Evaluating Forensic DNA Evidence: Where Do I Start??

So your client has been incriminated by a DNA test?  The lab report says incomprehensible things about “loci” and “alleles” and seems to imply that the odds of innocence are one in a godzillion?  And the last time you thought about science was when you took “Physics for Poets” as a college sophomore?  Well don’t panic.  You’ve come to the right place.  We’ll tell you what you need to know to get up to speed on DNA evidence, even if you are a complete novice.  And we’ll highlight and provide links to helpful materials in the Forensic Library.

First, realize that you have an important job to do.  Although DNA evidence is formidable, it often can be challenged successfully.  It doesn’t take a Ph.D. to do it—just some commonsense and a willingness to learn.  But it does take some time and effort.  There is no way to wing it.  If you don’t have the time to study and understand the DNA evidence, you will not be able to represent your client effectively. 

If you know nothing about DNA testing, we suggest you start by reading the article Introduction to DNA Testing, which provides an overview of the different types of DNA tests and mentions some of the key issues you need to consider.

Your key job is to EVALUATE the DNA evidence against your client to determine whether there were any weaknesses or problems and whether there are plausible alternative interpretations for the findings.  Start by obtaining copies of all laboratory report as well as the underlying laboratory notes.  You should examine and try to understand the entire history of each important biological sample in your case, from the time of collection to its current disposition.  You will need to obtain notes documenting the collection and initial processing of the samples as well as notes documenting any subsequent DNA testing.   

Examples of effective discovery requests can be found in the “Discovery Requests” section of the DNA library.   If you are having difficulty persuading the court of the need for discovery, see the section called “DNA Declarations,” which includes some helpful declarations about what defense lawyers need (in terms of time and discovery) to perform competently.

It often is useful to talk with the analysts who performed the tests in your case (if they are willing).  They can explain what the lab notes mean and help you understand what the test results mean.  Of course, forensic analysts sometimes identify strongly with the police and prosecution, and are not always the most critical reviewers of their own work.  So consulting an independent expert is also a good idea.  The section on “dna-experts” lists some independent consultants.  Defense lawyers sometimes have success recruiting experts from local universities.  Look for biochemists, molecular biologists, and statisticians who have some knowledge of genetics.    

There are five major lines of attack that defense lawyers have used against DNA evidence.  As you evaluate your case, consider whether the might be problems in each of these areas:

1. Sample Handling Error – In a surprising number of cases labs have mixed up, mislabeled, or cross-contaminated samples, thereby producing false results.  Think carefully about whether this might have happened in your case.  Pay special attention to circumstances in which reference samples and evidence samples were handled in close physical and temporal proximity—that is where mix-ups occur.

2. Knowing What Was Tested—In many cases it is crucial to know what kind of biological material produced a particular DNA result.  Was it really semen rather than blood, saliva or skin cells?  Sometimes labs do not know, or cannot say for sure, exactly what they tested that produced a particular result.  Look closely at the preliminary testing on samples.  Often labs do no preliminary testing, or perform only presumptive tests, leaving question marks about the true nature of the biological evidence.  The section of the Forensics Library on Serology contains some helpful references about blood and bodily fluid testing.

3. Knowing How It Got There—Current DNA tests are exquisitely sensitive, able to type tiny quantities of DNA.  Samples too small to see with the human eye can easily contain enough blood or skin cells to produce a result.  With the growing sensitivity of DNA tests, the potential for inadvertent DNA transfer has become a serious issue.  We all leave samples of our DNA behind us, as we travel through the world, and these DNA samples can sometimes be transferred inadvertently from one location to another.  Consider the possibility that the incriminating results in your case might be explained by such an event.  The following articles on DNA transfer are a helpful starting point for understanding this issue.

DNA Transfer Studies--Summary (msword, 217 Kb)

4. Misinterpretation of Test Results—DNA tests do not always produce clearcut results.  Often there is an element of subjective judgment in the “calls” that analysts make.  And experts don’t always agree.  Pay special attention in cases where there are discrepancies between the DNA profiles that are said to have a common source.   In cases involving STR and mtDNA testing, it often is useful to examine the underlying electronic data for evidence of unreported discrepancies between profiles, control problems, and evidence of unreported additional contributors.

5. Exaggerated Statistics—There is continuing controversy about the correct statistical characterization of DNA test results.  Particularly in cases involving mixed samples and partial (incomplete) profile matches, forensics labs tend to compute statistics in ways that may unfairly work against the accused.  It is useful to consult experts in such cases. 

As you learn more about DNA evidence, be sure to share your knowledge with other defenders by posting useful materials in the Forensic Library.