Scientific Annals of Polish Society of Animal Production - Vol. 11 (2015), No 3, 9-21 Genetic structure analysis of American Quarter Horses* Joanna Kania-Gierdziewicz, Maciej Gierdziewicz, Edyta Stankowska-Żołądź University of Agriculture in Krakow, Faculty of Animal Breeding and Biology, Department of Genetics and Animal Breeding, al. Mickiewicza 24/28, 30-059 Kraków; e-mail:
[email protected] The study investigated inbreeding and relationship in American Quarter Horses (AQ) kept or bred on the Roleski Ranch stud farm (Stare Żukowice, Małopolska region). The data consisted of pedigrees of 76 AQ horses (40 stallions and 36 mares) born in 1993-2012. Co- efficients of inbreeding (FX) and relationship (RXY) were calculated according to Tier. The effective number of founders (fe) and effective number of ancestors (fa) were calculated as well. Almost 91% of horses were found to be inbred – 90% of stallions and 91.67% of mares. In sex groups, FX averaged 0.0132 and 0.0142 for all stallions and all mares, and 0.0146 and 0.0155 for inbred stallions and inbred mares. About 97.37% of animal pairs were found to be related. Over 99% of male pairs were related, with RXY for all and inbred males averaging 0.0610 and 0.0641, respectively. In females, nearly 95% of pairs were re- lated, but RXY values were slightly lower, averaging 0.0590 and 0.0591 for all and related pairs, respectively. Among mixed male-female pairs 97.5% were related, with RXY values averaging 0.0607 and 0.0622 for all and related pairs, respectively.