Evidence for a major gene determining the resistance of Indonesian Thin Tail sheep against Fasciola gigantica

The genetic basis of the high resistance of Indonesian Thin Tail (ITT) sheep against Fasciola gigantica has been studied by measuring the resistance levels of ITT sheep, St. Croix sheep and their F 2 and F 3 crossbreds. All of the sheep were bred and reared in the same environment. Approximately half of the F 2 and F 3 crossbreds were as resistant as the ITT parental breed, one-quarter had the low resistance of the St. Croix parental breed and the rest had an intermediate level of resistance. It is concluded that the high resistance is determined by a major gene with incomplete dominance. The prevalence of the gene in the ITT sheep population is estimated to be about 90%. The resistance may be innate or acquired; but, regardless of the mechanism, it could be used to control fasciolosis caused by F. gigantica by breed substitution or by crossbreeding with other breeds of sheep.