The Meaning Of A Y Chromosome Match

Due to the fact that the Y chromosome is passed down unchanged (except for mutations) from father to son, the observation of a match with Y-STRs does not carry the power of discrimination and weight into court as an autosomal STR match. Peter de Knijff (2003) discusses some of the challenges of presenting Y-STR results in court. He concludes that information from the Y-STR Haplotype Reference Database (http://www.yhrd.org) should be seen as qualitative rather than quantitative because this database cannot provide a reliable frequency of the population at large. Thus, the fact that the Y-STR profile listed in the previous paragraph is found six times out of 23 597 haplotypes queried in the worldwide database does not necessarily mean that this minimal haplo-type profile is expected to be seen 0.025% (6/23 597) times from a random selection of unrelated males. Of course, this same Y-STR profile would also be seen in all brothers, male children, father, uncles, paternal grandfather, paternal cousins, etc., barring any mutations.

The following statement is an example of a conservative conclusion for a matching Y-STR profile as it might be reported to the court (de Knijff 2003): 'The Y-STR profile of the crime sample matches the Y-STR profile of the suspect. Therefore we cannot exclude the suspect as being the donor of the crime sample. In addition, we cannot exclude all patrilineal related male relatives and an unknown number of unrelated males as being the donor of the crime sample.' In spite of accuracy of the above conservative statement, courts are likely to require some kind of statistic to give meaning to a match (i.e., more than just a simple 'failure to exclude' statement). Budowle and co-workers (2003) advocate the counting method for this purpose because of its operational simplicity. Confidence intervals may be used to reflect the uncertainty involved in population database samplings of unrelated individuals, particularly since many possible rare haplotypes will not be observed with typical database sizes of hundreds to thousands of individuals. Thus, Y-STR profile frequency estimates with confidence intervals can be calculated in a similar fashion as mitochondrial DNA (see D.N.A. Box 10.3).

Using the counting method with an upper bound confidence limit and following the formulas laid out in D.N.A. Box 10.3, calculation for the above example profile that matched six times in a database of 23 597 yields 0.00025 + 1.96[{(0.00025)(1 - 0.00025)}/23 597]1/2 which equals 0.00045 (0.045% or 1 in 2000). Of course, the relevance of using an entire world survey of Y chromosomes rather than a specific ethnic or geographic group for a population comparison must also be considered within the scope of the case. Most likely a number of different population databases would be utilized in a case report for comparison purposes. Frequency estimates calculated with the counting method while not as powerful as those produced with unlinked autosomal STRs may nevertheless be informative in many forensic casework scenarios and provide another piece of evidence in the overall framework of a case.

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Stammering Its Cause and Its Cure

Stammering Its Cause and Its Cure

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