Trying to generate a reliable STR profile with only a few cells from a biological sample is similar to looking for an object in the mud or trying to decipher the image in a fuzzy photograph. Since the sensitivity of the STR typing assay is turned up so high, it is often not immediately clear if you have a reliable result or even a probative one. Recovered DNA profiles may not be associated with the crime event itself but rather have been left innocently before the crime occurred (Gill 2001). Secondary transfer of skin cells due to casual contact such as hand shaking has been demonstrated to occur in controlled laboratory settings (Lowe et al. 2002). This phenomenon occurs to a variable degree depending on what kind of a 'shedder' the individuals are (Lowe et al. 2002).
When LCN testing is performed at least three artifacts typically arise: (1) additional alleles are often observed from sporadic contamination in what is referred to as allele 'drop-in', (2) allele 'dropout' is common where an allele fails to amplify due to stochastic effects (see Chapter 4), and (3) stutter product amounts are enhanced so that they are often higher than the typical 5-10% of the nominal allele (Whitaker et al. 2001). Heterozygote peak imbalance is typically exacerbated due to stochastic PCR amplification, where one of the alleles is amplified by chance during the early rounds of PCR in a preferential fashion. Allele dropout can be thought of as an extreme form of heterozygote peak imbalance.
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