In general, forensic science incorporates a wide range of disciplines including chemistry, biology, mathematics, physics, botany, and ballistics. However, a crucial breakthrough in this field was made by the application of the analysis of DNA as a biological tool used to carry out a forensic investigation. At present, the forensic experts apply a number of DNA-based techniques that make them able to predict the physical attributes of the suspect while being a subject of ethical and public concern.
Common DNA-Based Methods of Human Identification
As a rule, human forensic investigations are performed with the application of developing DNA-based technologies. They include commercially cultivated autosomal STR multiplexes, DNA databases of autosomal STR profiles, autosomal SNP typing, Y-chromosomal analysis, mitochondrial DNA, and DNA phenotyping. Depending on the historical, legal, and social aspects, the application of these methods can differ between various jurisdictions.
Autosomal STR Profiling
Forensic DNA analysis based on short tandem repeat (STR) has been internationally accepted as a reliable method for individual identification and has significantly influenced the current criminal justice system. At the initial stages, multiplex was composed of four simple STRs. However, its high match probability of ̴ 1 in 10,000 contributed to its complementation by two highly variable complex STRs. In 2000, the developers added four extra loci to the second-generation multiplex and introduced it under the name of SGM Plus with the match probability less than 10-13. The analysis of multiplexes and their typing are performed by means of automated sequencing equipment. In this way, the geneticists want to reduce cost and increase throughput. The implementation of DNA methods has made it possible to reopen and solve ‘cold’ cases leading to the release of prisoners who were convicted by mistake.
DNA Databases of Autosomal STR Profiles
Similar to matching a profile of the putative criminal to the characteristics of an individual apprehended as a suspect, there is a possibility to match the discovered evidence to a DNA database of offender profiles. In 2004, the largest among these databases was the UK National DNA Database that comprised ~2.5 million reference profiles and ~200,000 crime-scene profiles. Up to 2004, the researchers also developed an alternative method of using this kind of databases presented as ‘familial searching’. This technique was based on the principle of obtaining a DNA profile at the crime scene and matching it with the DNA samples of potentially close relatives of the culprit distinguished by a geographically restricted search of the database. However, the searches of this kind did not correspond to the legal principles of some countries and became subject to a number of ethical questions.
Autosomal SNP Typing
The primary advantage of SNP typing is the small size of DNA template. While a successful STR profiling requires the DNA template sized ~300 bp, SNP typing may produce a DNA template not bigger than ~50 bp. Respectively, this method was applied in the investigation of the technical challenges of the World Trade Center disaster.
The analysis of Y chromosomes is based on the mutation that leads to the diversification of STR haplotypes. Since Y chromosomes are present only in males, their identification at the crime scene enables to determine the number of male offenders as well as specific information about them. Apart from STR mutation, the same Y haplotype is present in all male relatives of a suspect which needs to be taken into account during the assessment of the evidence's credibility. Y-chromosomal analysis is particularly useful in the investigation of rape caseworks where the offenders are usually male.
Mitochondrial DNA is similar to the Y chromosome in a number of aspects. Its non-recombining nature does not allow the SNP markers to segregate independently thus reducing their diversity. In addition, this haplotype is usually inherited and is present in all members of a matriline. However, mitochondrial DNA has a significant advantage lying in its copy number that is from ~200 to 1,700 per cell. Consequently, it has more chances to survive in comparison with nuclear DNA. In the forensic science, mitochondrial DNA is taken into consideration in cases involving old or severely damaged samples that are low in DNA and are frequently examined in historical criminal cases.
Forensic DNA Phenotyping (FDP) is an innovative technique developed as an alternative to the traditional comparative DNA profiling and mass DNA dragnets. The primary purpose of using this method is to deduce the externally visible attributes from DNA samples discovered directly at the crime scene or detected in unknown bodies. FDP can also serve as an effective tool for the identification of missing individuals in cases when reference DNA profile from assumed ante-mortem samples or putative relatives is not available. Among the variety of externally visible characteristics, the FDP analysis is available only for those attributes that are marked by pigmentation, namely dissimilarities in the human iris coloration, hair color, and, to a lesser extent, skin color.
Eye color identification. The first DNA-based eye color prediction system developed for forensic investigations was IrisPlex. This system was published in 2010/2011 and included a sensitive testing of multiplex genotyping of the six SNPs that predicted eye color. In the course of further testing, the IrisPlex system was used to predict eye color in 940 DNA samples worldwide. Throughout this study, the system managed to predict different eye colors only in European samples and to a lesser extent in samples from neighboring areas. In 2014, the developers introduced the advanced IrisPlex prediction model for eye color on the basis of samples obtained from nearly 9100 people residing in eight parts of Europe. As stated by Kayser the system achieved AUCs of 0.95 for brown, 0.94 for blue, and 0.74 for intermediate eye color. The success of this system was followed by the development of multitude of tools used to identify the eye color of the suspect.
Hair color determination. After the decade of research, the first DNA test system for multicategorial prediction of hair and eye colors was introduced to public in 2013. The HIrisPlex system involved a single multiplex genotyping test for 24 eye and hair color predicting SNPs, six of which were from previously developed IrisPlex. In the worldwide sample, different hair colors were predicted only in samples from Europe and neighboring regions. The application of the HIrisPlex system to DNA samples obtained from ancient and old bones and teeth demonstrated its effectiveness in the analysis of degraded DNA.
Skin color prediction. The global distribution of skin color variations has contributed to the slower development of the skin color prediction techniques. Classical techniques of gene mapping including GWAS are not suitable for the analysis of genetically heterogeneous study populations. For instance, previously conducted analysis of skin color in Europeans and Asians was not successful due to the limited skin color dissimilarities in different continental groups. The first in-depth skin color prediction analysis was published in 2014 and comprised the investigation of 59 SNPs previously referred to eye, hair, and skin color in a small sample of 280 European and non-European people. The techniques for more accurate skin color prediction are still at the stage of development.
Ethical Concerns and Current Issues
Although the majority of people are aware of the application of DNA analysis as a tool of forensic investigation, there are still multitude of misconceptions related to it. Since the majority of people gain their knowledge of the forensic application of DNA from television news, documentaries, fiction, or drama, their understanding of the procedures of DNA storage and profiling is partially distorted. While some people treat DNA databases as an intrusion of privacy, a number of respondents are convinced that their DNA samples may be used not only for crime investigations but also for some other purposes. Another matter of concern is familial analysis of DNA that has no legal basis in many jurisdictions. The opponents of this issue claim that this type of search violates the privacy of an individual who provides the sample and his biological relatives and may reveal the previously unknown biological connections as well as deny the accepted ones. Therefore, many people do not approve the use of their DNA for any scientific or forensic purposes.
In the course of the international conference dedicated to artifacts and errors in DNA profiling, the participants discussed a variety of issues ranging from the accreditation of laboratories to the principles of the evaluation of DNA profiles. In one of the lectures, Peter Gilled addressed the controversial issue of the relevance of the evidence. The spokesman indicated that the introduction of highly sensitive instruments and multiplexes enabled the investigators to get full, high-quality profiles based on the low-level DNA samples. Consequently, the forensic experts should be extremely cautious while inferring any activity to identify the DNA profile obtained from a knife handle or any other surface.
To conclude, the DNA analysis is one of the basic techniques applied in the process of forensic investigation. The most common methods of genetic evaluation include autosomal STR multiplexes, DNA databases of autosomal STR profiles, autosomal SNP typing, Y-chromosomal analysis, mitochondrial DNA, and DNA phenotyping. At present, the genetics can predict eye, hair, and, to some extent, skin color of the suspect on the basis of DNA-involving technologies. In spite of the existence of specific concerns and prejudice concerning the DNA use, these methods have proven to be highly effective and progressive.