Forensic Science Consultants

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DNA

DNA

Bericon are DNA experts and have vast experience in the review of cases involving DNA and typically can be asked to review:

  • Whether the scientist has allowed appropriate procedures in a case
  • From what starting point did the scientist base their conclusions?
  • The quantity and quality of the DNA detected in this case, to what extent is it or may it be consistent with transfer by another person or object that has some of the defendant’s DNA on it when coming into contact with the relevant exhibits.
  • Whether it is possible that a close relative of a defendant may have contributed to the reported DNA results.
  • Provide an explanation regarding why the DNA results are reported as “complex” and how this influences the scientists’ conclusions.
  • Explain how the uncertainty of the number of contributors affects the reliability of the reporting scientists’ conclusions.

In addition, Bericon DNA experts have access to UKAS accredited testing facilities that enable additional DNA testing work to be undertaken where, for example, samples may not have been previously tested or where more powerful analytical methods are required.

DNA profiling can be useful in determining whether a person was present at a crime. If a DNA profile obtained from a scene sample matches that of a suspect, that DNA could have come from the suspect or from someone else who happens, by chance, to have the same DNA profile. However, not all DNA profiles carry the same evidential value. Some may provide extremely strong evidence of association while others may be of poor quality and of limited evidential value.

It is very important that DNA evidence is examined by a suitably experienced and qualified scientist who is able to critically evaluate the DNA results themselves and also to consider their significance in the context of the particular case being considered. This is particularly so in the case of mixed DNA profiles, which may be complex, and in the case of DNA profiles obtained using Low Copy Number or other highly sensitive techniques which may be open to subjective interpretation.

DNA Profiles

Deoxyribonucleic Acid (DNA) is a chemical found in an individual’s cells that carry genetic information. This information is inherited from both parents. The DNA in each cell of an individual is the same; i.e. the DNA in a skin cell is the same as the DNA in a cell from another part of the body of the same individual.

When STR profiling is carried out, the whole of the person’s DNA is not examined. Rather, specific regions (loci) of the DNA which are known to vary greatly between individuals are examined. These loci are areas of the DNA which contain varying numbers of repeating sequences known as short tandem repeats (STRs). It is the number of these repeating units which can differ between individuals. If there are differences between profiles obtained from different samples, the two samples cannot have come from the same person. If, however, the profiles match, then it follows that the samples could have originated from the same person or from any other person who happened to have the same STR profile.Â

The components of the STR profile are represented as data consisting of a series of peaks. For each location (locus) along the DNA molecule there will usually be two peaks, one from each parent, representing STR components (alleles) with differing numbers of repeats. If an allele with the same number of repeats is inherited from both parents, only one peak will be present.

By comparing the arrangement and distribution of the peaks, it is possible to compare profiles of known origin with a profile from a crime sample. If the two profiles match, databases are used to estimate the probability of obtaining a matching profile from a person selected at random; this is referred to as thematchprobability. Closely related individuals are more likely to share DNA profiles than more distantly related individuals.

Mixed DNA Profiles

When a sample of biological material contains DNA from more than one person, this can result in a “mixed DNA profile”. In such profiles, there may be a reduced amount of useful information regarding whether or not a specific person could have contributed to this sample. This could be because the contributors may share one or more DNA alleles, resulting in the masking of the DNA of one person by that of the other.

The results of mixed DNA profiles may therefore provide reduced match probabilities when compared with non-mixed profiles. It may be possible for a scientist to be able to assess the relative amount of DNA contributed by different donors in a DNA mixture. If one person has contributed a clear and distinct majority of the DNA detected, that part of the profile may be referred to as the “Major Contribution”.

Similarly, if a person has contributed a clear and distinct minority of the DNA detected, that part of the profile may be referred to as the “Minor Contribution”. However, if DNA from one or more people is present in a mixed DNA result in roughly equal quantities, any statistic relating to the likelihood that any one particular person may have contributed to the DNA profile is necessarily reduced in value due to the inherent uncertainties regarding which DNA components may have come from either contributor.

The situation is made even more complex if it is considered that three or more people may have contributed to a particular DNA result. Often, in such cases, it is not possible for a scientist to undertake a reliable statistical evaluation of the mixed DNA result. If the DNA result indicates that a very low level of DNA has been detected, it is recommended that the reporting forensic scientist consider the possibility that the result may have been derived from a very low level of DNA from more than one person, some of the components of which may be missing from the DNA result because of the low level of DNA present

Partial DNA Profiles

The SGM Plus system of DNA analysis targets ten loci, each of which contains two alleles. These are the “short tandem repeats” that vary between individuals. In addition, a further locus is targeted that acts as in indicator of the sex of the donor. A “full” DNA profile is one in which all of these loci have produced a reliable and reportable result. Occasionally, the processes used to target some of these loci fail, resulting in an incomplete or “partial” DNA profile. The most common reasons for such failure are either that a very small amount of DNA was present in the sample, the DNA may have become degraded, or that substances may have been present in the sample that may have inhibited the analysis process. Depending on the degree of success of the DNA analysis, the match probability calculated from a partial DNA profile may be reduced below the 1 in 1 billion that would be obtained from a full profile.

Low Copy Number DNA Analysis

Some DNA analysis uses the Low Copy Number(LCN) method. This is a modification of the more commonly used SGM Plus method of analysis. The advantage of LCN analysis over standard SGM Plus is its extreme sensitivity; however this is also a disadvantage. The effects of cross contamination are more prevalent in LCN analysis, and due to various effects observed when amplifying very small amounts of DNA any LCN profile should be interpreted with caution.

Following the judgement in the case of R v Hoey (20th December 2007; neutral citation number NICC 49, ref. We17021), the use of LCN was reviewed after a number of concerns had been raised by the Judge regarding the reliability and validity of evidence derived by this technique.

The recently published review highlighted a number of key points and the use of this technique continues albeit with a number of additional safeguards and improved practices and protocols.

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What our clients say

Due to Dr Short preparing a report on DNA at short notice our client was found not guilty of a serious sexual offence
David Hay, Abraham Solicitors