Pedigree analysis of four decades of Quarter Horse breeding.

Pedigrees of randomly selected Quarter Horses born in each of the years 1946, 1956, 1966 and 1976 and of winning halter, cutting and race horses born in the same years were evaluated and compared. Average inbreeding and inter se relationship levels and relationships of influential ancestors to the sample were calculated for each sample. The amount of Thoroughbred influence and the average generation interval were also determined for each random sample. The levels of inbreeding found in the random samples were low, ranging from 1.3% in 1956 to 2.6% in 1966; however, these levels were higher than would be expected if mating were random. Show and race winners born in 1976 appeared to be less inbred than random horses of the breed. The estimated average inter se relationship within the random samples increased from 0% in 1946 to 3% in 1966, decreasing again to 0% in 1976. Horses in the elite samples appeared to be more closely related to each other than those in the random samples. Fifteen ancestors of horses in the random samples were identified as influential to the Quarter Horse breed. Many of these same ancestors were influential in the halter and cutting samples, but only one was influential in the race samples. The percentage of pedigree lines in the random samples that contained a Thoroughbred ancestor were as follows; 1946, 27.5%; 1956, 19.2%; 1966, 23.2% and 1976, 31.4%. The average generation interval fluctuated from approximately 8 yr to approximately 10 yr for the random samples.