The standard studies that are conducted to become 'SWGDAM Validated' are listed below with the original TWGDAM validation guidelines (1995) numerical headings shown in parentheses. The purpose of each study is also enumerated. Although SWGDAM approved an updated version of these validation guidelines in July 2003 (SWGDAM 2004), the 1995 version is the basis for most validation studies to date with STR typing kits.
■ Standard specimens (188.8.131.52). DNA is isolated from different tissues and body fluids coming from the same individual and tested to make sure that the same type is observed. These studies are important because the blood from a suspect might be used to try and match semen found at a crime scene.
■ Consistency (184.108.40.206). The measurement technique is evaluated repeatedly to assess the reproducibility of the method within and between laboratories. The power of DNA testing is only fully realized when results can be compared between laboratories in different areas or database samples that were analyzed some time before a crime was committed. Thus, results must be comparable across both distance and time. The use of internal sizing standards and allelic ladders has greatly improved the consistency of STR typing.
■ Population studies (220.127.116.11). A set of anonymous samples that have been grouped by ethnicity is analyzed to determine allele frequencies for each major population group that exists in a forensic laboratory's vicinity. These allele frequencies are then used in reporting population statistics and calculating the probability of a random match (see Chapter 21).
■ Reproducibility (18.104.22.168). Dried blood and semen stains are typed and compared to DNA profiles obtained from liquid samples. Samples from the same source should match. Obviously, this fact is important since a crime scene stain should match the reference blood sample of a suspect if he or she is the perpetrator of the crime.
■ Mixed specimen studies (22.214.171.124). The ability of the DNA typing system to detect the various components of mixed specimens is investigated. Evidence samples in forensic cases often originate from more than one individual and thus it is essential that typing systems can detect mixtures. Several studies are typically conducted to define the limitations of the DNA typing system. Genomic DNA from two samples of known genotype is often mixed in various ratios ranging from 50:1 to 1:50. The limit of detection for the minor component is determined by examining the profiles of the mock mixtures (see Chapter 7). Studies are also performed to examine the peak height ratios of heterozygote alleles within a locus and to determine the range of stutter percentages for each allele of each locus (see Chapter 6). The results of these relative peak height measurements can then be used to establish guidelines for separating a minor component of a mixture from the stutter product of a single source sample.
■ Environmental studies (126.96.36.199). Samples of known genotype are environmentally stressed and examined to verify that the correct genotype is obtained. The environmental studies reflect the situations typical of a forensic case (i.e., exposure to sunlight, humidity, and temperature fluctuations).
■ Matrix studies (188.8.131.52). Samples of known genotype are examined after contact with a variety of substrates commonly encountered in forensic cases. For example, blood and semen may be deposited on leather, denim, glass, metal, wool, or cotton as well as mixed with dyes and soil. DNA profiles from samples exposed to these substrates are carefully examined for non-specific artifacts and amplification failure at any of the loci studied.
■ Non-probative evidence (184.108.40.206). DNA profiles are obtained from samples that are from forensic cases that have already been closed. These samples demonstrate that the DNA typing system being examined can handle real casework situations.
■ Non-human studies (220.127.116.11). The DNA typing system being evaluated is subjected to non-human DNA to see if other biological sources could interfere with the ability to obtain reliable results on samples recovered from crime scenes. Primates, such as gorillas and chimpanzees, are typically tested along with domestic animals, such as horses, cattle, dogs, and cats. Bacteria and yeast, which can be prevalent at some crime scenes, are also tested. Most STR loci used for human identity testing are primate-specific, that is, they amplify in gorillas and chimps but not dogs or cats. Bacteria, yeast, and most non-primates do not yield any detectable products with the STR kits currently available (Applied Biosystems 1998, Micka et al. 1999). The sex-typing marker amelogenin does amplify in a number of other species but with DNA fragments that are slightly smaller in size than the standard 106 and 112 bp for human X and Y alleles (Buel et al. 1995).
■ Minimum sample (18.104.22.168). The minimum quantity of genomic DNA needed to obtain a reliable result is typically determined by examining a dilution series of a sample with a known genotype. For example, 10 ng, 5 ng, 2 ng, 1 ng, 0.5 ng, 0.25 ng and 0.1 ng might be evaluated. Most protocols call for using at least 0.25-0.5 ng genomic DNA for PCR amplification to avoid allele dropout from stochastic effects during the PCR step or poor sensitivity during the detection phase of the analysis.
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