Data are presented showing the incidence of halothane reaction in 10657 British Landrace tested in a commercial nucleus herd, in which positive reactors (HP) animals were not allowed to breed. Incidence of halothane reaction declined from 0.42 to 0.16 over a period of 4 years, but the calculated gene frequency only changed from 0.58 in non-selected pigs to 0.43 in halothane negative (HN) pigs over two generations of selection. Estimated penetrance for the recessive homozygotes was 0.8 to 0.9, and 0.0 to 0.018 for the heterozygotes. There was some evidence to suggest that heterozygotes had a selective advantage, and that this was leading to stabilising selection. The study demonstrates that halothane screening can successfully lower the incidence of halothane reaction under field conditions, although the rates at which this is achieved may be slow.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume X. Breeding programs for swine, poultry, and fish., , 174–181, 1986
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