DNA Applications in Forensic Science

Locard’s Principle of Exchange states that when any person comes into contact with an object or another person, a cross-transfer of physical evidence occurs. The identification of that physical evidence has been the pivotal point in forensic work since Francis Galton developed a system of fingerprint classifications in 1892. Forensic science has rapidly evolved from using fingerprint identification to using a person’s genetic makeup through a whirlwind of technology that has reshaped the Criminal Justice System.

DNA analysis has become the most important tool for human identification and sorting unidentified remains by cataloging genetic markers. This paper will first examine how analyzing genetic markers first found in blood has evolved to analyzing DNA for direct mutations and how using amplification techniques have allowed the smallest biological samples to be identified. Second, this paper will review how the acceptance of these technological advances throughout many jurisdictions has necessitated the standardization of qualification and validation methods to leave no room for doubt of the reliability of data in a court of law.

Third, the movement from retaining samples as evidence into creating a computerized data banking systems will be explored and the policies it has affected in most large U. S. cities and countries across the globe. Finally this paper will explore the criticism of the data banking systems from civil rights groups, convicted felons and politicians. Blood Typing Systems In the late 19th century Karl Landsteiner discovered that human blood could be distinguished into four main groups using the Antibody/Antigen interactions: Groups A, B, AB, and O.

There are two antigens and two antibodies that are responsible for the ABO types. The specific combination of these four components determines an individual’s type. A blood test is used to determine whether A, B and/or O characteristics are present in a blood sample, and though it is not possible to determine the exact genotype from a blood test the results do reveal the individual’s phenotype. The blood-group typing was implemented in crime labs and has remained useful in many crime solving techniques today.

Advances in blood typing have concentrated on the different enzymes and proteins associated with the major blood groups because they have the characteristic of being polymorphisms, which means they exist in several forms and variants, so each one of them have subtypes. The early work on blood proteins was performed on enzymes such as Phosphoglucomutase (PGM), Haptoglobin (Hp) and Adenylate Kinase (AK). Each of these protein and enzyme variants, as well as all blood subtypes, has known distributions in a population.

The goal of typing using enzymes is to reduce the frequency of possible matching blood types to the smallest number by examining the multiple markers such as subtypes, proteins and enzymes available in a typed blood sample. For instance, the ABO-typing system used with the PGM system, which has 3 common and 31 rare phenotypes, can be used to determine a possible source of a blood sample ( Percin, 1998). An example would be an unknown blood sample is tested and yields type A blood, which has a 35% frequency in a Caucasian population.

Then, the same blood sample is typed with a PGM enzyme system and yields a type 1+ factor, a 19% frequency. When calculated the percentage of possible donors of the sample obtained from an elected ethnic population, Caucasian, with type 1+ A blood is: 35% x 19% = 6. 7%. This result would become smaller with more markers added. Forensic DNA typing seeks the same goal as blood typing of reducing the amount of possible donors to a lowest amount (Rudin, 2002). DNA Typing Systems

The first DNA typing system was Restriction Fragment Length Polymorphism analysis (RFLP). The history of RFLP began in 1980 when David Botstein and co-workers began developing the small variations at the genetic level found between people as markers to construct a human genome map (Rudin, 2002). Unique differences in the banding pattern of DNA fragments from different individuals are observed when subjected to restriction enzyme analysis. These variations in the DNA are used as markers on both physical and genetic linkage maps.

RFLP uses enzymes to expose genetic variations through size differences of DNA fragments at specific loci. The standard process used a probe to locate the marker that is specific to a single clone/restriction enzyme combination. The results are plainly interpreted but the results take four to six weeks for a positive identification. Samples need only be the size of a dime to be amplified using the RFLP process. In 1984 Sir Alec Jeffreys discovered sets hypervariable DNA patterns he referred to as “minisatellite” sequences.

These sequences consisted of tandemly repeated DNA core sequences later to be known as Variable Number Tandem Repeat (VNTR) loci (OTA, 1989). The term “DNA fingerprinting” was coined when he described the implications of this discovery stating, “A probe based on a tandem-repeat of the core sequence can detect many highly variable loci simultaneously and can provide an individual-specific DNA ‘fingerprint’ of general use in human genetic analysis” (Jeffreys, 1985). A typical VNTR loci contain tandem repeated units that span 15 to 35 base pairs and flanked by endonuclease restriction sites.

The overall length of the restriction fragment produced by this type of genetic locus is proportional to the number of oligonucleotide core units. The VNTR also varies with each allele, which means different alleles can be identified according to their repeated lengths. VNTR’s also have a high mutation rate that contributes to the length variations and the number of possible genotypes found at a locus is greater than the actual number of alleles. When several loci are combined for examination the genetic variability is impressive (NRC, 1996).

Since there are many different numbers of repeat units for any one locus among different people, the fragment pattern revealed by analysis of multiple VNTR loci constitutes a nearly unique genetic profile for every individual. A scientist will measure the size differences at multiple loci to identify the distinct DNA patterns, but is not able to detect direct sequence variations (OTA, 1989). The discriminating power of the VNTR typing system makes it the most advantageous for DNA identification due to its high frequencies and large variety of the genotypes recognized.

Another method of DNA typing is Short Tandem Repeat (STR), which uses shorter repeat units of DNA nucleotide sequences of 2 to 5 base pair units in length. They are found throughout the chromosomes in vast quantities, increasing the availability …

Blood is one of the most important fluids in the human body. It helps carry nutrients such as oxygen to the cells in the body and transports out wastes like carbon dioxide and other metabolites from these cells. Since blood …

The current applications of genetic fingerprinting and how they have helped society – 2000) The number and position of bands formed on each lane of gel is the actual genetic fingerprint of that DNA sample. The characteristics of certain segments of …

Enzymes are biological catalysts that operate at the temperatures, pressures, and moderate pH values found in living organisms. They help in a faster chemical reaction in our human body. Besides this, enzymes also play a huge role in various industries. …

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