorn Champion of England would 

 fall in this class. 



LINKAGE OF CHARACTERS. 



Emphasis has been laid on the usual 

 custom of declaring prepotency on the 



