Polymerase Chain Reaction

Despite many issues of reliability of LCN analysis, this technique is already well established. The question in validity of LCN typing lies not with the technique itself but how DNA is handled to avoid contamination. Several methods have been reinforced in order to increase reliability of LCN typing. There are: Increasing PCR cycle numbers to achieve sensitivity enough for right results; Strengthening the fluorescent dye signal; using the higher purity formadine in a sample preparation for electrophoresis; negative control are used on every step to show absence of contamination; the results are compared against the stuff database. An addition, all practices reduced a lab contamination by: using hermetic rooms; very sterile labs; changing clothing at every sample during analysis; materials and instruments are free of other DNA; lab equipment treated with bleach and irradiated with UV light; post PCR cleaning. These rules, also, apply during gathering and packaging touch DNA at crime scene and transportation. Hence, the routine procedure designed with minimum risk of data to be failed. Moreover, DNA evidences are typically meaningful by statistics. The most common statistic is Random Match Probability(RMP) that highly impressive because of billions and trillions estimates. Even in cases of mix DNA, the using of combined probability of inclusion and exclusion are still remain reliable. In that case, most lab scientist endorse 2-3 analysis of LCN typing, and allele must be observed twice, so when more replicates show the same allele, the more validate result is. Even more, the high trained forensic stuff are well familiar with the interpretation methodology of data and statistics that give additional confidence in validity. Altogether, I argue in favor of the value of LCN evidence presented in this Court. The tragic terrorist attack in Omagh, Ireland in 1998 raised the question of reliability of LCN typing for the first time.

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The penetrator was arrested, and then released due to lack of validity of how LCN testing was provided. Criticism named this method as “robust” and “fit to purpose”. Hence, LCN evidence presented today in Court should not be accepted as evidence due to technique causing many debates and lack of reliability of this test. There are some flows. ​Sensitivity​ of touch DNA and Low Template DNA can lead to higher possibility of errors. For example, the sensitivity increases the risk of ​contamination​ of DNA which is a pretty small molecule(less than 100-200 bp), and phenomena as allele ​drop-in ​when allele is not part of the typing profile is seen. We know that LCN requires more numbers of PCR cycles from 28-30 to 34. Rising PCR cycles creates allele drop-in as random event which may not be very supportable. In addition, after a crime, the contamination can occur on each step during detecting, gathering and transferring LCN by investigators, and accepting by lab workers, their efforts on extraction and amplification in PCR machine. The very sterile environment and accuracy have to be keeped on the each step because LCN(or LTDNA) may be contaminated even by touching a test equipment by skin, or breathing on a sample of DNA. Environmental conditions such as temperature and humidity are, also, important factors that affect the final results. Moreover, the data interpretation is more complicated due to ​stochastic threshold ​that more noticeable during amplification. There are possibility of allele ​drop-out​ and ​peak imbalance​ to be occurred when peaks from heterozygote is imbalancing, and a heterozygote appears as a homozygote. Further, sometimes it is hard to find out how individual’s DNA came because the individual could not be present at scene at all. Tiny amounts of DNA can be passed through handshake, or in a high speaking area. Hence, the presence of a suspect’s DNA does not mean that a suspect was present at crime scene. This situation gives a rise to the problematic interpretation of a ​mix DNA​. Consequently, we should see all evidences, as a whole picture, and the data has to be interpreted with caution and all knowledge in order to be accepted.

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