Major histocompatibility complex (MHC) class I and class H loci are highly polymotphic in a majority of species including all major farm animals. There is clear evidence in farm animals as well as in other species that this polymorphism is maintained by some form of balancing selection. Although not yet formally proven, the cause of this selection is most likely pathogen infections. The mechanism promoting MHC diversity is overdominant selection and/or frequency dependent selection. The occurrence of balancing selection at MHC loci implies that it is important to maintain MHC diversity in farm animal populations. It is unlikely that it will be possible to improve general disease resistance by selective breeding for certain MHC haplotypes. It is suggested that a major challenge for future MHC research in farm animals will be to provide a better understanding of the cause for selection at MHC loci.

Leif Andersson

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 21. Gene mapping; polymorphisms; disease genetic markers; marker assisted selection; gene expression; transgenes; non-convention, , 177–181, 1994
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