The Basics of Line Breeding
There are basically two kinds or patterns of selective breeding. One is line breeding and the other is outcross breeding. Line breeding involves breeding to fairly close relatives and thus is a kind of inbreeding, whereas outcrossing involves breeding to non relatives or to very distant relatives, those to which there is no known genetic link.
Each of these breeding patterns has its advantages and disadvantages. Line breeding can accomplish several goals: 1) produce a more consistent and uniform offspring; 2) identify breeding populations that have no genetic flaws, or identify existing flaws so they can be eliminated; 3) accentuate and sustain more consistent excellence; 4) develop linebred progeny that will provide a "genetic kick" or hybrid vigor when they are outcrossed with non relatives.
The advantage of outcrossing is basically what people call hybrid vigor. This gives a genetic kick to offspring that are from parents that are as genetically distant from each other as possible within a given species. This is essentially the advantage derived from cross breeding, but that advantage declines in each successive generation. It is not as sustainable as the strengths developed from line breeding.
The difference between line breeding and outcrossing within a species can be illustrated on the macro level by the differences between breeding purebreds in one breed as opposed to breeding across breeds in the bovine species. At the macro level, all Herefords are linebred and any breeding of Herefords to other breeds is outcrossing or cross breeding.
Within the Hereford breed or line of purebred Herefords, there are two types of line breeding possible. One is to create a closed line of Herefords whose offspring are all bred to other animals or their linebred descendents in the original genetic pool. This pattern concentrates on accentuating the best traits that exist in the original genetic population, and the goal of this closed line pattern of breeding is to develop a very consistent and uniform group of excellent cattle. On the macro level that is how the "breeds" we have today came into existence. All Herefords trace to a small group of similar type of cattle developed in Herefordshire, and all registered Herefords trace to those animals in the original Herdbook. That is why Herefords of today that are purebred or closed line have the unique color patterns and composition associated with purebred Herefords. Those patterns have been "locked in" by closed line breeding.