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In the days before people discovered the value of fingerprinting to identify criminals, mistakes were made when innocent people looked similar to the real felons.
It is easy to take your own fingerprints, though it's a bit messy with the ink! With enough care, you can find out what sort of fingerprints you have. You can also identify your friends from their fingerprint!
First take a latent (hidden) fingerprint. Press your right thumb on a sheet of glass or plastic film. Gently shake some talcum powder over your sweaty thumbprint, and blow off the excess. Make sure you don't blow the talcum powder into your eyes. Smooth a piece of sticky tape over your thumbprint, then peel it off carefully. Stick the tape onto a labelled piece of black card. Hand it in to the Police Officer!
Fingerprint record chart
Collect a blank Record Chart (a simple version is shown below) and ink pad from the Police Officer. Roll your index finger lightly on the ink pad, then roll your inked finger onto the correct space on your Chart (you may have to practise this technique until you produce a clear fingerprint). Â Repeat this method for each finger on your right hand. Wash your hand!
Fingerprint record chart
Central pocket loop
In the lab:
Black ink pads (washable ink)
Rolls of clear sticky tape
Pieces of black card, approx. 10cm x 10cm
Pieces of clean glass (use propanone for cleaning)
Fingerprint Record Sheets
(Redirected fromÂ DNA profiling)
Genetic fingerprinting,Â DNA testingÂ andÂ DNA profilingÂ is a technique to distinguish between individuals of the same species using only samples of their DNA. Its invention by SirÂ Alec JeffreysÂ at theÂ University of LeicesterÂ was announced inÂ 1985.
Two humans will have the vast majority of their DNA sequence in common. Genetic fingerprinting exploits highly variable repeating sequences calledÂ microsatellites. Two unrelated humans will be likely to have different numbers of microsatellites at a givenÂ locus. By usingÂ PCRÂ 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 inÂ 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,Â 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 humanÂ diasporaÂ inÂ prehistoricÂ 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 aÂ search warrantÂ orÂ court order. Several jurisdictions have also begun to assemble databases containing DNA information of convicts. TheÂ United KingdomÂ currently has the most extensive DNA database in the world, with well over 2 million records as ofÂ 2005. The size of this database, and its rate of growth, is giving concern toÂ civil libertiesÂ 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. A common method is aÂ buccalÂ swab.
Next,Â restriction fragment length polymorphismÂ (RFLP) analysis is performed by using aÂ restriction enzymeÂ to cut the DNA into fragments which are separated into bands duringÂ agarose gel electrophoresis. Next, the bands of DNA are transferred via a technique calledSouthern blottingÂ 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.
Recently, an additional technique for genetic fingerprinting has been introduced: AFLP, or amplified fragment length polymorphism. This new technique is similar to RFLP analysis, but introduces a few other features, like two rounds of amplification and specially made primers. AFLP analysis is now highly automated, and allows for easy creation of phylogenetic trees based on comparing individual samples of DNA.
One of the most modern and widely accepted methods for producing DNA fingerprints in criminal cases, is that ofÂ polymerase chain reactionÂ (PCR). PCR involves the amplification of specific regions of DNA that are known to be highly variable from one individual to another. This amplification process allows the scientist to start with a very small amount of material, and the outcome is a highly discriminating outcome, with the chance of a random match being in the 1 in a billion region. PCR is by far the most common method for presenting DNA evidence in a forensic context.
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 defence 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 theÂ prosecutorHYPERLINK "http://www.chemistrydaily.com/chemistry/Prosecutor's_fallacy"'HYPERLINK "http://www.chemistrydaily.com/chemistry/Prosecutor's_fallacy"s fallacy.
Nowadays, more testing is carried out so that the theoretical risk of a coincidental match is 1 in 100 billion (100,000,000,000). However, the rate of laboratory error may be much 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 inÂ preciselyÂ 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 concernÂ . The cautious juror should not convict on genetic fingerprint evidence alone if other factors raise doubt.
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?
- History, Fingerprints, Genetic Fingerprints, Evidence And Tools Used In Forensic Science
Forensic science reflects multidisciplinary scientific approach to examining crime scenes and in examining evidence to be used in legal proceedings. Forensic science techniques are also used to verify compliance with international treaties and resolutions regarding weapons production and use.
Forensic science techniques incorporate techniques and principles ofÂ biology,Â chemistry, medicine,Â physics, computer science,Â geology, andÂ psychology.
Forensic science is the application of science to matters of law. Both defense and prosecuting attorneys sometimes use information gleaned by forensic scientists in attempting to prove the innocence or guilt of a person accused of a crime.
A basic principle of forensic science is that a criminal always brings something to the scene of a crime, and he or she always leaves something behind. The "some-thing" left behind is the evidence that detectives and criminalists (people who make use of science to solve crimes) look for. It might be fingerprints, footprints, tooth marks,Â blood, semen, hair, fibers, brokenÂ glass, a knife or gun, a bullet, or something less tangible such as the nature of the wounds or bruises left on the victim's body, which might indicate the nature of the weapon or the method of assault. Careful analysis of evidence left at the scene of a crime often can be used in establishing the guilt or innocence of someone on trial.
Forensic Science - History
Archimedes, who proved that his king's crown was not pure gold by measuring its density, was perhaps the world's first forensic scientist. However, it was Sir Arthur Conan Doyle's fictional stories of Sherlock Holmes, written in the late nineteenth century, that first anticipated the use of science in solving crimes in the twentieth century. At about the same time, Sir Francisâ€¦
Forensic Science - Fingerprints
Although fingerprints have been used by crime investigators for more than a century, they remain one of the most sought after pieces of evidence. All human beings are born with a characteristic set of ridges on our fingertips. The ridges, which are rich in sweat pores, form a pattern that remains fixed for life. Even if the skin is removed, the same pattern will be evident when the skin regenerateâ€¦
Forensic Science - Genetic Fingerprints
The nuclei within our cells contain coiled, thread-like bodies called chromosomes. Chromosomes are paired, one member of each pair came from your father; the other one from your mother. Chromosomes are made of deoxyribonucleic acid, often called DNA. It is DNA that carries the "blueprint" (genes) from which "building orders" are obtained to direct the growth, maintenancâ€¦
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The next evolutionary step in high-speed structural screening was theÂ fingerprint, a more abstract relative of the structural key.
The structural keys described above suffer from a lack of generality. The choice of patterns included in the key has a critical effect on the search speed across the database: An effective choice will screen out virtually all structures that aren't of interest, greatly increasing search speed, whereas a poor choice will cause many "false hits," which slows searching to a crawl. The choice of patterns also depends on the nature of the queries to be made: A structural key used by a group of pharmaceutical researchers might be nearly worthless to a group of petrochemical researchers.
FingerprintsÂ address this lack of generality by eliminating the idea of pre-defined patterns. A fingerprint is a boolean array, or bitmap, but unlike a structural key there is no assigned meaning to each bit. Your own fingerprint is very characteristic of you, yet there is no meaning to any particular feature. Similarly, a pattern's fingerprint characterizes the pattern, but the meaning of any particular bit is not well defined.
Unlike a structural key with its pre-defined patterns, the patterns for a molecule's fingerprint are generated from the molecule itself. The fingerprinting algorithm examines the molecule and generates the following:
a pattern for each atom
a pattern representing each atom and its nearest neighbors (plus the bonds that join them)
a pattern representing each group of atoms and bonds connected by paths up to 2 bonds long
... atoms and bonds connected by paths up to 3 bonds long
... continuing, with paths up to 4, 5, 6, and 7 bonds long.
For example, the moleculeÂ OC=CNÂ would generate the following patterns:
Â Â C
Â Â O
Â Â N
Â Â OC
Â Â C=C
Â Â CN
Â Â OC=C
Â Â C=CN
Â Â OC=CN
The list of patterns produced is exhaustive:Â EveryÂ pattern in the molecule, up to the pathlength limit, is generated. For all practical purposes, the number of patterns one might encounter by this exhaustive search is infinite, but the number produced for anyÂ particularÂ molecule can be easily handled by a computer.
Because there is no pre-defined set of patterns, and because the number of possible patterns is so huge, it is not possible to assign a particular bit to each pattern as we did with structural keys. Instead, each pattern serves as a seed to a pseudo-random number generator (it is "hashed"), the output of which is a set of bits (typically 4 or 5 bits per pattern); the set of bits thus produced is added (with a logicalÂ OR)Â to the fingerprint.
Note that because each set of bits is produced by a pseudo-random generator, it is likely that sets will overlap. For example, suppose we are in the middle of generating a fingerprint, and it happens that 1/4 of the bits are already set. If the next pattern generates a set containing 5 bits, the probability that all 5 bits will be unique is (3/4)5, or about 24%. Likewise, the probability that all 5 bits willÂ notÂ be unique are (1/4)5, or about 0.1%.
In spite of the difference between the meaning of a fingerprint's bits and a structural key's bits, fingerprints share an important feature with structural keys: If a pattern is a substructure of a molecule,every bit that is set in the pattern's fingerprint will be set in the molecule's fingerprint.Â This means that, like structural keys, we can use simple boolean operations on fingerprints to screen molecules as we search a database, making a fingerprint comparison an extremely fast screen for substructure searching.
The best way to think of the bits of a fingerprint is as "shared" among an unknown but very large number of patterns. Each pattern generates its particular set of bits; so long as at least one of those bits is unique (not shared with any other pattern present in the molecule), we can tell if the pattern is present or not. A structural key indicates with certainty that a particular pattern is present or absent. Fingerprints are not so definite: if a fingerprint indicates a pattern is missing then it certainly is, but it can only indicate a pattern's presence with some probability. Although a fingerprint doesn't indicate with 100% certainty that a particular pattern is present, it contains far more patterns total than a structural key, the net result being that a fingerprint is a far better screen than a structural key in almost all situations.
Fingerprints have several advantages over structural keys:
Since fingerprints have no pre-defined set of patterns, one fingerprinting system serves all databases and all types of queries.
More effective use is made of the bitmap. Structural keys are usually very "sparse" (mostly zeros) since a typical molecule has very few of the patterns that the structural key's bits represent. Although a mathematical analysis of fingerprint density is beyond the scope of this introduction, it turns out that fingerprints can be relatively "dense" (20-40% ones) without losing specificity. The result is that a fingerprint can be much smaller than a structural key with the same discriminating power.
The patterns that go into a fingerprint are highly overlapped - except for "lone atoms", each pattern shares portions of itself with at least one other pattern (the example above illustrates this). The result is that the more complex a molecule gets, the more accurately its fingerprint characterizes it.
6.1.3 Variable-sized Fingerprints
The next evolutionary step in screening was the concept ofÂ foldingÂ a fingerprint to increase information density.
In the discussion above we mentioned theÂ sparsenessÂ of a fingerprint, which is directly related to itsinformation density. A fingerprint's information density can be thought of as the ratio how much information it actually holds to how much it could hold. As a practical definition, we measure theÂ bit density, the ratio of "on" bits to the total number of bits (e.g. the bit density of "11000000" is 0.25).
Fingerprints for small molecules and "featureless" molecules (such asÂ CH4Â orÂ C40H82) have less information in them than those for large or "rich" molecules. But the fingerprinting mechanisms discussed so far require a fixed fingerprint size for all molecules. If we choose to use small fingerprints, the fingerprint of large or complex molecules will be "black" - nearly all ones - and will not discriminate well (there is more information than the fingerprint can hold). On the other hand, if we use very large fingerprints, most molecules' fingerprints will be "white" - nearly all zeros - and will waste space. In both cases we have low information density; the "black" fingerprint because it is too dense and the "white" one because it is too sparse.
Ideally, we would like to choose a particular discriminatory power (e.g. "For a typical pattern, 98% of the database is screened out by the pattern's fingerprint") and compute the fingerprint density needed to achieve that discriminatory power on a case-by-case basis. Although we can not actually do this, the process of "folding" achieves nearly the same performance and size as the "ideal" case.
The folding process begins with a fixed fingerprint size that is quite large - large enough to accurately represent any molecule we expect to encounter. The fingerprint is thenÂ folded: we divide it into two equal halves then combine the two halves using a logical OR. The result is a shorter fingerprint with a higher bit density. We can repeatedly fold the fingerprint until the desired information density (called theÂ minimum density) is reached or exceeded.
As long as two fingerprints are the same size (even if created with different sizes), they are compatible. To see why, consider the fingerprints of a pattern P and a molecule M. If the screen is initially positive (e.g. all bits in the P's fingerprints are also in M's) then the same will be true after folding. On the other hand, a negative screen (at least one bit in P's fingerprint is not in M's) might be converted to a positive screen after folding. But this is ok - converting "correct negative" to a "false positive" doesn't violate the rules of screening: a screen is only required to sayÂ PÂ Â«inÂ»Â MÂ with 100% reliability. With each fold, we increase the chances of a false positive but save half of the space needed to store the fingerprint.
Fingerprint folding allows us to optimize the information density in a set of fingerprints, thus optimizing screening speed. Rather than choosing one fingerprint size for the entire database, we choose the size of each molecule's fingerprint individually, according to the complexity of the molecule and the desired success rate of the screening process. In most real databases, optimizing the information density greatly reduces the amount of data stored, and increases the screening speed correspondingly.