The prevalence of mixed DNA profiles in fingernail samples taken from couples who co-habit using autosomal and Y-STRs.

Physical contact can result in the transfer of DNA from one individual to another. In cases of sexual offence this can often be by means of an aggressive or sexual act. Biological material can accumulate under the fingernail hyponychium of both the victim and/or the suspect and has the potential to provide evidence and intelligence information to the police. The incidence of mixed DNA profiles obtained from fingernail samples of couples who co-habit was explored using AmpFlSTR SGM Plus (SGM Plus) (Applied Biosystems). It was predicted that the higher incidence of contact from co-habitation and therefore the greater opportunity for DNA transfer would result in an increased level of foreign DNA from the partner under the donor's fingernails. Fingernail swabs were taken from all fingers of the left and right hands of 12 couples on three separate occasions. DNA profiles were compared to both the donor and partner's reference DNA profiles. The number of reportable mixed DNA profiles obtained (17%) was higher than in previous reports. In this study, the majority of non-donor alleles matched the partner's reference DNA profile, indicating that co-habitation affected non-donor DNA obtained from fingernail swabs. The results demonstrated that as the couples spent increasing amounts of time together, the incidence of mixed DNA profiles increased. The large variation in the incidence of foreign alleles observed within and between couples suggested that a combination of lifestyle factors were having a significant effect on whether mixed DNA profiles were observed. Further DNA profiling using the AmpFlSTR Y-filer (Yfiler) PCR amplification (Applied Biosystems) was carried out to determine whether female fingernail samples that had previously given single donor profiles contained low levels of male DNA that may be detected by targeting the Y-chromosome. It was found that 63% of the samples analysed using Yfiler produced full or partial Y-STR profiles. The Y-STR profiles were compared to partners' profiles and searched through two haplotype databases to determine the evidential value of such samples.