Forensic efficiency of microsatellites and single nucleotide polymorphisms on the X chromosome

Polymorphisms located on the X chromosome are being increasingly used in forensic studies. However, they have not been studied as extensively as the autosomal and Y-linked polymorphisms. Therefore, we planned to study the population genetics of two sets of X-linked markers, including short-tandem repeats (STRs) and single nucleotide polymorphisms (SNPs), and particularly analyze the differences at the microgeographical level. Six X-linked STRs (DXS 9895, DXS 7132, DXS 9898, DXS 6789, GATA 172D05, and DXS 7130) and ten SNPs (rs1229078, rs1544545, rs4442270, rs1874111, rs5968332, rs1166756, rs12849634, rs5932595, rs203648, and rs611711) were studied in two population samples from Cantabria, northern Spain, a mixed coastal population and a relatively isolated small population in the Pas valley. There were statistically significant differences in allelic frequencies of the six STRs studied between both populations. On the other hand, only one out of ten SNPs studied showed between-population differences. Overall Fst values were 0.4-2.9% and 1.8-2.2% for the STRs and the SNPs, respectively. The overall power of discrimination for female samples was higher than 99.99% for both groups of markers. Therefore, these sets of X-linked STRs and SNPs seem to be potentially useful in forensic genetics, but care should be taken when interpreting results from cases that originate from small and relatively isolated populations.