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How does forensic identification work? Any type of organism can be identified by examination of DNA sequences unique to that species. Identifying individuals within a species is less precise at this time, although when DNA sequencing technologies progress farther, direct comparison of very large DNA segments, and possibly even whole genomes, will become feasible and practical and will allow precise individual identification. To identify individuals, forensic scientists scan 13 DNA regions that vary from person to person and use the data to create a DNA profile of that individual (sometimes called a DNA fingerprint). There is an extremely small chance that another person has the same DNA profile for a particular set of regions. Some Examples of DNA Uses for Forensic Identification
Is DNA effective in identifying persons? DNA identification can be quite effective if used intelligently. Portions of the DNA sequence that vary the most among humans must be used; also, portions must be large enough to overcome the fact that human mating is not absolutely random. Consider the scenario of a crime scene investigation . . . Assume that type O blood is found at the crime scene. Type O occurs in about 45% of Americans. If investigators type only for ABO, then finding that the "suspect" in a crime is type O really doesn't reveal very much. If, in addition to being type O, the suspect is a blond, and blond hair is found at the crime scene, then you now have two bits of evidence to suggest who really did it. However, there are a lot of Type O blonds out there. If you find that the crime scene has footprints from a pair of Nike Air Jordans (with a distinctive tread design) and the suspect, in addition to being type O and blond, is also wearing Air Jordans with the same tread design, then you are much closer to linking the suspect with the crime scene. In this way, by accumulating bits of linking evidence in a chain, where each bit by itself isn't very strong but the set of all of them together is very strong, you can argue that your suspect really is the right person. With DNA, the same kind of thinking is used; you can look for matches (based on sequence or on numbers of small repeating units of DNA sequence) at a number of different locations on the person's genome; one or two (even three) aren't enough to be confident that the suspect is the right one, but four (sometimes five) are used and a match at all five is rare enough that you (or a prosecutor or a jury) can be very confident ("beyond a reasonable doubt") that the right person is accused. Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products. In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions (markers). Scientists find the markers in a DNA sample by designing small pieces of DNA (probes) that will each seek out and bind to a complementary DNA sequence in the sample. A series of probes bound to a DNA sample creates a distinctive pattern for an individual. Forensic scientists compare these DNA profiles to determine whether the suspect's sample matches the evidence sample. A marker by itself usually is not unique to an individual; if, however, two DNA samples are alike at four or five regions, odds are great that the samples are from the same person. If the sample profiles don't match, the person did not contribute the DNA at the crime scene. If the patterns match, the suspect may have contributed the evidence sample. While there is a chance that someone else has the same DNA profile for a particular probe set, the odds are exceedingly slim. The question is, How small do the odds have to be when conviction of the guilty or acquittal of the innocent lies in the balance? Many judges consider this a matter for a jury to take into consideration along with other evidence in the case. Experts point out that using DNA forensic technology is far superior to eyewitness accounts, where the odds for correct identification are about 50:50. The more probes used in DNA analysis, the greater the odds for a unique pattern and against a coincidental match, but each additional probe adds greatly to the time and expense of testing. Four to six probes are recommended. Testing with several more probes will become routine, observed John Hicks (Alabama State Department of Forensic Services). He predicted that, DNA chip technology (in which thousands of short DNA sequences are embedded in a tiny chip) will enable much more rapid, inexpensive analysis using many more probes, and raising the odds against coincidental matches. What are some of the DNA technologies used in forensic investigations? Restriction Fragment Length Polymorphism (RFLP) RFLP is one of the original applications of DNA analysis to forensic investigation. With the development of newer, more efficient DNA-analysis techniques, RFLP is not used as much as it once was because it requires relatively large amounts of DNA. In addition, samples degraded by environmental factors, such as dirt or mold, do not work well with RFLP. PCR Analysis STR Analysis Mitochondrial DNA Analysis All mothers have the same mitochondrial DNA as their daughters. This is because the mitochondria of each new embryo comes from the mother's egg cell. The father's sperm contributes only nuclear DNA. Comparing the mtDNA profile of unidentified remains with the profile of a potential maternal relative can be an important technique in missing person investigations. Y-Chromosome Analysis The answer to this question is based on information from Using DNA to Solve Cold Cases - A special report from the National Institute of Justice (July 2002). Some Interesting Uses of DNA Forensic Identification
National DNA Databank: CODIS CODIS utilizes computer software to automatically search its two indexes for matching DNA profiles. Law enforcement agencies at federal, state, and local levels take DNA from biological evidence (e.g., blood and saliva) gathered in crimes that have no suspect and compare it to the DNA in the profiles stored in the CODIS systems. If a match is made between a sample and a stored profile, CODIS can identify the perpetrator. This technology is authorized by the DNA Identification Act of 1994. All 50 states have laws requiring that DNA profiles of certain offenders be sent to CODIS. As of January 2003, the database contained more than a million DNA profiles in its Convicted Offender Index and about 48,000 DNA profiles collected from crime scenes but which have not been connected to a particular offender. As more offender DNA samples are collected and law enforcement becomes better trained and equipped to collect DNA samples at crime scenes, the backlog of samples awaitning testing throughout the criminal justice system has increased dramatically. In March 2003 President Bush proposed $1 billion in funding over 5 years to reduce the DNA testing backlog, build crime lab capacity, stimulate research and development, support training, protect the innocent, and identify missing persons. For more information, seethe U.S. Department of Justice's Advancing Justice Through DNA Technology. More on CODIS
Ethical, Legal, and Social Concerns about DNA Databanking The primary concern is privacy. DNA profiles are different from fingerprints, which are useful only for identification. DNA can provide insights into many intimate aspects of a person and their families including susceptibility to particular diseases, legitimacy of birth, and perhaps predispositions to certain behaviors and sexual orientation. This increases the potential for genetic discrimination by government, insurers, employers, schools, banks, and others. Collected samples are stored, and many state laws do not require the destruction of a DNA record or sample after a conviction has been overturned. So there is a chance that a person's entire genome may be available --criminal or otherwise. Although the DNA used is considered "junk DNA" (STRs-single tandem repeated DNA bases), in the future this information may be found to reveal personal information such as susceptibilities to disease and certain behaviors. Who is chosen for sampling is also a concern. In the United Kingdom, for example, all suspects can be forced to provide a DNA sample. Likewise, all arrestees --regardless of the degree of the charge and the possibility that they may not be convicted--can be compelled to comply. This empowers police officers, rather than judges and juries, to provide the state with intimate evidence that could lead to "investigative arrests." In the U.S., arresting people on less than probable cause just to obtain DNA evidence raises the question of Fourth Amendment violations against unreasonable search and seizure. Practicality also is a concern. An enormous backlog of over half a million DNA samples waits to be entered into the CODIS system. The statute of limitations has expired in many cases where the evidence would have been useful for conviction . Potential Benefits of DNA Databanking Arrestees
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