Journal of Heredity 6: 17-20 (1915)
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
Between "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 extremely 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.
|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 Génétique 1911, pp. 221-256.|
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 performance, and as has been shown in numerous studies,1 there is actual difference in the breeding power of individuals. Davenport 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 narrowing of the bloodlines, either through inbreeding, linebreeding, or some form of pedigree selection. Prepotency is assumed to be the result 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 communication 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.
|2 The italics are mine.|
"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, 1/2, 1/4, 1/8, 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 individuals 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, regardless 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 prepotency, and it is all there is of it." 2
While this idea is widely held by animal husbandmen, the man who has conducted genetic experiments and watched the segregation of individual factors cannot help but feel that the conception is manifestly outside of the facts. The behavior of hereditary characters as though controlled by unit factors in the germ plasm leaves no room for a cumulative effect of ancestry (unless the increased opportunities to bring about homozygosis through selection in hereditarily limited stock be thus considered).
NATURE OF THE GERM PLASM.
Even the parent himself has no effect on his progeny in an hereditary way, as inheritance does not really consist in the passing on of characters from one generation to the next. The similar characters of parent and progeny develop because both parent and progeny arise from qualitatively the same germ-plasm (as far as the particular characters are concerned). When one mixes lime chloride and sulphuric acid he will obtain lime sulphate and muriatic acid. These are the end products of the reaction. If he sets apart a portion of the lime chloride and sulphuric acid the first day and a few days later mixes them, he will get the same result. The results of the first day have no effect on the results of the later day. When the germ cell of the animal begins development it corresponds to the first chemicals, the developed body to the end-product of the reaction. Characters that are alike in parent and offspring arise because they come from similar origin, but the body, an end-product of development, no more affects the germ cells that produce the progeny, than the end-products of the chemical reaction affected the reaction of the later day. This shows by analogy how far outside the facts of inheritance the conception of a cumulative ancestry lies.
|3 There are two classes of red roans corresponding to bays and chestnuts, but each carrying the roan pattern.|
To indicate the principal difference between the prepotent and non-prepotent sire, the roan Belgian referred to by the writer in a previous paper, that sired only red roan colts (256 in number) from various colors of mares may be compared to another roan Belgian therein mentioned that produced about half roan colts and the other half grays, bays, browns, blacks and chestnuts. Obviously the first horse was the more prepotent of the two. He must have transmitted red roan in every germ cell. To do this, he must have been homozygous or pure for the dominant characters of roan pattern and hay color; that is, he must have received both roan and hay from sire and from dam. While there is no way of exactly determining what was transmitted in the germ cell, the fact that both parents were bay (red) roans3 and that only bay roan colts were produced would show that there was only one chance out of approximately 18,062,500,000,000,000,000 that the assumption is wrong.
The second horse, on the other hand, was sired by a blue roan stallion out of a bay mare. This indicates at once that he cannot be homozygous for roan since he received it from only one parent, nor could he be pure for bay since it came only from the dam. Yet he sired about 50% of red roans and blue roans, a performance which some breeders might also consider prepotent.
PREPOTENCY AND PURE BREEDING.
This indicates the first essential of prepotency, homozygosis in a dominant character. Of course, the breeder believes that prepotency is a property of the individual and not of the character. But in almost every instance the idea of prepotency is based on some superficial and striking character like color, and it is assumed that since this character appears with fair uniformity, the rest of the characters must appear also. As a matter of fact, it is highly improbable that there ever occurred the ideally prepotent animal described by the breeder; that is, one which is able to impress most of his characters upon his progeny in spite of the females to which he is mated. The livestock man reads with interest of that great line of Clydesdale sires from Darnley down through Topgallant, Sir Everard, the long lived and redoubtable Baron's Pride, the $47,500 Baron o' Buchlyvie, and the sensational five-year-old sire, Dunure 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 individual, and their condition as to homozygosis 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 number 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 characters and also more desirable combinations 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 numerous 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 hereditary influence is concerned.
Thus far the discussion has applied only to simple qualitative characters that depend on relatively few and easily recognizable factors. When one approaches quantitative characters such as size, vigor, etc., from which dominance is probably absent, the problem becomes more complex. It seems very doubtful if the principle differs here, but the presence of larger numbers of factors or of factors for greater development must be assumed to take the place of the dominant factors already discussed. 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 characters 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 quantitative 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 history records many prepotent sires that bred better than themselves. 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 basis of some striking superficial character. Attention was directed to the fact that it is probable in the main that this character is transmitted alone and independently of other characters, vet such need not by any means be the case, since the phenomenon of linkage and coupling assures us of a mechanism whereby totally unrelated characters physiologically may he part of one hereditary complex. Thus T. H. Morgan and his associates have demonstrated in the pomace-fly, Drosophila, the tendency of miniature wings, yellow body, and white eyes to be grouped together, in segregating out of crosses with the normal fly that has the gray body, red eyes, and long wings. Many other combinations of similarly linked characters have also appeared in their work, some linked to sex as the characters just mentioned, and some uninfluenced by the sex determining factor. They have interpreted these results on the basis of the factors in the germ cells that produce these characters somatically, having practically a common locus (probably a chromosome). If such be the case in the domestic mammals, and there is no more reason for doubting it with them than with Drosophila, there is really a genetic foundation for the belief of the breeder that the superficial characters on which the degree of prepotency is determined also indicate the transmission of other characters, at least characters that find their origin at the same germ locus (chromosome).
As a result of this, one can see very clearly that the distinction between the "breeder of breeders" and the "breeder of performers" is almost entirely a question of character linkage, the "breeder of performers" ending his function when he has contributed a number of characters to his progeny, so distributed among different hereditary complexes that the segregations in germ cell formation separate the necessary interacting factors. The "breeder of breeders" must bear his factors that interact to produce performers in one hereditary complex only. The "breeder of performers" possibly has similar factors to the "breeder of breeders," but they are located in two or more complexes, thus permitting segregation and separation.