REPRESENTATION OF MARES BROUGHT TO OLD SORREL 1921 - 1943

A sketch of Old Sorrel surrounded by various imaginary broodmares, some of Thoroughbred type, others of Billy type, still others more or less mixed. They represent the variety of mares actually brought to Old Sorrel between about 1921 and his last foal crop, sired in 1943 (Old Sorrel covered a total of about 70 different mares in his lifetime, so this diagram is a simplification). I have assigned each mare a letter which can be thought of as a “mare family” or “mare bloodline.” Pink circles in the next part of the chart represent mares and fillies while blue squares represent colts. Mares A, B, C, D, E, and F may be covered more than once so that they bear one, two, or three foals, ideally over a short span of years. Mare B, for example, bears two foals, one filly and one colt. Since all the mares mated to Old Sorrel are unrelated to him, all foals represent outcrosses. The breeder will therefore concentrate on culling undesirable foals—one example given here is filly D1, which has “too much white.”

This is not a deleterious trait but simply the breeder’s preference. He also eliminates filly D3 because its conformation is “insubstantial.” Accident—bad luck—may intervene at any time by pure chance, as in colt F1 who “breaks a leg” and is euthanatized. Mare family E is eliminated from the program—at

least temporarily, until such time as breeding to an uncle or half-sibling is desired—because she has borne only a colt.

In the next stage, females who survive the initial round of culling are brought back to Old Sorrel for cover. This represents a round of intensive inbreeding. Mares A1, A3, B1, C2, D2, and F3 bear viable foals. Because these foals are inbred, a high rate of genetic disease and the exposure of other deleterious traits are expected, and the necessary culling acts to purge the gene pool of a large number of deleterious alleles.

When they mature, fillies who survive this round of breeding can be brought back to an uncle (E1 in this example) for another round of inbreeding, or else to an unrelated male (G1) for outcrossing in hopes of achieving a desirable “nick” genetically fueled by heterosis and positive pleiotropism.