In-Breeding, Line-Breeding, and Out-Crossing
'In-breeding means the mating of closely related parents, the commonest being full brother and sister, sire to daughter, dam to son. In-breeding does not produce any new characters. It does intensify such characters as exist, good or bad, whether these be apparent through dominance or hidden in recessives. This intensifying of characters is accomplished by decreasing the lines of inheritance. Such characters as emerge come from the reshuffling of the genes already present, but there are fewer to shuffle and a greater degree of uniformity is certain to result.
In-breeding has been regarded as dangerous. By some it has been regarded as immoral. There is a quite commonly held opinion that it leads certainly to decrease in vigor and in size and a general deterioration of the inbred strain. People who hold these opinions usually admit the benefits of in-breeding but regard the risk involved as too great to justify the practice. It is true that many of the disadvantages feared have resulted, but it now seems probable that these faults arise more from lack of their timely recognition by the breeder than from anything inherent in the practice itself.
Unwanted results arise from the recessives that are not visible in the original parents, but can be located by back-crossing, and animals showing these faults should be discarded immediately upon their recognition, else they may become dominant. With their elimination the desirable characteristics are likely to reappear in intensified measure. Small animals have been inbred in laboratories for more generations than cattle have been known to exist. In many cases they show no lack of size, physical vigor, or reproductive power. On the other hand it cannot be denied that loss of size and vigor, and abnormalities have some times occurred in long continued in-breeding. When two inbred strains showing these ill effects have been themselves crossed, size and vigor have been restored immediately and even intensified.
It seems certain to this writer that most rapid progress in cattle improvement may be expected to come from the practice of intelligent in-breeding accompanied by rigid selection on the part of individual breeders who then may, when they feel that they have the characteristics they are seeking well established, resort to cross-breeding with other inbred lines of other breeders who are seeking similar objectives. It is neither necessary nor desirable to resort to the so-called “cold out-cross.” Neither is it necessary that the animal eliminated from the inbred line be sent to the butcher. Such an animal in other herds might well work greater improvement than would an individually superior animal of mixed inheritance.
Line-breeding is the mating of related animals of less close relationship than that of in-breeding, from which it differs only in degree. Such matings are half-brother to half-sister, cousins, aunts, uncles, etc. Line-breeding may hope to achieve, in time, approximately the same results as in-breeding. It is obvious that this system introduces into the strain greater numbers of probable variations. It is certain, therefore, that the breeder using this practice will need more years in which to achieve his goal. Of course, he may, get from some collateral ancestor unexpectedly favorable results, but he is no more certain to do this than he is to get unfavorable results. Which system is best to use will depend upon the skill-and luck-of the breeder.
It has been determined by such a mass of experimentation and evidence that it can scarcely be doubted that such characteristics as appear in an individual are usually, in practice, inherited from the first half dozen or fewer generations of ancestors. Atavism, which is the going back to a more remote ancestor, does occur. Recessives in the germ plasm do emerge, but they are comparatively uncommon.
Out-crossed animals are those which show in their pedigree no common ancestor within the first four generations. Lines of inheritance drawn from so many different ancestors cannot be expected to produce a uniform herd. It is quite common for breeders who perceive some unwanted characteristics within their herds, to seek to correct this immediately by the use of a bull with entirely different bloodlines and an extreme development of the individual characteristics which it is desired to correct.
This is what is called a “cold out-cross.” It rarely works. The first generation may show (it also may not) an average midway between the characters of the two parents and consequent improvement of the characters desired. It is probable that subsequent generations will show a great degree of variation, and the resultant herd, in the long run, will be a group of dissimilar individuals. The man who practices this system is likely, at the end of more years than he may have left, to find that he has been a cattle multiplier rather than a breeder.
Correction of unwanted tendencies can be brought about with more speed and certainty through the use of satisfactory individuals within the breeder's own strain; or by the use of close-bred individuals from some other breeder whose animals happen to be satisfactory and whose general objectives are similar to those of the breeder wishing to make the correction. Then, when it is corrected, it will stay corrected.'
From "Dual-Purpose Cattle" by Claude H. Hinman, 1953.