For an introduction to the potential and a case summary* of the use of Y-STR haplogroups in forensics, see:Getting back to the 2013 paper by Vecchiotti and Zoppis, available at:
DNA and the law in Italy: the experience of “the Perugia case” - PMC
pmc.ncbi.nlm.nih.gov
Table 1 of the paper compares the interpretations of the electropherograms (graphs) of the Y chromosome STRs - the Y chromosome DNA profiling results for the bra clasp - by the prosecution's technical consultant (Stefanoni) and by the Hellmann court's independent technical consultants (Conti and Vecchiotti).
There are 16 STRs listed; the DYS prefix on the locus identifiers stands for DNA - Y chromosome - sequence number; these identifiers are assigned by an international organization that sets standards for human gene nomenclature.
According to Table 1:
For 15 of the STR loci, Stefanoni reported only 1 peak, but for 1 STR she reported 2 peaks. She concluded that the Y-STR results showed the DNA profile of one male and that the profile was compatible with Sollecito.
For 4 of the STR loci, Conti & Vecchiotti reported only 1 peak, but for 9 STRs they reported 2 peaks, and for 3 STRs they reported 3 peaks. All the additional peaks were above the manufacturer's threshold (50 RFU), with the heights of the peaks not reported by Stefanoni ranging from 59 to 201 RFU. This last peak reached over 32% of the height of the Stefanoni-reported peak, while the others were also of significant relevant height. The peaks not reported by Stefanoni were not in stutter position. Therefore, since the peaks were above threshold and not in stutter position they were not artifacts of the test and, following accepted international DNA profiling standards should have been reported. The C & V results show that three males contributed DNA to the bra clasp sample.
Why did some of the STRs have only 1 or 2 peaks rather than 3? A likely reason is that the Y chromosome STRs of one or both of the males that were the source of the DNA that had not been reported by Stefanoni shared allele repeat values with the DNA profile attributed to Sollecito for those STR loci. "Allele sharing" - a person having some STRs with the same number of repeats as another, even unrelated, person is not uncommon. Persons who are in the same haplogroup although extremely very distantly related will be likely to share some STR alleles*. On the other hand, other technical reasons that are DNA profiling artifacts, for example, drop-out, may be responsible. Yet another possibility is that the two unknown male DNA deposits had degraded during the time (deposited up to 46 days prior to collection and subsequent profiling) that the bra clasp lay on the floor of the murder room, while possibly the DNA compatible with Sollecito's profile was deposited later, at the time the clasp was collected by the obviously dirty and pnossibly DNA contaminated gloves of the scientific police. (The recommended method of collection is to use DNA-free sterile disposable forcepts.)
The conclusion is that the DNA profiling results, and the known failings to follow appropriate storage, chain of custody, and collection methods strongly suggest the male DNA on the bra clasp are the result of contamination. Thus, under Italian law, CPP 192, paragraph 2, no fact against an accused could be lawfully inferred from the DNA profiling of the bra clasp.
*See: https://en.wikipedia.org/wiki/Human_Y-chromosome_DNA_haplogroup
*A 16-year-old girl, Marianne Vaastra, from a small village in Friesland, the Netherlands, was raped and murdered on her way home from another village, in 1999. DNA profiling of autosomal and Y-chromosomes of semen on the victim was conducted.
Some initial suspicion in the case was directed at asylum-seekers from the Middle East who lived relatively near the site of the crime. A mass canvas of DNA profiling of male volunteers and suspects was begun. It became clear, after some time, that the Y chromosome haplotype indicated that the rapist/murderer was a person of northwest European ancestry, excluding the asylum-seekers. The rapist/murderer, after some years of the DNA profiling canvas, was identified by a match of autosomal and Y-chromosome STRs. This individual was a man of Dutch ancestry who lived in a village about 2.5 km from the murder site, was one of the DNA profile volunteers, and confessed to the rape and murder after his arrest.
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