Farm Animals - Breeding And Selection
( Originally Published 1912 )
To say that the breeding of stock is fundamentally and chiefly a matter of selection is to repeat a truism. The primitive germ cells have been seen to go through important changes that determine what part of their contents shall be reconveyed to the next generation. Any tendencies or characters not represented in the material contained in the mature ovum and spermatozoon that unite in fertilization can not by any possible means be found in the new animal resulting from that union. Our only opportunity of controlling the make-up of the parental contributions to the offspring lies in becoming assured that in the whole hereditary substance of either parent there is nothing representative of objectionable characters; this being true, no matter what enters the embryo, the result is good.
Realizing that the parent does not draw from the various parts of its own body the components of the germ plasm we must not allow ourselves to regard any parent's germ cell as a recapitulation or even as a representation of itself. Viewing the ancestral source of the germ plasm and its comparative independence of the influence of the body, the offspring then becomes an offshoot from the same stream that gave off the parent. Parent and offspring are similar because they have a common origin.
The numerous and distant sources from which any animal receives its inheritance are suggested in Fig. 7. The heavy lines from B and C, which enter A, represent the actual hereditary material contribution by those parents from the store in their own bodies, which was also implanted in each by their respective parents, the grandparents of A. A has not inherited and cannot transmit any tendency or quality that has not been contributed through his parents or grandparents. Of course it is possible and not improbable that a part of all of C's inheritance from G may happen to be represented in the polar bodies that perished when the ovum from which A developed underwent maturation, and thus A's inheritance through C may be stronger from F than from G or it may be the reverse or equal. This is indicated in the figure by the two lines of C's inheritance entering separately from F and G, the individuals contributing them, while the stream issuing from C draws from the combined supply an amount the same as entered from each of F and G, shown by the line leading from C being no larger than either of those coming from F and G.
A will be able to transmit good qualities in accordance with the degree of merit that was conveyed to him from his innumerable and distant progenitors through those that stand nearest him in descent. What possibilities were carried by these channels of inheritance in their devious windings and what was subtracted from the stores for each germ cell cannot be shown however far back we may undertake to trace the stream. To be sure the fundamental characters do not vary, but the features that give value to domestic animals are really minor ones so far as the re-semblance of any individual to his race is concerned.
For the breeder's purpose it is sufficient to know the character of the material in those nearer courses that are most likely to contribute to what has been received by the individual in question. The nature of the inheritance, reaching A from E and J, can be shown by the development that resulted in the conformation of the bodies of those ancestors. It can also be judged by the development in the bodies of other animals to whose inheritance these individuals contributed. J's inheritance may have been a mixed one and some of his offspring may have exhibited undesirable features. If, however, it is known that E was a really good individual and produced mainly good offspring of which B was one, we may consider the stream as having been purified from inferiority in that part of its course. If examination of other lines shows that the flow from the sources of good inheritance has been added to only by other individuals whose superiority is attested by the merit of their off-spring, we are assured that the individual in which these streams unite must transmit the excellence of his strain. This enables us to understand the strong breeding powers of animals whose inheritance traces exclusively through ancestors similar to each other in excellence. The stream of germ plasm has come to be of a pure and homogeneous makeup, and when mixed with that of an animal whose inheritance was not so restricted the pure material is able to dominate the miscellaneous tendencies from a mixed ancestry, and we have a pure-bred especially potent in stamping his likeness upon his off-spring.
This figure also aids in explaining the phenomenon spoken of as atavism or reversion. The dam of H may have been of a red color, while the sire and all the other individuals to which B traces were black, and produced only black offspring. The same may be true on the maternal side except that N has an inheritance of red which has been present in both G and C, but held in check by stronger tendencies to black. If an ovum produced by C and containing a strong infusion from N of tendency toward the red color is fertilized by a spermatozoon from B that also happens to carry the remnants of a tendency toward red, then such a union may hold in check the tendencies toward the black color. It is unlikely that the same parents would produce similarly endowed germ cells at another mating, and thus their subsequent progeny would be of the usual color, as is commonly observed to be the case with parents of red Angus calves.
To be assured of having a breeding animal that will transmit the maximum of the good with the minimum of bad it is then necessary to select one that individually exhibits such an inheritance, and that has had no ancestors' from whom it might have received either active or dormant material to produce inferiority. As we proceed backward the probabilities of inheritance from any one ancestor diminish, but the possible preservation and recurrence of the contribution of that ancestor must al-ways be reckoned with. The only practical method of directing heredity is to select for mating those animals that carry hereditary material of the desired potency and this can only be secured by the further selection of approved ancestors. This suggests an explanation of the preference some breeders of note have had for breeding the sires they used. In so doing it was possible for them to be more fully familiar with the ancestry and more competent to determine with what other descents matings should be made. The foregoing suggests as an ideal practice the selection of good individuals from good stock. These two factors, individuality and pedigree, are the subjects of the next chapters.