Review: Forensic Identification of Gender from Fingerprints
Emily C. Lennert
Biology, Chemistry, Patterned Evidence
fingerprint, male, female, identification, gender, biological, sex, amino acid, protein, extraction, enzyme, assay
- Huynh, C.; Brunelle, E.; Halámková, L.; Agudelo, J.; Halámek, J. Forensic identification of gender from fingerprints. Analytical Chemistry. 2015, 87, 11531-11536.
The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
Fingerprints have long been used for comparison and matching in forensic analysis as well as a source of DNA. However, in the absence of a reference sample to allow for comparison of the fingerprint or DNA, these samples may be of little use. Biological materials contained in fingerprints may provide additional information for investigators that could be used in the identification of a suspect. In this study, the authors provide an alternate method of fingerprint analysis to allow for the differentiation of males and females using the biochemical composition of sweat contained in fingerprints.
Sweat, which is deposited with a fingerprint, contains various biochemical including amino acids. Amino acids are compounds found within the body that serve as the “building blocks” for proteins and play a role in metabolism. Previous studies have shown that amino acid levels differ considerably between males and females, with females, on average, possessing higher total levels of amino acids. Based on this difference in amino acid concentration, a fingerprint may be used to identify whether the depositor was male or female.
The method presented uses biocatalytic assay, i.e. an enzyme cascade interaction, to produce a visible color change in solution. The enzyme L-amino acid oxidase type IV (L-AOO) reacts with amino acids, producing hydrogen peroxide (H2O2). Another enzyme, horseradish peroxidase type VI (HRP) consumes the H2O2, leading to the oxidation of the o-dianisidine dye that produces a color change. The intensity of color corresponds to the concentration of amino acids present in the sample. The more intense the color, the higher the concentration of the amino acid. The solution may be visually observed as well as spectroscopically analyzed. A SpectraMax Plus384 spectrometer was used to analyze samples at a wavelength of 436 nm, which corresponds to the absorbance wavelength of the dye. This allows the instrument to read the intensity of the color, since it is at the correct wavelength.
Simulated samples of amino acid mixtures were examined first. Concentrations of 23 amino acids were chosen for each sample by random generation with statistical software. The resulting mixtures, containing varying concentrations of amino acids, were prepared: 25 mixtures representing male samples and 25 mixtures representing female samples. The amino acid content followed published amino acid distributions for males and females, and thus, the authors stated, should correspond to potential real world samples. The samples were processed with the biocatalytic assay and analyzed. The absorbance of each sample at 436 nm was collected and graphed as a function of time, up to 1800 seconds. Female samples were observed to produce higher absorbance values compared to male samples with little overlap between male and female samples. A cutoff absorbance value of 0.439 was determined, with absorbance values above the threshold being attributed to female samples and values below being attributed to male samples. The performance of the assay was evaluated to estimate the probability of accurately determining whether the sample was from a male or female based on amino acid content in the simulated samples. A receiver operating characteristic (ROC) curve was generated, which plots specificity against sensitivity to determine the probability that the method will correctly differentiate between male and female samples. Sensitivity refers to the true positive rate, e.g. the sample is identified as male and is truthfully male. Specificity refers to the false positive rate, e.g. the sample is identified as male and is truthfully female. When plotted against one another, the area under the curve (AUC) can be used to determine the probability of a correct determination by the method. A generalized depiction of a ROC curve can be seen below. Using simulated samples, the method was determined to have a 99% probability of accurately distinguishing male and female samples.
Authentic fingerprint samples were then examined. To eliminate the possibility of race as a factor, samples were obtained from Caucasian males and females. Samples were deposited on polyethylene film and extracted in a 0.1 M hydrochloric acid solution, which was heated to 40 C for 20 minutes. Extracts were processed via biocatalytic assay and analyzed. Resulting signal intensities were notably lower than that of simulated samples, which the authors attributed to dilution from the extraction procedure. Despite the dilution of the samples, authentic male and female samples could be distinguished clearly due to the higher amino acid content present in female samples.
Extraction surfaces were then examined for authentic female samples to determine the effect of sampling surface on amino acid recovery, i.e. whether the surface would lead to false positive identification of the female samples as male due to low amino acid recovery. Male samples were not used in this test due to the low absorbance values obtained in the previous authentic samples. Fingerprints were deposited onto a doorknob, laminate desktop, composite benchtop, and computer screen. Polyethylene film was used to remove fingerprints from the sample surfaces, and samples were extracted following the previously defined protocol. Extracts were then prepared with the biocatalytic assay and analyzed. Results indicated that samples were successfully identified as female, regardless of the sampling surface that the fingerprint was removed from.
- Experiments with simulated fingerprint samples resulted in a model with a 99% probability of correct male/female determination.
- Results obtained from genuine fingerprint samples supported the discriminatory capability of the method.
- The extraction and bioassay method allowed for accurate identification of female samples obtained from multiple surface types.
In the absence of a reference material, such as a reference fingerprint or DNA profile, a fingerprint and other evidence obtained from it may provide little actionable evidence until a suspect is identified and reference samples are collected. By using the amino acid content to determine whether the fingerprint originator was male or female, the fingerprint may be used to narrow down a pool of suspects and aid in the identification of a suspect.
- In the absence of reference samples, a fingerprint may be analyzed by this method and used to identify the depositor as male or female.
- By identifying whether the originator of the print was male or female, the fingerprint may aid in the identification of a suspect from which reference samples may be collected.