Prop.Devenport the author of “The Principles of Breeding” further states; Line breeding carried to its limits involves the breeding together of individuals closely related. When it involves the breeding together of sire and offspring or of dam and offspring or of brother and sister, it becomes inbreeding, or “breeding in and in” It is line breeding carried to its limits, and of course possesses all the advantages and disadvantages of that form of breeding carried to their utmost attainable degree.
Forms of inbreeding.
Three forms of inbreeding are possible among animals, namely:
- Breeding the sire upon his daughter, giving rise to offspring three fourths of whose blood lines are those of the sire, a practice which, if followed up ,soon results in offspring with but one line of ancestry, thus practically eliminating the blood of the dam. This form of breeding is practiced when it is desired to secure all that is possible of the blood of the sire.
- Breeding the dam to her own son or sons successively,thus increasing the blood line of the female side. This form of practiced when it is the dam’s blood lines that are to be preserved and condensed. Both systems are necessarily limited to the lifetime of the individuals involved. Either system can of course be approximated by the use of granddaughter or grandson, which would by common consent be called inbreeding, but relationship more remote would generally be regarded merely as line breeding.
- Breeding together of brother and sister, —a form of inbreeding which preserves the blood lines from both sire and dam in equal proportions. It is inferior to either of the others as a means of strengthening previously existing blood lines,but it is freely employed when the combination has proved exceptionally successful ,virtually establishing a new type
It has all the dangers of the other two ,and in a larger degree, because we have practically no acquaintance with the new combination, whereas in strengthening the proportion of one line of ancestry over another ,whether it be that of the sire or that of the dam, we are dealing with previously existing blood lines known to be harmonious.
Advantage of inbreeding
Nobody claims advantages in inbreeding per se but it is the acme of line breeding, and when superior individuals are at hand it is the most powerful method known of making the most of their excellence. It is the method by which the highest possible percentage of the blood of an exceptional individual or of a particularly fortunate “nick” can be preserved, fused into and ultimately made to characterise an entire line of descent on both sides.
If persisted in, the outside blood disappears by the same law that governs grading, and the pedigree is speedily enriched to an almost unlimited extent by the blood of a single animal,in practice, generally that of the sire. It is a method not so much of originating excellence as of making the most of it when it does appear,and it is not too much to say that a large proportion of the really great sires have been strongly inbred.
An inbred animal is of course enormously prepotent over everything else. Its half of the ancestry, being largely of identical blood,is almost certain to dominate the offspring. Inbreeding is, therefore, recognised as the strongest of all breeding, giving rise to the simplest of pedigrees,—-an advantage quickly recognised when we recall the law of ancestral heredity. In this respect it is all that line breeding is and more.
A second advantage is that successful associations of characters are preserved intact and not shattered by the infusion of new strains . If the breeder were dealing with but a single character he could readily find its equal, and there would be little need for inbreeding.
Disadvantages of inbreeding
Tradition everywhere has it that inbreeding, if long continued,is practically certain to end in loss of vigor and of fertility, and plenty of instances are given to “prove” it .
Now a rational consideration of the principles of transmission has already led us to expect that bad characters as well as good will be intensified. We could not expect so powerful a method to work only to our advantage and to grant immunity from disadvantage in all cases.
What we want to know is whether ,in respect to trouble, we are to look out for likelihood or for certainty; whether disaster is inevitable, or only extremely probable. This question has been much befoffed by certain catchy statements such as, “Nature abhors incestuous breeding”, all of which confuse an ethical and social question with the biological one in which only we are interested.
Lack of vigor and low fertility the two most common defects.Few individuals are fully fertile, that is free and regular breeders, and fewer yet are both fertile and vigorous. Shortcomings in these two respect may be call distinguishing defects of both plants and animals under domestication. In nature they constitute the chief points of attack of natural selection, but in domesticated animals we commonly select for other points, even colour, trusting to luck for vigor and fertility. Is it any wonder that these lurking evils have crept upon us until they often constitute an insurmountable bar to inbreeding, and have invaded even our most carefully outbreed line of birds. As inbreeding is the supreme test of a race, so it is a character: if a character suffers by inbreeding it is a sign of natural defectiveness and should be accepted as such, and not laid up as an additional instance and a weapon with which to abuse a system with a history of laudable achievement in the past and rich with possibilities for the future.
No man has bred Berkshires more successfully than N.H. GENTRY, of Sedalia, Missouri, and no American breeder has been credited with a freer use of inbreeding. This veteran breeder writes as follows;
My experience in inbreeding is that you do good ,or fail in proportion to the quality in the strain of blood; that is, that you intensify what you have, let it be good or bad, let it be weak or strong in constitution. The theory advanced by the mass of people, to the effect that you degenerate size and weaken constitution, is all wrong unless the strain you are inbreeding lacks size as a rule, or lacks constitution. Animals that have plenty of size and a vigorous constitution can have these traits intensified as certainly as you can lessen these traits by inbreeding with strains lacking these essential traits. If you can intensify the one it seems to me as reasonable that you can the other; so a man’s success in inbreeding will depend upon what he has to inbreed with. Rightly and intelligently done I have never been able to detect any bad results whatever from inbreeding.