Posted by Sarah Rackley
Edited by Dr. Maher Noureddine, forensic DNA expert
The SBI lab started analyzing “touch DNA” around 2006. Now that the lab is capable of analyzing this type of evidence, investigators are collecting it more often, and I am hearing about it coming up in more cases.
So, what is touch DNA and what does it mean for your case? Touch DNA refers to the DNA that is left behind from skin cells when a person touches or comes into contact with an item. Trace DNA would be a more accurate way to characterize this evidence, as small amounts of DNA can be left behind simply from the shedding that our bodies do naturally. An average size human will shed thousands of skin cells every minute, and each cell will contain a full copy of that individual’s genome. Small amounts of DNA can also be left behind from a cough or a sneeze.
So, when the SBI lab or investigators refer to “touch DNA,” you should think – very small amounts of DNA. With “touch DNA,” physical contact is sometimes assumed or hypothesized. Therefore, you should also think about the physical environment from which this evidence was collected. Always consider alternative explanations for why someone’s DNA would be expected to be found in a given area or on a given surface.
Because such a small amount of evidence is being analyzed (as few as 5-10 cells) contamination and transfer of DNA are issues that should be considered through each step in the analysis. Evidence collection, extraction of DNA, amplification and interpretation are four areas where attorneys should be aware of possible problems.
Evidence collection:
Trace DNA can be collected either in the field by law enforcement officers or crime scene investigators, or it can be collected from a piece of evidence (such as a gun) at the crime lab. Attorneys should ask:
- What is the collection technique?
- The use of evidence collection devices on multiple samples (such as fingerprint brushes) can be a source of contamination. How was this limited?
- What type of tape or swab is used? Wet or dry swab? How many times was the piece of evidence swabbed?
- Are swabs from different areas kept separate? (i.e., the swab from trigger kept separate from swab from other parts of gun)
Extraction of DNA:
Before the DNA can be analyzed, the sample that was collected has to be removed (“extracted”) from the tape or swab.
- What technique was used to extract the potential DNA sample from the tape or swab?
- Was DNA extracted from the swab while it was still wet or was it frozen or allowed to dry first?
- Has the analyst been proficiency tested in collection techniques?
- Was the “touch DNA” sample kept away from standard samples such as blood cards or cheek swabs to minimize cross contamination?
- Are there additional measures used in the lab to prevent contamination?
- Was the analyst aware that he/she was dealing with a “touch DNA” sample?
Amplification:
At this stage, exact copies of the DNA are made so that there will be a sufficient amount of DNA to analyze.
- Is quantity of DNA measured prior to amplification? Although the starting DNA material recovered from a “touch” sample can be miniscule, current standard laboratory techniques are capable of providing a measure of such small samples.
- Is any portion of the sample retained for possible further analysis?
- In amplification, are Low Copy Number techniques used, such as increasing the number of cycles of amplification? Increasing cycle number has drawbacks and can result in artifacts.
- Did the analyst conduct simultaneous reaction standards and appropriate negative controls to show that no cross contamination occurred in the lab?
Interpretation:
This is the most subjective phase of DNA analysis. It is possible for different analysts to reach different conclusions about interpretation, especially where such small amounts of DNA are involved. Consider asking a qualified DNA expert to validate the lab’s data and interpretation.
- What are the interpretation guidelines? What are the guidelines and procedures for interpreting mixtures? What were the threshold levels for validating a peak as a true allele?
- How are problems such as allele drop out and allele drop in dealt with?
- Did the analyst or lab supervisor properly annotate artifact peaks (peaks that are not true genotypic alleles), off-ladder peaks, and other rare allele variants that sometimes show up in the data?
- How does the analyst account for the potential for transfer of the material and possible artifacts that may confuse the true result?
If touch DNA plays a role in one of your cases, a good starting point to learn more about it is the article, Roland AH van Oorschot et al., Forensic trace DNA: a review, 1 Investigative Genetics 14 (2010).
Click here to access a database of experts in DNA analysis.
Visit the DNA Resources page of the IDS Forensics website for additional information.