Genetic fingerprinting, DNA testing and DNA profiling are techniques used to distinguish between individuals of the same species using only samples of their DNA. Its invention by Sir Alec Jeffreys at the University of Leicester was announced in 1985.

Two humans will have the vast majority of their DNA sequence in common. Genetic fingerprinting exploits highly variable repeating sequences called microsatellites. Two unrelated humans will be likely to have different numbers of microsatellites at a given locus. By using PCR to detect the number of repeats at several loci, it is possible to establish a match that is extremely unlikely to have arisen by coincidence.

Genetic fingerprinting is used in forensic science, to match suspects to samples of blood, hair, saliva or semen. It has also led to several exonerations of formerly convicted suspects. It is also used in such applications as studying populations of wild animals, paternity testing, identifying dead bodies, and establishing the province or composition of foods. It has also been used to generate hypotheses on the pattern of the human diaspora in prehistoric times.

Testing is subject to the legal code of the jurisdiction in which it is performed. Usually the testing is voluntary, but it can be made compulsory by such instruments as a search warrant or court order. Several jurisdictions have also begun to assemble databases containing DNA information of convicts. The United Kingdom currently has the most extensive DNA database in the world, with well over 2 million records as of 2005. The size of this database, and its rate of growth, is giving concern to civil liberties groups in the UK, where police have wide-ranging powers to take samples and retain them even in the event of acquittal.

DNA fingerprinting process
DNA fingerprinting begins by extracting DNA from the cells in a sample of blood, saliva, semen, or other appropriate fluid or tissue. Reference samples are often collected using a buccal swab.

When DNA fingerprinting first began, restriction fragment length polymorphism (RFLP) analysis was used, though it has been almost completely replaced with newer techniques. RFLP analysis is performed by using a restriction enzyme to cut the DNA into fragments which are separated into bands during agarose gel electrophoresis. Next, the bands of DNA are transferred via a technique called Southern blotting from the agarose gel to a nylon membrane. This is treated with a radioactively-labelled DNA probe which binds to certain and specific DNA sequences on the membrane. The excess DNA probe is washed off. An X-ray film placed next to the nylon membrane detects the radioactive pattern. This film is then developed to make a visible pattern of bands called DNA fingerprinting. The primary drawback of RFLP is that the chromasomal locations or exact sizes of the bands are unknown, and comparison is done in a purely qualitative manner. This means that it was impossible to determine individual bands frequencies reliably, something that has become critical in modern DNA fingerprinting.

More Books about DNA

Another technique, AFLP, or amplified fragment length polymorphism was also put into practice during the early days of DNA fingerprinting. This technique is similar to RFLP analysis, but introduces a few other features, like two rounds of amplification and specially made primers. AFLP analysis can be highly automated, and allows for easy creation of phylogenetic trees based on comparing individual samples of DNA.

With the invention of polymerase chain reaction (PCR), DNA fingerprinting took huge strides forward in both discriminating power and ability to recover information from very small starting samples. PCR involves the amplification of specific regions of DNA using a cycling of temperature and a thermostable polymerase enzyme along with sequence specific primers of DNA. Commercial kits that used single nucleotide polymorphisms (SNPs) for discrimination became available. These kits use PCR to amplify specific regions with known variations and hybridize them to probes anchored on cards, which results in a colored spot corresponding to the particular sequence variation.

Another DNA fingerprinting technique based on PCR, the most prevalent in use today, is the use of short tandem repeats (STR). This method uses highly polymorphic regions that have short repeated sequences of DNA (the most common is 4 bases repeated, but there are are lengths in use, including 3 and 5 bases). Because different people have different numbers of repeat units, these regions of DNA can be used to discriminate between individuals. These STR loci (locations) targeted with sequence-specific primers and are amplified using PCR. The DNA fragments that result are then separated and detected. There are two common methods of separation and detection, capillary electrophoresis (CE) and gel electrophoresis.

CE works by electrokinetically (movement through the application of an electric field) injecting the DNA fragments into a thin glass tube (the capillary) filled with polymer. The DNA is pulled through the tube by the application of an electric field, separating the fragments such that the smaller fragments travel faster through the capillary. The fragments are then detected using fluorescent dyes that were attached to the primers used in PCR. This allows multiple fragments to be amplified and run simultaneously, something known as multiplexing. Sizes are assigned using labeled DNA size standards that are added to each sample, and the number of repeats are determined by comparing the size to an allelic ladder, a sample that contains all of the common possible repeat sizes. Although this method is expensive, larger capacity machines with higher throughput are being used to lower the cost/sample and reduce backlogs that exist in many government crime labs.

Gel electrophoresis acts using similar principles as CE, but instead of using a capillary, a large polyacrylamide gel is used to separate the DNA fragments. An electric field is applied, as in CE, but instead of running all fo the samples by a detector, the smallest fragments are run close to the bottom of teh gel and the entire gel is sacnned into a computer. This produces an image showing all of the bands corresponding to different repeat sizes and the allelic ladder. This approach does not require the use of size standards, since the allelic ladder is run alongside the samples and serves this purpose. Visualization can either be through the use of fluorescently tagged dyes in the primers or by silver staining the gel prior to scanning. Although it is cost effective and can be rather high throughput, silver staining kits for STRs are being discontinued. In addition, many labs are phasing out gels in favor of CE as the cost of machines becomes more managable.

The true power of STRs is in its statistical power of discrimination. In the U.S.A., there are 13 loci (DNA locations) that are currently used for discrimination. Because these loci are independent (having a certain number of repeats at one locus doesn't change the likelihood of having any number of repeats at any other locus), the power rule can be applied. This means that if someone has the DNA tpye of ABC, where the three loci were independent, we can say that the probability of having that DNA type is the probability of having type A times the probability of having type B times the probability of having type C. This has resulted in the ability to generate match probabilities of 1 in a quintillion (1 with 18 zeros after it) or more.

Considerations when evaluating DNA evidence
In the early days of the use of genetic fingerprinting as criminal evidence, juries were often swayed by spurious statistical arguments by defense lawyers along these lines: given a match that had a 1 in 5 million probability of occurring by chance, the lawyer would argue that this meant that in a country of say 60 million people there were 12 people who would also match the profile. This was then translated to a 1 in 12 chance of the suspect being the guilty one. This argument is not sound unless the suspect was drawn at random from the population of the country. In fact, a jury should consider how likely it is that an individual matching the genetic profile would also have been a suspect in the case for other reasons. The false assumption that a 1 in 5 million probability of a match automatically translates into a 1 in 5 million probability of innocence is known as the prosecutor's fallacy.

When using RFLP, the theoretical risk of a coincidental match is 1 in 100 billion (100,000,000,000). However, the rate of laboratory error is almost certainly higher than this, and often actual laboratory procedures do not reflect the theory under which the coincidence probabilities were computed. For example, the coincidence probabilities may be calculated based on the probabilities that markers in two samples have bands in precisely the same location, but a laboratory worker may conclude that similar -- but not precisely identical -- band patterns result from identical genetic samples with some imperfection in the agarose gel. However, in this case, the laboratory worker increases the coincidence risk by expanding the criteria for declaring a match. Recent studies have quoted relatively high error rates which may be cause for concern [1].

STRs do not suffer from such subjectivity, and provide much better power of discrimination (many orders of magnitude). However, with any DNA technique, the cautious juror should not convict on genetic fingerprint evidence alone if other factors raise doubt. Chimerism is one such instance where a lack of a genetic match may unfairly exclude a suspect.

When evaluating a DNA match, the following questions should be asked:

Could it be an accidental random match?
If not, could the DNA sample have been planted?
If not, did the accused leave the DNA sample at the exact time of the crime?
If yes, does that mean that the accused is guilty of the crime?
[edit]
Fake DNA evidence
The value of DNA evidence has to be seen in light of recent cases where criminals planted fake DNA samples at crime scenes. In one notorious case, a criminal even planted fake DNA evidence in his own body: Dr. Schneeberger of Canada raped one of his sedated patients in 1992 and left semen on her underwear. His DNA was tested on three occasions, never showing a match. It turned out that he had surgically inserted a Penrose drain into his arm and filled it with foreign blood and anticoagulants.

Cases
In 1988, British baker Colin Pitchfork was the first person to be convicted using DNA evidence.

In 1989, Florida rapist Tommie Lee Andrews was the first American to be convicted as a result of DNA evidence, for an assault committed in 1987.

In 1991, Allan Legere was the first Canadian to be convicted as a result of DNA evidence, for four murders he had committed while an escaped prisoner in 1989. During his trial, his defense argued that the relatively shallow gene pool of the region could lead to false positives.

In 1992, DNA evidence was used to prove that Nazi doctor Josef Mengele was buried in Brazil under the name Wolfgang Gerhard.

The science was made famous in the United States in 1994 when prosecutors heavily relied on — and through expert witnesses exhaustively presented and explained — DNA evidence allegedly linking O. J. Simpson to a double murder. The case also brought to light the laboratory difficulties and handling procedure mishaps which can cause such evidence to be significantly doubted.

In June of 2003, because of new DNA evidence, Dennis Halstead, John Kogut and John Restivo won a re-trial on their murder conviction. The three men had already served eighteen years of their thirty plus year sentences.

The trial of Robert Pickton is notable in that DNA evidence is being used primarily to identify the victims, and in many cases to prove their existence.

In March 2003, Josiah Sutton was released from prison after serving four years of a twelve year sentence for a sexual assault charge. Questionable DNA samples taken from Sutton were retested in the wake of the Houston Police Department's crime lab scandal of mishandling DNA evidence.

In December 2005, Robert Clark was proven innocent of a 1981 attack on an Atlanta woman after serving twenty four years in prison. Mr Clark is the 164th person in United States and the fifth in Georgia to be freed using post-conviction DNA testing.
The source of this article is Wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL

 

Web www.best-discount-gifts.com

Real Ways to Get Rich on the Internet!

Medical Resources:

Post nasal drip, lose weight, diabetes, Alzheimer's, more

Guides for Better Living

How to Cope with Life's Problems

Household Tips:

how to clean athletic shoes, get rid of roaches

Help for Senior Citizens:

Avoid scams, more...

*This site does not provide medical advice.
The contents of this site, such as text, graphics, images, information and other material ("Content") contained on the this site, is for information purposes only. The Content is not intended to be a substitute for professional medical advice, diagnosis or treatment (and the Site does not render medical, nursing, or professional health-care advice in any jurisdiction). Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or medical treatment. Never disregard professional medical advice or delay in seeking it because of something you have read on this Site.

If you think you may have a medical emergency, call your doctor, or 911 or other emergency service number immediately. Our site does not recommend or endorse any specific tests, products, procedures, opinions or other information that may be mentioned on the Site. Reliance on any information provided by this Site is solely at your own risk.