In paternity cases involving close blood-relatives as suspects, the exclusion power of STRs is substantially reduced and ChrX STRs may be superior to AS markers. For example, if two alleged fathers are father and son, they would not share any X-chromosomal alleles identical by descent (ibd) so that ChrX markers would be more efficient than AS markers. Brothers, in contrast, share a given maternal ChrX allele with probability 0.5, which corresponds to the probability of exactly one allele shared identical by descent at an AS locus.
For four unlinked ChrX loci, the chance of sharing alleles identical by descent would be 0.54 = 0.0625. Unfortunately, since the ChrX length is not more than 198 cM this chromosome can host a maximum of four, but strictly only three unlinked marker regions. When the markers are closely linked, they do not segregate independently. As with AS markers, they would instead represent a single haplotype that is shared with a probability of 0.5. The ChrX contains four linkage groups located at Xp22.2, Xq12,Xq26 and Xq28 that can provide independent genotype information (Figure 7.1). At present, we propose that it is preferable to use clusters DXS10135-DXS837, DXS7132- DXS10074, HPRTB-DXS10101 and DXS7423- DXS10134 to define haplotypes in forensic practise. Typing of these four marker pairs can be done by using the PCR kit Mentype® Argus X-8 that is now commercially available. Alternatively, other cluster haplotypes such as DXS101-77424, DXS6801-DXS6809-DXS6789 and DXS10076-DXS10077-DXS10078 may be chosen on the basis of Table 7.2 and Figure 7.1.
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