Direct responses to selection are reviewed from 12 sheep and 3 cattle experiments in New Zealand and Australia, where the primary trait was resistance (R) or susceptibility (S) to “disease”. Apart from three single generation experiments, the life of the remaining experiments averaged 14.9 years to 1997 inclusive. Single-record heritability, from the base population or from a restricted maximum likelihood analysis of all years and all flocks/herds in each experiment, averaged 0.28. This was no lower than for production traits such as milk yield, body weight or fleece weight. Direct responses (on a transformed scale) between R and control lines, or between S and control lines, averaged 0.085 phenotypic standard deviations per year, higher than corresponding responses (0.065 units) in cattle selection studies of weight or gain or efficiency, reviewed by Mrode (1988). Absolute rates of direct response may depend on the mean incidence of disease in the control, and on overall fitness. The ultimate rate of response in an R line may diminish as the line reaches fixation or as phenotypic variation is reduced, or both. Major genes have been implicated for resistance or susceptibility to three different disease traits. In most cases the disease selection lines are also providing a resource for studies of the underlying causes of resistance/susceptibility

C. A Morris

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 27: Reproduction; fish breeding; genetics and the environment; genetics in agricultural systems; disease resistance; animal welf, , 295–302, 1998
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