Microsatellites or simple sequence repeats are tandemly repeated DNA sequences found in varying abundance in all human chromosomes. In forensic science the term short tandem repeat (STR) is ingrained. Including ChrX the overall STR density is comparable in all chromosomes (Subramanian et al., 2003). Within the chromosomes the density of STRs, however, shows significant variations. Tri- and hexanucleotide repeats are more abundant in exons, whereas other repeats are more abundant in non-coding regions. Moreover, as has been shown (McNeil et al., 2006), a striking enrichment (>10-fold) of [GATA]n is revealed throughout a 10 Mb segment at Xp22 that escapes inactivation (Lyon, 1961), and is confirmed by fluorescence in situ hybridization. A similar enrichment is found in other eutherian genomes. These findings clearly demonstrate sequence differences relevant to the novel biology and evolution of the X chromosome. Furthermore, they implicate simple sequence repeats, linked to gene regulation and unusual DNA structures, in the regulation and formation of facultative heterochromatin.
The analysis of tri-, tetra- and penta-STRs has become widespread in forensic medicine and STRs located on autosomes (Brinkmann, 1998; Urquhart et al., 1994) were used long before application of Y-chromosomal (de Knijff et al., 1997; Jobling et al., 1997; Kayser et al., 1997a, 1997b; Roewer et al., 2001) and X-chromosomal STR markers. Although the existence of ChrX STRs, i.e. HPRTB (Hearne and Todd, 1991; Edwards et al., 1992) and ARA (Edwards et al., 1992; Sleddens et al., 1992) and DXS981 (Mahtani and Willard, 1993), was reported relatively early, the desire to use such markers as tools for forensic application came up later.
The International Society for Forensic Haemogenetics guidelines for the forensic use of microsatellite markers (Bär et al., 1997) apply to both AS and ChrX STRs. However, some specific molecular and formal genetic aspects need to be taken into account when dealing with ChrX markers. The forensic application of microsatellite markers can be done in practise if they are in Hardy-Weinberg equilibrium and have a high enough degree of polymorphism. The application of coding STRs such as ARA should be avoided. Some ethical considerations are discussed below.
Figure 7.1 and Table 7.2 review the main forensic ChrX repeat markers known to date. Most of them show no specific peculiarities in terms of their
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