PREPOTENCY 



A Quality Belonging to Characters Rather Than Individuals — Something More 

 Than a Result of Inbreeding — Linkage or Coupling of Separate Factors in 

 Heredity Explains Observed Prepotency, and the Difference Be- 

 tween "Breeders of Breeders" and "Breeders of Performers." 



Edward N. Wentworth 

 Professor of Animal Breeding, Kansas State Agricultural College, Manhattan, Kans. 



ACCORDING to the opinion of 

 / \ leading live stock men, the chief 

 / \ essential to success in breeding 

 is the possession of prepotent 

 breeding animals (particularly sires). 

 Very few of these men will attempt to 

 explain what this prepotency is and 

 fewer still can give more than an ex- 

 tremely fragmentary idea. Yet the fact 

 is self-evident to the student of pedigrees 

 that there are marked differences among 

 breeding animals, since only 3 to 5% 

 of the breeding stock existing a few 

 generations back is represented in the 

 tables of ancestry of the present-day 

 individuals. 



Just what factors are responsible for 

 this condition? Without doubt, fashion 

 and advertising play an important part 

 in determining the blood lines that shall 

 survive, but the breeder in final analysis 

 is a business man demanding perform- 

 ance, and as has been shown in niunerous 

 studies,^ there is actual difference in the 

 breeding power of individuals. Daven- 

 port in his "Principles of Breeding" 

 sums up a distinction common among 

 practical breeders when he points out 

 "breeders of breeders" and "breeders 

 of performers," or those that transmit 

 performance through more than one 

 generation and those that simply confer 

 the good qualities upon their offspring. 

 It has been customary in defining 

 prepotency to state the manner in which 

 the breeder thinks it occurs. According 

 to popular idea, prepotency depends 

 upon the presence of a " high percentage 



of blood" of some particular individual. 

 The means by which prepotency is 

 brought about is by a supposed narrow- 

 ing of the bloodlines, either through 

 inbreeding, linebrceding, or some form 

 of pedigree selection. Prepotency is 

 assumed to be the restdt of a cumulative 

 effect of ancestry. In correspondence 

 conducted by the writer a few years ago 

 to obtain prevalent ideas on the nature 

 of prepotency, the following communi- 

 cation was received from Dean Eugene 

 Davenport of the University of Illinois. 

 It is of interest in that it very clearly 

 expresses the popular idea on the 

 subject. 



"Prepotency, of course, is a corollary 

 of the law of ancestral heredity. That 

 parent that has behind him the largest 

 mass of back ancestry selected to the 

 same characters, will, of course, be 

 prepotent. If you take the series of 

 fractions, ]/2, 3^> ^. etc., and divide 

 them by 2 , representing the contribution 

 of the sire and dam, you will obtain 

 the possibilities of each expressed in 

 fractional form so far as prepotency is 

 concerned. ... If, now, all the indi- 

 viduals represented by these fractions 

 have been selected to the same standard, 

 then, of course, the sire himself backed 

 by his ancestors will control one-half of 

 the possibilities of the offspring, regard- 

 less of what the female will be. Of 

 course this same would be true for the 

 female under like conditions. This, 

 it seems to me, is the essence of pre- 

 potency, and it is all there is of it.'' "^ 



1 "Distribution of Prepotency," Francis Galton. Nature LVIII, 246-247. "Principles of 

 Breeding," Eugene Davenport. Pp. 551-567. "Prepotency of Different Plants," W. W. Tracy. 

 Proc. Soc. Prom. Agr. Sci. 1900, pp. 57-59. "Result of Selecting Fluctuating Variations," F. M. 

 Surface, Conf. Internationale Genetique 1911, pp. 221-256. 



^ The italics are mine. 



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