In forensic science, X-chromosomal short tandem repeats (ChrX STRs) bear the potential to efficiently complement the analysis of other genetic markers (autosomal, Y-chromosomal or mitochondrial). We review the population genetic properties and forensic utility of selected ChrX markers, and discuss the problems and limitations arising with their practical use. Formulae required to assess the evidential power of individual markers in different contexts are summarised and applied to ChrX STRs of interest. Since linkage and linkage disequilibrium between markers affect the inferential interpretation of genotype data, practically relevant information regarding the co-localisation and haplotypic association of ChrX STRs is provided. Finally, two examples of complex kinship testing are presented which serve to highlight the particular importance of ChrX STRs for solving deficiency cases and cases involving blood relatives.

Short tandem repeat (STR) markers DXS6801 (GATA41B11), DXS6809 (GATA69B129) and DXS6789 (GATA31F01) are located in a 3-Mb region on human chromosome Xq21, spanning approximately 3-6 cM. Theoretically, this cluster could give rise to 1,144 different haplotypes in the German population. In fact, genotyping of 806 males revealed the presence of 207 different haplotypes. Since the three STRs have been shown to be in strong linkage disequilibrium (LD), haplotype frequencies cannot be computed on the basis of single locus allele frequencies alone, but have to be estimated directly instead. In this work, we present data on linkage, haplotype frequencies and LD in the German population. To highlight the potential of the STR cluster for forensic analysis, we also report two examples of its successful application in pedigree-based kinship testing.

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The evaluation of the short tandem repeat (STR) markers DXS10079, DXS10074 and DXS10075 was amended to establish a STR cluster spanning a genetic distance<1 cM. These three STRs are located within a 280-kb region at Xq12 and provide stable haplotypes useful for solving complex kinship cases. Theoretically, this cluster could give rise to 2,548 different haplotypes in the German population and the genotyping of 781 men revealed the presence of 172 haplotypes. Since the three STRs were shown to be in strong linkage disequilibrium (LD), haplotype frequencies cannot be computed on the basis of a single locus allele frequency alone but have to be estimated directly. Here, we present data on linkage, haplotype frequencies and LD in a German population. Further clusters from other regions of the X chromosome will be published in the future to cover the chromosome with a well-structured network of highly informative sites.

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