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Techniques of DNA Fingerprinting

The DNA sequence that makes up a human genome is 99.9 percent identical in all human beings. It is this 0.1 percent that makes each of us unique. DNA fingerprinting is the only forensic tool that has been proven scientifically valid to connect a suspect with a crime. Since its first development in the mid-1980's, techniques for DNA fingerprinting have rapidly evolved and have made it one of the most powerful crime-fighting tools available.

  1. Discovery

    • Dr. Alec J. Jeffreys, an English geneticist, discovered that individuals could be identified based on studying the repetition of their unique DNA sequencing. He published his findings in Nature magazine in 1985. He called this technique "DNA fingerprinting" (later known as "DNA profiling"). The following year, he used this technique to help police successfully capture the murderer of two young girls and prove the innocence of a suspect. This was documented as the first use of DNA to solve a crime. In the following decades, the techniques for DNA fingerprinting have evolved in efficiency, practicality and purpose.

    RFLP Analysis

    • Restriction Fragment Length Polymorphism (RFLP) is perhaps the oldest DNA fingerprinting technique. The process involves the separation and binding of DNA fragments with radiation to study their sequencing. The final sample reveals unique patterns that are compared with known and unknown samples for identification. RFLP is not able to produce viable results from degraded DNA samples. It also requires larger biological samples for analysis (no smaller than the size of a quarter). This process has been replaced by more modern and practical procedures.

    PCR Analysis

    • Polymerase Chain Reaction (PCR) allows for the generation of millions of identical copies of DNA ("amplification") from a single biological sample. This technique is able to produce viable DNA for analysis from degraded or minute biological samples.

    STR Analysis

    • Short Tandem Repeat (STR) is a technique used by law-enforcement agencies like the FBI to examine an isolated region ("loci") of the nuclear DNA. The FBI uses a standard set of loci taken from the sample and then compares it with millions of other stored DNA samples in CODIS ("Combined DNA Index System," an interagency DNA database) for identification.

    mtDNA Analysis

    • Mitochondrial DNA (mtDNA) Analysis is used when RFLP and STR analysis is not possible. This process enables the extraction of DNA from a cell's mitochondrion in hair, bone and teeth samples. It is most often used in "cold cases" that have gone unsolved for many years, and in cases for identifying the remains of missing persons.

    Y-Chromosome Analysis

    • This technique focuses on examining the genetic markers of the male Y-chromosome. It's most often used to determine if a biological sample has several male contributors, and distinguishes the multiple parties.

    Accuracy

    • Despite its proven effectiveness in identification, there is much controversy surrounding the accuracy of DNA testing. Although government crime laboratories are subjected to federal guidelines, there is no uniform regulation or enforcement of quality-control procedures for private laboratories where analysis is conducted. Some states have more stringent requirements than others. The lack of federally regulated guidelines for these private labs enables practices that lead to contaminated, inconclusive and erroneous results.

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