by Emil Romando Jr.
E-Mail: eromando@taconic.net

The Thoroughbred racehorse is a hybrid animal which has considerable variation in both its genotype and its typology. Each racehorse has a genetic profile that traces back to lineages common to the very origins of the breed. From these confined roots, the ancestral condition of the Thoroughbred horse has diverged into many racing aptitudes and levels of talent. The challenge to every breeder is to recognize how the variations of individual performance can be maintained and transmitted to succeeding generations. We need help in understanding the historical narratives that are found in the pedigrees of horses we breed. Pedigrees are genetic codes. They consist of many ancestors some especially valued for exemplary racing achievement or their ability to pass on favorable traits. Only highly valued ancestors and their best representatives can meet the exacting criteria of quality demanded to help upgrade for racing performance. Indisputably, every two or three generations working lines from these superior individuals tend to fade. No matter what the accomplishments from racing or breeding programs, nature smoothes out all inconsistencies that deviate from the accepted norm. In an attempt to counteract this slide toward mediocrity, we can give a brief description of four methods of breeding Thoroughbreds. They are inbreeding, linebreeding, outbreeding and outcrossing.

Inbreeding is the duplication of one particular ancestor, common to both the sire and dam, within four generations of a pedigree. Horses have inbred pedigrees when an ancestor is represented at least twice anywhere within the first four removes of its sire and dam. Inbreeding can induce dramatic results from matings between animals of sound and superior constitutions. It is a somewhat controversial practice because specific traits are magnified for both positive and negative genetic expressions. This type of mating is perhaps the most problematic one for breeders offering pitfalls as well as possibilities for outstanding potential.

Linebreeding includes 4x4 matings of a common ancestor in both the sire and dam and continues to the sixth generation. Multiple combinations of one ancestor that edge over into the seventh generation are also linebred. Multiple duplications to one or more ancestors of high class merit in the 4th, 5th and 6th generations offer a less obvious agenda for top flight racing performance but are successful especially if these highlighted ancestors are themselves either inbred or genetically compatible.

Outbreeding consists of one parent of a mating being inbred, linebred or both. An effort is made to breed away from the parent that has an intense concentration of duplications by selecting a partner which is genetically different. Too close a build up of genetic ancestors permits recessive genes to block favorable results. Such a pedigree needs to be opened up in order to renew possibilities for subsequent generations. Incidentally, mares with strong concentrations of same ancestors, genetic relatives or even genetically compatible lines (nicks) tend to make for desirable breeding animals.

Where there are no duplications within six generations of a pedigree, outcrossing has taken place. The assumption here is that hybrid vigor has been maintained from the mating of two genetically dissimilar parents. While this assumption is correct, the other methods of breeding also contribute to hybrid vigor so long as they are not taken to the extreme. Constant outcrossing runs the risk of diluting the cohesive genetic pool unique to the Thoroughbred. Understandable, we need to vary the methods of breeding through an alternating sequence of expanding and contracting, as required, the distinctive genetic modalities of individual horses. In this way, we are better able to obtain specific results from superior animals that allow us to breed with confidence.