As with mitochondrial DNA information that will be discussed in the next chapter, the size of the database matters in trying to estimate the rarity of a Y-STR profile. The lack of recombination between Y chromosome markers means that
Y-STR results have to be combined into a haplotype for searching available databases as well as estimating the rarity of a particular haplotype.
Generally speaking there are three possible interpretations resulting from a Y-STR test: (1) exclusion because the Y-STR profiles are different and could not have originated from the same source, (2) inconclusive where there are insufficient data to render an interpretation or ambiguous results were obtained, or (3) inclusion or failure to exclude as the Y-STR results from two samples are sufficiently similar and could have originated from the same source.
When the evidence and suspect do not match, then Y-STR typing is helpful in demonstrating the exclusion. However, estimating the strength of a match when a suspect's Y-STR haplotype cannot be excluded is more problematic (de Knijff 2003). Yet it is common practice to place some significance on the likelihood of a random match. Thus, statistics derived from population data would usually be applied.
Three approaches to evaluating the rarity of a coincidental match using Y-STR markers are reviewed by Budowle et al. (2003). These include (1) the counting method, (2) a Bayesian approach (Roewer et al. 2001, Krawczak 2001), and (3) the use of a 'mismatch' distribution of haplotypes present in a reference database to evaluate how often two randomly selected haplotypes would be at a molecular distance as close as the two matched haplotypes found in the case analysis (Pereira et al. 2002).
Within the European Y-STR Haplotype Database (http://www.ystr.org), the most frequent minimal haplotype occurs approximately 3% of the time (Roewer et al. 2001). This most common type has the following alleles: DYS19 (14), DYS389I (13), DYS389II (29), DYS390 (24), DYS391 (11), DYS392 (13), DYS393 (13), and DYS385a/b (11,14). As will be discussed in more detail in a later section, the use of additional markers beyond the minimal haplotype has been shown to help differentiate individuals that have this most common type (Redd et al. 2002, Schoske et al. 2004).
A query of the Y-STR Haplotype Database (http://www.yhrd.org) on 12 June 2004 with a Y-STR minimal haplotype of DYS19 (14), DYS389I (13), DYS389II (29), DYS390 (24), DYS391 (11), DYS392 (14), DYS393 (13), and DYS385a/b (11,15) revealed six matches in a worldwide sample of 23 597 haplotypes from a set of 211 populations. These matches can be broken down into the following population statistics in various subpopulations: 1/147 (Bogota, Colombia), 1/185 (Central Portugal), 1/135 (Cologne, Germany), 1/661 (Leipzig, Germany), 1/81 (Liguria, Italy), and 1/247 (London, England).
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