Forensic biology
Forensic biology is the application of biological principles and techniques in the investigation of criminal and civil cases.
Forensic biology is primarily concerned with analyzing biological and serological evidence in order to obtain a DNA profile, which aids law enforcement in the identification of potential suspects or unidentified remains. This field encompasses various sub-branches, including forensic anthropology, forensic entomology, forensic odontology, forensic pathology, and forensic toxicology.
History
The first recorded use of forensic procedures dates back to the 7th century when the concept of using fingerprints as a means of identification was first established. By the end of the 7th century, forensic procedures were being used to determine the guilt of criminals.An early pioneer in criminal identification through biology was Alphonse Bertillon, also known as the "father of criminal identification". In 1879, he introduced a scientific approach to personal identification by developing the science of anthropometry. Anthropometry involves the use of a series of body measurements to distinguish human individuals.
Karl Landsteiner, in 1901, introduced the categorization of human blood into groups: A, B, AB, and O. From this discovery, blood typing, became a fundamental tool in forensic science.
After this, advancements were made which contributed to the ease of using and detecting blood found at crime scenes, expanding the use of blood analysis in forensic biology. Leone Lattes discovered a method to determine the blood group of dried bloodstains in 1915. Subsequently, Albrecht H.O., a German chemist, developed luminol in 1928, which is used to detect trace blood stains at crime scenes.
Alec Jeffreys developed DNA fingerprinting in 1984, which examines variations in DNA that can identify individuals. This has become eminently useful not only in forensic science, but also in resolving paternity and immigration disputes.
In 1983, Kary B. Mullis expanded the use of DNA profiling by developing PCR, which amplifies DNA segments in-vitro, even in trace amounts. DNA samples found in crimes scenes are often found in minute amounts and degraded states, and sometimes mixed with various body fluids from multiple individuals. Using PCR, these DNA samples can be amplified for analysis when they otherwise would be useless. Beyond forensics, PCR has made an impact on a wide range of fields, including disease diagnosis and virus detection.
DNA analysis
, is one of the most popular pieces of evidence to recover at a crime scene. Evidence containing DNA is regarded as biological evidence, and is recognized as the "golden standard" in forensic science.DNA analysis has numerous applications, such as paternity testing, identification of unknown human remains, breakthroughs in cold cases, as well as connecting suspects and/or victims to a piece of evidence, a scene, or another person. Nuclear DNA evidence can be recovered from blood, semen, saliva, epithelial cells and hair. Furthermore, Mitochondrial DNA can be recovered from the shaft of hair, bone, and the roots of teeth.
To be used, biological evidence must be initially visually recognized at the crime scene. To aid in this, alternative light sources, or an Advanced Light Source, are used. Once a potential source is identified, presumptive tests are conducted to establish if there is a specified biological presence. If positive, samples are collected and submitted for analysis to a laboratory, where confirmatory tests and further tests are performed.
For most forensic DNA samples, STR analysis of autosomal short tandem repeats is performed in an attempt to individualize the sample to one person with a high degree of statistical confidence. Here, STR markers for autosomal STR are used in forensic DNA typing to track down the missing, verify family connections, and potentially connect suspects to crime sites.
Laboratory analysis of DNA evidence involves the sample DNA being extracted, quantified, amplified, and visualized. There are several methods of DNA extraction possible including organic extraction, Chelex extraction, and differential extraction.
Quantitation is commonly conducted using a form of the polymerase chain reaction, known as real-time PCR, quantitative PCR. qPCR is the preferred method of DNA quantitation for forensic cases because it is very precise, human-specific, qualitative, and quantitative. This technique analyses changes in fluorescence signals of amplified DNA fragments between each PCR cycle without needing to pause the reaction or open the temperature-sensitive PCR tubes. In addition to the components necessary for a standard PCR reaction, qPCR reactions involve fluorescent dye-labelled probes that complement and anneal to the DNA sequence of interest that lies between the two primers. A "reporter" dye is attached at the 5' end of the fluorescent probe, while a "quencher" dye is attached at the 3' end. Before the DNA strands are extended by the polymerase, the reporter and quencher are close enough in space that no fluorescence is detected by the instrument. As the polymerase begins to extend the strand, the 5' end of the probe is degraded by the polymerase due to its exonuclease activity. The reporter dye is released from the 5' end. It is no longer quenched, thus enabling fluorescence detection. A graph is constructed for the sample DNA comparing the presence of fluorescence to cycle number of the qPCR process. This is then compared to a standard curve of the cycle fluorescence threshold versus the log of known DNA concentrations. By comparing the sample data to the standard curve, one may extrapolate the DNA concentration in the sample, which is essential to move forward with PCR amplification and capillary electrophoresis to obtain a DNA profile. DNA profiles are produced as an electropherogram. The obtained profile can be compared to known samples in CODIS to identify a possible suspect. Based on known frequencies of the genotype found in the DNA profile, the DNA analyst may place a statistical measure of confidence on DNA match.
Mitochondrial DNA analysis
is used instead of nuclear DNA when forensic samples have been degraded, are damaged, or are in very small quantities. In many cases, these may be older human remains, sometimes ancient, and the only options for DNA collection are the body's bone, teeth, or hair.mtDNA can be extracted from degraded samples since its presence in cells is much higher than nuclear DNA. There can be more than 1,000 copies of mtDNA in a cell, while there are only two copies of nuclear DNA. Nuclear DNA is inherited from both the mother and the father but mtDNA is passed down from only the mother to all of her offspring. Due to this type of inheritance, mtDNA is useful for identification purposes in forensic work but can also be used for mass disasters, missing persons cases, complex kinship, and genetic genealogy.
The main advantage of using mtDNA is its high copy number. However, there are a few disadvantages of using mtDNA as opposed to nuclear DNA. Since mtDNA is inherited maternally and passed to each offspring, all members of the maternal familial line will share a haplotype. A haplotype "is a group of alleles in an organism that are inherited together from a single parent". Sharing this haplotype among family members can cause an issue in forensic samples because these samples are often mixtures that contain more than one DNA contributor. The convolution and interpretation of mtDNA mixtures is more difficult than that of nuclear DNA, and some laboratories choose not to attempt the process Since mtDNA does not recombine, the genetic markers are not as diverse as autosomal STRs are in the case of nuclear DNA. Another issue is that of heteroplasmy — when an individual has more than one type of mtDNA in their cells. This can cause an issue in interpreting data from questioned forensic samples and known samples that contain mtDNA. Having adequate knowledge and understanding of heteroplasmy can help ensure successful interpretation.
There are some ways to improve success of mtDNA analysis. Preventing contamination at all testing stages and using positive and negative controls is a priority. In addition, the use of mini-amplicons can be beneficial. When a sample of mtDNA is severely degraded or has been obtained from an ancient source, the use of small amplicons can be used to improve the success of amplification during PCR. In these cases primers amplifying smaller regions of HV1 and HV2 in the control region of mtDNA are used. This process has been referred to as the 'ancient DNA' approach.
The first use of mtDNA as evidence in court was in 1996 in State of Tennessee v. Paul Ware. There was only circumstantial evidence otherwise against Ware so the admittance of mtDNA from hairs found in the victim's throat and at the scene were key to the case.
In 2004, with the help of the National Center for Missing and Exploited Children and ChoicePoint, mtDNA was used to solve a 22-year-old cold case where the nuclear DNA evidence was not originally strong enough. After mtDNA analysis, Arbie Dean Williams was convicted of the murder of 15-year-old Linda Strait, which had occurred in 1982.
In 2012, mtDNA evidence allowed investigators to establish a link in a 36-year-old investigation into the murders of four Michigan children. Hair fibers found on the bodies of two of the children were tested and the mtDNA found to be the same for each sample. For the investigators this was a big break because it meant that the murders were likely connected.