Small effective population size and genetic homogeneity in the Val Borbera isolate.

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Population isolates are a valuable resource for medical genetics because of their reduced genetic, phenotypic and environmental heterogeneity. Further, extended linkage disequilibrium (LD) allows accurate haplotyping and imputation. In this study, we use nuclear and mitochondrial DNA data to determine to what extent the geographically isolated population of the Val Borbera valley also presents features of genetic isolation. We performed a comparative analysis of population structure and estimated effective population size exploiting LD data. We also evaluated haplotype sharing through the analysis of segments of autozygosity. Our findings reveal that the valley has features characteristic of a genetic isolate, including reduced genetic heterogeneity and reduced effective population size. We show that this population has been subject to prolonged genetic drift and thus we expect many variants that are rare in the general population to reach significant frequency values in the valley, making this population suitable for the identification of rare variants underlying complex traits.