STR genotyping of exogenous hair shat DNA

Most hairs found at crime scenes yield low quality and/or low quantities of nuclear DNA. This DNA is further depleted when stringent hair cleaning procedures are applied in the laboratory, suggesting that detectable DNA exists exogenously. The phenomenon of exogenous hair DNA is the subject of this study. DNA was extracted from washed and unwashed hairs and the resulting Profiler™ Plus STR genotypes were compared with those of reference (buccal) swabs from the hair donors. The DNA extraction procedure involved no prior cleaning of the hair sample and no dissolution of the hair during digestion, in contrast to standard procedures. The STR genotyping success was measured by recording the two dominant alleles at each locus and comparing them with the reference DNA profile. The effect of hair cleanliness was examined by leaving donors’ hair unwashed for periods of 1, 3 and 7 days before sampling. It was found that the genotyping success for unwashed hair was significantly higher than that for freshly washed hair, with the majority of clean hair samples producing little or no DNA. Genotyping success was also lower for donors with cosmetically treated hair compared with those having untreated hair. Although the quality of STR profiles (i.e. allele dropout, differential amplification) from hair shafts or telogen hair clubs is reduced compared with those from other biological sources, the genotypes obtained in this study may be usable and are certainly discriminating if alternative interpretational methods are applied