Heterozygosity excess at the cattle DRB locus revealed by large scale genotyping of two closely linked microsatellites

A method for MHC DRB typing in cattle based on two closely linked and highl y polymorphic microsatellites is described. The two microsatellites DRBP1ms and DRB3ms are located in intron 2 of the corresponding DRB gene. The very strong linkage disequilibrium between the two loci made it possible to est ablish DRB microsatellite haplotypes. The typing results with this method o n reference samples followed closely that obtained with RFLP and direct seq uence analysis of DRB3 exon 2. The method is well suited for large scale ge notyping and was successfully applied for typing more than 600 unrelated an imals representing 23 breeds. The data were used to test whether the observ ed DRB allele frequency distributions were consistent with that expected fo r selectively neutral alleles in populations at mutation-drift equilibrium. A significant heterozygosity excess was detected and there was an obvious trend across breeds towards a more even allele frequency distribution than expected. The deviation may be due to balancing selection acting on the DRB locus or by recent population bottlenecks.