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Trial Preparation:


Please note that the most effective way to contact SFR is via e-mail:, and all phone contact should go through the number: 575-640-2360

Discovery Requests for Vital Case Documents: Maximizing the effectiveness of case review by SFR will require a request for more than just one or two brief crime lab reports. SFR will assist you with executing a thorough examination of these reports, as well as a myriad of supporting case file documents. Copies of the following documents, where applicable, should be requested via discovery:

  • Evidence Screening/Examination (Serology) Report
  • DNA Profile Analysis Report
  • Evidence Item Lists - Examined Items & Unexamined Items
  • Laboratory Case Notes/Analyst Bench Notes
  • Evidence Screening/Examination (Serology) Worksheets
  • DNA Worksheets-Extraction, Quantification, PCR Analysis, etc.
  • STR Typing Data - Electropherogram printouts: IMPORTANT NOTE: Each electropherogram used by the crime lab analysts in the process of reporting the comparative conclusions MUST include RFU values corresponding to every allelic or non-allelic peak visible on the e-gram printout
  • Genetic Loci STR Allele Call Summaries
  • Allelic tables
  • DNA Mixture Analysis
  • Population Statistical Calculations
  • Positive and Negative Controls
  • Laboratory Standard Operating Procedures-Particularly the reporting lab's RFU limits for Capillary Electrophoresis
  • Chain of Custody For Pertinent Case Items
  • Pertinent Reports From Law Enforcement or Medical Facilities
  • Communications between Analysts, Supervisors, Investigators, Prosecutors, etc. This includes any corrective action documents issued from the crime lab during the previous FIVE years—pertaining to improper calculation of population statistics
  • Any additional corrective action reports relevant to this case, or issued within the time frame of case evidence processing


Forensic academians should educate themselves relative to two areas of misinterpretation that frequently occur during forensic DNA testimony at criminal proceedings. These are: DNA Transfer Events and DNA Mixture Examinations. The subtle misinterpretations-relevant to these areas-have been the topic of numerous recent scientific debates and the focus of intense research efforts.

Understanding DNA Transfer Events

Over a century ago in France, Professor Edmond Locard established the world's first forensic science lab for the Lyons Police Department. This great scientist was the first to formally propose the importance of transfer events in the investigation of crimes. Dr. Locard's ideas evolved into the time-tested Locard Exchange Principal, which states that "Every contact leaves a trace."

Amazingly, Locard's principal become universally accepted over forty years before James Watson and Francis Crick described the first accurate model of the DNA double helix.

Today's state-of-the-art DNA detection technology can produce a full DNA profile from less than a BILLIONTH of a gram of DNA. One way to grasp the extraordinarily tiny amounts of DNA is to begin by visualizing the mass of material in a standard packet of artificial sweetener. These packets contain one gram. Imagine setting aside 1/1000th of this material and disposing of the remainder. The spec of powder set aside would weigh one milligram. Now imagine setting aside 1/1000th of this milligram and discarding the remainder. You now have one microgram of material (which is 1/1 millionth of the original sweetener packet). This amount of material cannot be clearly seen without the use of a microscope. By some means, you must now set aside 1/1000th of your microgram of artificial sweetener-this is one nanogram of material. The astonishing sensitivity of forensic DNA typing technology does not diminish the fact that we are indeed working with a profoundly tiny mass of DNA.

Scientific debates focusing on the mechanisms of casual/incidental transfer events, involving such tiny amounts of DNA, are far from settled. In December 2010, some of the world's most renowned authorities on forensic trace DNA (Oorschot, Ballantyne, and Mitchell), published a REVIEW in Investigative Genetics. Quoting the authors directly from the "Transfer Issues" section of the review:

"Greater effort needs to be made by police/crime investigators to investigate how a DNA sample arrived at the location where it was found, as well as by scientists to better understand the impact of activities on the relative amounts of DNA from particular sources at a crime scene. In some instances, it is possible to derive the chain of events that led to a trace DNA sample being present at a crime scene - for example, prior visits to the scene or the known use of an item. Awareness of these variables, and their impact on transfer events, will assist in weighting the likelihood of proposed alternative scenarios."

In 2010, Allan Jamieson and Georgina Meakin of The Forensic Institute (Glasgow, UK), published an ARTICLE in The Barrister Magazine entitled "EXPERIENCE IS THE NAME THAT EVERYONE GIVES TO THEIR MISTAKES"

The following is a quote from this article:

"The examination of evidence for handler DNA can reveal DNA of people who have, or have not, handled the item; the stronger profile may, or may not, be the person who last handled the item; An inference of direct contact between an individual and the item may or may not be supportable, depending on the circumstances of the case. In other words, we did not know enough to make any sensible scientific judgements as to how DNA came to be on an item." 

Later, the article continues as follows:

"Frequently, the underlying hypothesis is that touching, or direct contact, is a more likely scientific explanation for the finding of a DNA profile on an item than indirect contact. This to the extent that it may be described as providing ‘extremely strong’ support for direct versus indirect transfer.  In our view, such an opinion on DNA transfer is not supportable based on case experience or on the available scientific research."

In July 2013 Dr. Jamieson and Dr. Meakin published the most important recent REVIEW on this important area of forensic biology/DNA. In Forensic Science International: Genetics, the article is entitled: "DNA transfer: review and implications for casework".

Finally, a 2009 ARTICLE in Law Officer (a journal for police and law enforcement) by Suzanna Ryan, arrived at the conclusion:

"Obviously, the inadvertent transfer of DNA is an area that should be further studied. Since so many of the available journal articles present conflicting information, more work is needed to see how likely it is to both transfer and detect DNA in a secondary or even a tertiary fashion, especially considering the sensitivity of modern forensic DNA analysis."

It is vital to keep in mind that the average adult human sheds approximately 36,000 skin cells every 60 seconds. This number varies broadly among individuals, as there are profound differences between those who can be characterized as 'good shedders', and others who are 'poor shedders'. A single drop of saliva, expelled during a cough or a sneeze, will contain approximately 500,000 salivary epithelial cells. Forensic Bioligists can attest to the fact that 500 to 10,000 nanograms of DNA are routinely recovered by oral swabs. Again, ONLY ONE NANOGRAM is optimal for generating a complete DNA profile. This mass of DNA can be readily extracted from as few as 200 cells. This tiny number of cells could sit-nearly invisible-upon the very tip of a toothpick. A sobering question: How many falsely accused individuals have been wrongfully imprisoned as a consequence of a few hundred cells finding their way to an incriminating location?




Among the cases involving disparities associated with DNA analysis, the presence of complex DNA mixtures on key evidence items is often at the center of the controversy. For this reason, Spence Forensic Resources has outlined a policy on scrutinizing and interpreting DNA mixtures observed by the reporting laboratory. The steps for these interpretations are as follows:

Step 1: Analysis of electropherogram data to ensure that the profile in question is, indeed, a mixture. When a single source DNA profile from an evidence item matches the profile of a known reference standard, with the exception of ONE additional allele, it is my opinion that no statistical conclusions should be drawn from that single allele. This is especially true when the single allelic peak height is relatively low, or can potentially be attributed to an artifact of the PCR process or capillary electrophoresis.

Step 2: In the event that a genuine mixture is being called, SFR will evaluate the mixture for the possible presence of a major profile, plus one or more minor profiles. The alternative to this would be the apparent presence of NO major profile, ONLY minor, indistinguishable sources of DNA. This evaluation is dependent upon a detailed assessment of the peak height landscape of the entire mixture profile electropherogram-with consideration of the potential contributing reference standard profiles. A final determination will be at the discretion of the analyst, based on years of experience in evaluating hundreds of DNA profiles.

Step 3: If three alleles are present at two or more loci, the mixture includes at least two individuals. If five alleles are present at one locus or more loci, the mixture includes at least three individuals. If seven alleles are present at one locus or more loci, the mixture includes at least four individuals, and so on.

Step 4: On a case by case basis, SFR will evaluate of the capacity of each mixture to be interpreted through sound, reasonable, scientific methods. A variety of factors can contribute to the conclusion that a DNA mixture is simply unsuitable for a reliable interpretation. These factors include, but are not limited to the following: 1) The number of contributors to the mixture-the greater the number of total alleles observed, the less potential there is for a reliable statistical conclusion. 2) When indistinguishable minor contributors are relatively equal donors of DNA, little can be gained from scrutinizing allelic imbalances. 3) When alleles overlap, such as might be expected from contributors who are genetically related or from the same ethnic population pool. 4) When one or more unknown individuals have contributed to the mixture profile, i.e. they have donated DNA alleles that cannot be correlated with the available known reference standards. 5) Allelic dropout has to be assumed in order to make non-exclusion calls.

Step 5: In the event that a genuine mixture is called, the degree of allelic dropout may be cautiously evaluated for the potential exclusion of each suspected contributor. When a key suspect or victim reference standard profile is to be compared to a DNA mixture, the absence of ONE allele from the mixture could be evidence for potential exclusion. An example of this would be that Suspect A has a 9, 11 DNA profile at a given locus. In the mixture profile, a 9 allele-with a substantial peak height-is observed that cannot be attributed to any other suspected contributors to the mixture. Meanwhile, there is a clear absence of an 11 allele in the mixture. Observation of additional dropout events corresponding to Suspect A would further strengthen the cause for exclusion. By assessing the peak height landscape of the DNA mixture electropherogram and the number of apparent allelic dropout events, SFR will utilize years of experience-evaluating hundreds of DNA mixture profiles to arrive at decisions on exclusion or non-exclusion.

Step 6: When a mixture is judged to be suitable for interpretation and there is insufficient evidence for exclusion, the probability of exclusion will be calculated for the mixture. If there is evidence of allelic dropout at a particular locus, that locus will not be used in the probability of exclusion calculation for the mixture.



Once all of the pertinent case documentation has been carefully examined, all potential DNA transfer events characterized, and all DNA mixtures have been interpreted, SFR might recognize the need for additional documentation from the crime lab. Areas requiring further review might include any of the following: Lab quality assurance documents and technical manuals; additional review of standard operating procedures (SOPs); validation studies; lab diagrams demonstrating separation of work areas; qualifications for personnel conducting the case supervision, analysis, or review; proficiency test results; copies of computer data files; genetic databases and allelic frequency tables used for statistical analysis; lab records for equipment maintenance, troubleshooting, reported errors, and corrective actions; lab accreditation and audit documents demonstrating compliance or non-compliance with standards for forensic DNA testing laboratories.

Once the review process is complete, SFR will generate a report. This report will summarize the strengths and weaknesses of the data recovered from the case evidence. The report will also identify any issues associated with the reliability of the analysis, the accuracy of the data, or the significance of the conclusions. In some instances, it might be useful for SFR to summarize complex DNA data and the significance of analytical conclusions in the form of a PowerPoint presentation. Informal discussions, official reports, and visual presentations prepared by SFR, all serve as effective tools in the event that courtroom testimony becomes necessary.


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