Linebreeding

Abstract

Linebreeding is a form of inbreeding (usually mild) directed toward keeping the offspring closely related to one ancestor (usually a much admired one). All inbreeding not necessary for holding this relationship high is avoided as far as possible. It is practiced to conserve, among the descendants, the good traits of an outstanding sire or dam, increasing those descen- dants in numbers without lessening their resemblance to this ancestor even for many generations after that ancestor’s death. The more superior a breeder’s herd or flock is to the average merit of its breed, the more reason he has to practice linebreeding to his very best animals or to the very best of their recent ancestors. Breeders of grades cannot often afford to do much linebreeding. The closeness of relationship and the intensity of inbreeding are presented in figs. 1 and 2 and table I for some of the more common types of mating related animals. A breed can be improved most rapidly when at least its better herds are practicing linebreeding at about the rate which would oceur if it were divided into isolated groups of herds with about 3 to 5 sires in each group. Several plans are suggested whereby an individual may start on a linebreeding plan with some assurance that he will not have to give it up soon in order to avoid extreme linebreeding to ordinary ancestors. All these plans involve cooperation with other breeders in some way or other except in those rare cases where a single herd is large enough to use economically three to five sires at all times. The more meritorious the foundation stock is and the more skillful the breeder is in his selections, the more extremely he can linebreed with safety. From the genetic point of view, linebreeding is a form of inbreeding devised to hold whatever has already been gained by selection while trying to make still further gains by additional selection. The selection practiced in linebreeding is aimed primarily at deciding which proved ancestors are to have their influence perpetuated. There is no magie about the linebreeding process which will automatically produce good results. If selection is not practiced, a breeder would do well to avoid linebreeding altogether. But a breeder starting with good stock and directing the line- breeding toward the best of its recent ancestors can effect more improvement by selection while holding the improvement he already has than would be possible if he were continually outcrossing.