SEWALL WRIGHT’S TABLE OF RELATEDNESS
Several important insights may be gleaned from this table. The degree of relatedness, expressed as a percentage, increases with the number of generations over which inbreeding continues to be carried out. The rate of increase is higher when matings occur between more closely related individuals—it is higher between parent and offspring than between uncles and nieces. All inbreeding plans ultimately lead to fixation of recessive alleles. In the “Plan A” breeding scheme, a single male is mated to two half-sisters in each generation. Under “Plan B,” a single male is mated to many half-sisters in each generation.
This table shows the degree of relatedness that results from different breeding plans, which is a measure of the number of identical alleles found in related individuals. Degree of relatedness is vastly different than the inbreeding coefficient, which geneticists designate by the letter “F” (or sometimes “f”), which is a measure of the degree of inbreeding and is usually a low number because it assumes every horse with a unique name in a pedigree is unrelated to all others.
The F value of the Quarter Horse breed as a whole is about 1.2 percent, while the coefficient for the King Ranch Quarter Horse breeding herd is much higher, about 8.6 percent. In terms of degree of relatedness, the most closely related King Ranch Quarter Horses have about 62 percent of their alleles in common. Numerous scientific studies have shown that F values higher than about 9 percent, or relatedness greater than that between first cousins (25 percent of alleles in common) is associated with high incidence of genetic disease, loss of fertility, foaling problems, metabolic syndromes, immune deficiencies, conformational defects, and behavioral difficulties. James Clement III, current head of the King
Ranch horse breeding division, confirmed in a recent conversation that Bob Kleberg’s policy of avoiding excessive inbreeding is still in effect. “Whenever we think the inbreeding is getting a little too tight,” Clement observed, “we go to an outcross.”