DNA Analysis

Forensic DNA Experts

Bericon’s team of DNA experts, based in Staffordshire UK, have vast experience providing forensic DNA services nationwide. In reviewing cases involving DNA profiles, experts typically review the following aspects:

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

DNA profiling can be useful in determining whether a person was present at a crime. If a scene sample’s DNA matches a suspect’s, it might be the suspect’s or someone else’s with the same profile by chance. 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.

DNA Profiles

If the profiles match, they could be from the same person or anyone with the same STR profile.

The data representing the components of the STR profile consists 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 both parents pass on an allele with the same number of repeats, you will see only one peak.

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 several concerns had been raised by the Judge regarding the reliability and validity of evidence derived by this technique.

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