"Forensic science" is a broad term encompassing many disciplines of science and technology that are geared toward investigating cases in the criminal justice system. Forensic pathology is a subset of forensic science, and its focus is on the autopsy to aid in determining identity and the cause and manner of death. There are many other disciplines within the forensic sciences which are often included as part of a "crime laboratory." Some medical examiners have or may be affiliated with a crime laboratory.
Some of the nonpathology forensic sciences directly aid the medical examiner in making cause/manner of death determinations or in establishing identity. Other forensic sciences analyze evidence taken by the medical examiner from decedent remains in order to aid law enforcement in other aspects of the criminal investigation.
Legal Issues Pertaining to Forensic Science
Because the results and testimony regarding scientific casework has far-reaching implications in judicial proceedings, various criteria have been established regarding the admissibility of forensic sciences and related expert testimony. Frye versus United States ruled that the technique must have "general acceptance" within the appropriate scientific community, for example, by publication or case precedent. 
Rule 702 of the Federal Rules of Evidence does not necessarily require general acceptance, but it does require that the testimony be based upon sufficient facts or data, reliable principles and methods, and that those principles and methods are appropriately applied to the facts of the case. The US Supreme Court ruled in Daubert versus Merrell Dow Pharmaceutical, Inc, that the admissibility of the scientific evidence and testimony is based on the judge hearing each individual case.  The criteria suggested to judges for making these decisions are as follows:
Whether the technique has been tested;
Whether it has been subjected to peer review and publication;
Whether it has an established error rate;
Whether there are performance standards in place; and
Whether the technique has gained acceptance in the scientific community.
Some disciplines in forensic science have become so commonplace as to be routinely be accepted in court. In some cases, however, there may be a pretrial hearing to establish whether results and testimony relating to a particular field will be admissible.
Forensic odontologists are forensically trained dentists who primarily perform 2 main functions. They compare antemortem dental records, primarily dental charts and radiographs, against postmortem examinations in order to assist the medical examiner in establishing the identity of unknown decedents. These investigators also analyze bite marks on victims or objects and compare them to dental molds of a suspect in order to determine whether the bite marks may have been inflicted by that person. However, many forensic odontologists no longer perform bite-mark analysis, as there has been controversy regarding its specificity. The American Board of Forensic Odontology (ABFO) certifies forensic odontologists.
Forensic anthropologists examine skeletal remains to determine whether they are of human origin, and if so, to characterize sex, age, race, and stature. This process aids the medical examiner in the identification of unidentified remains. Forensic anthropologists also evaluate skeletal remains for the presence of disease processes and/or injuries.
Forensic entomologists utilize known succession patterns of insect activity on human remains in order to estimate postmortem interval. Most insects exhibit predictable life cycles, with known times spent in each cycle. By examining the species and developmental stage of insects (usually flies) collected from decedent remains and correlating their findings with other environmental factors, such as geographic region and temperature, forensic entomologists can estimate the time of death. When such analyses are desired, the medical examiner generally provides 2 types of samples to the forensic entomologist. The first is live maggots, which are then allowed to grow to maturity in order to determine the species. The second type of evidence collected is killed exemplars of maggots, which are used to estimate the current stage of their life cycle, which is important for estimation of time of death.
Forensic psychiatrists evaluate whether an individual accused of a crime is mentally competent to stand trial. They also formulate "psychological profiles" of a perpetrator based on information about a crime scene or murder victims. This process is largely based on databases of information collected from crime scene observations and characteristics of known perpetrators.
Toxicology deals with the detection of drugs in biologic specimens. Hospital-based toxicology laboratories generally employ qualitative analysis on urine, because this is a rapid method to determine the presence of drugs in a critically ill, often unconscious, patient. Forensic toxicology is not used for such an acute situation in which it is more important to obtain quantitative information than to obtain the results right away. Therefore, blood is generally used for testing, as this represents the drugs present in the decedent's body at the time of death. Utilizing a variety of types of analyses, hundreds of drugs and medications may be detected and quantitated. The results of forensic toxicology testing commonly aid the forensic pathologist in determining the cause of death.
Trace Evidence Examination
Trace evidence analysis involves virtually any substance that can be transferred to or from a decedent or crime scene. Common examples include hair, fibers, paint, glass, soil, gunshot residue, and explosive residue. In general, the analysis involves identifying the substance, determining whether it has unique characteristics, and whether it could have come from the same source as another source of the same substance found elsewhere. This can be very useful in law enforcement investigations to connect a suspect with a crime scene or a victim.
Analyses employed may include microscopic visualization, chemical methods, spectroscopy, chromatography, elemental analysis, refractive index, electron microscopy, and others. Many of these analyses are performed on evidence collected from a decedent by the medical examiner; however, the results of these analyses are generally more useful to the law enforcement investigation than they are to the medical examiner. One exception to this is the analysis of gunshot residue, which may aid the medical examiner in estimating the range of fire.
Hair analysis was once routinely used to attempt to link individuals to a crime by matching either a hair recovered from a crime scene to a suspect or a hair recovered from something belonging to a suspect with a victim. The analysis of hair is primarily a visual technique, using microscopy to characterize its various physical characteristics. Obviously, this is a subjective technique, and DNA analysis of hairs has demonstrated significant error rates in these examinations.  One well-known example of this is the case of the 1989 New York City Central Park jogger rape/assault, in which the convictions of 5 men on the basis of hair comparisons were overturned when DNA testing contradicted the conclusions of the hair analyses. Currently, hair analysis is used to determine whether the hair is human or to rule out a suspected match when the hairs being compared are nothing alike.
Firearms and Toolmarks Examination
Firearms and toolmarks analysis is based on 2 premises. The first is that when 2 surfaces are forced together, marks will be left on the softer of the 2 surfaces. The second premise is that if those marks have sufficient individual characteristics, it can be concluded that the 2 surfaces were at one time in contact with one another. For example, comparisons can be made to determine whether a specific pair of bolt cutter was used on a particular padlock. Firearms analysis can compare the striations left on bullets by the lands in a gun barrel to determine whether 2 bullets were fired from the same gun or whether a particular bullet was fired from a particular gun. There are also databases into which data can be entered so that future queries can be made in order to find a match.
Document examination focuses on the validation of various types of documents, especially those that are handwritten. Handwriting analysts compare a questioned sample of writing to a known exemplar from a specific person to opine whether that person is the author of the exemplar, for example, a suicide or ransom note. Document examiners also assess authenticity of various documents. For example, inks can be differentiated by chemical means; this is useful for detecting forged or altered documents.
No 2 fingerprints have ever been found to be exactly alike, even among identical twins. Friction ridges remain constant during life and exhibit general patterns that can be systematically classified. Prints may be visible (grease, blood, etc), plastic (impressions in soft materials such as soap, or latent (invisible oils on the skin); these last are not visible to the naked eye and must be detected by other means, such as with the use of powders, fumes, or alternative light sources. Comparisons are based upon various ridge characteristics in order to aid the medical examiner in establishing the identity of an unknown individual. Centralized databases exist for the purpose of querying a print or set of prints in order to identify decedents.
Serology and DNA Analysis
Serology evaluates bodily fluids for identifying characteristics. Chemical tests can be used to identify human blood and semen. However, serology lacks sufficient power to identify a specific individual. Once a specimen is verified to be of human origin, DNA analysis can be performed to attempt to identify an individual or to link a specific individual to a certain piece of evidence.
DNA analysis is a powerful tool in part because of the principle of the polymerase chain reaction (PCR), which amplifies minute amounts of DNA into amounts that may then be used for analysis. The primary method of DNA analysis involves the analysis of short tandem repeats (STRs). These are short regions of DNA that are repeated a certain number of times. After amplifying a sample of DNA, 13 STR loci are analyzed. The allele of the locus is the number of times that particular sequence is repeated. Because there are 13 STR loci  and the number of repeats at each of these loci is between 7 and 30, and because each individual has 2 chromosomes for each locus, the statistical power of a match is astronomic.
When comparing an unknown/evidence sample with a known (victim/suspect) sample, there are 3 possibilities. If all alleles match, then a statistic must be generated describing the strength of that match. This is done by observing the frequency in the population of all of the STR alleles. Those frequencies are multiplied together to generate a random match probability or the probability that another individual could share all of the same alleles by random chance. As an example, the random chance of another person having the same STR profile of the author of this chapter is 1 in 19 quadrillion. Another way of stating this is that a population roughly 3 million times that of planet Earth would have to be analyzed in order to randomly find a profile that matched that of the author!
If 2 sources of DNA exhibit at least 1 difference, the samples can be said to not have come from the same source. The third possibility is that the DNA samples match, but the statistics are not powerful enough to conclude that the sources of DNA are the same.
There are also STRs on the Y chromosome, which are shared among male members of a family.  These are analyzed in the same way as the somatic STRs. Y STRs are useful in cases involving mixed samples (eg, from a sexual activity kit in which there is both a male and female component of DNA in the mixture). Because only males have Y STRs, this is the only DNA that is amplified, and DNA from the female component will not obscure that of the male component.
Mitochondrial DNA (mtDNA) is inherited maternally; that is, all children of one mother will share the same mtDNA. There are 2 regions of DNA that are amplified and sequenced. The allele is simply the linear sequence of nucleotides in these areas and is expressed as any differences in that sequence as compared to a consensus sequence known as the Anderson sequence.  Because, by definition, mtDNA is inherited from mother to offspring, it cannot be used to identify one specific individual. However, because there are many mitochondria per cell, and mitochondrial DNA is protected by the membrane of the organelle, obtaining mtDNA is often possible in degraded samples or in samples with a low copy number, such as hairs and teeth.
There are several DNA databases into which DNA profiles of missing persons and convicted offenders are entered. These may be queried when there is an unknown sample of DNA an individual is attempting to identify in order to get a "cold hit." In order to establish identify, DNA standards may also be taken from alleged family members of unidentified decedents who are not visually identifiable, usually because of decomposition or severe injury.
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- Forensic Science
- Legal Issues Pertaining to Forensic Science
- Forensic Odontology
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- Trace Evidence Examination
- Firearms and Toolmarks Examination
- Document Examination
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