Foundation Requirements and Daubert Standard
Georgia courts require the prosecution to establish proper foundation before DNA evidence reaches the jury, including proof of lawful collection, unbroken chain of custody from scene to laboratory, accredited laboratory analysis, and qualified examiner testimony. Under O.C.G.A. Section 24-7-702, as extended to criminal cases by HB 478 (2022), the proponent of scientific evidence must demonstrate that the evidence is the product of reliable principles and methods applied reliably to the facts of the case under the Daubert standard. DNA evidence based on polymerase chain reaction (PCR) and short tandem repeat (STR) methodologies generally satisfies Daubert reliability because these methods have been extensively validated.
Consider this scenario: DNA found at a crime scene is a partial match to your profile. The prosecution’s expert says the probability of a random match is one in ten million. But what if the DNA was a mixture from multiple people, or was transferred to the scene by secondary contact?
However, foundation failures at any step, including collection errors, documentation gaps, or unqualified analysts, can render otherwise reliable DNA evidence inadmissible. The U.S. Supreme Court in Melendez-Diaz v. Massachusetts, 557 U.S. 305 (2009), held that the Confrontation Clause requires the analyst who performed the testing to testify in person, and in Bullcoming v. New Mexico, 564 U.S. 647 (2011), extended this holding to prohibit surrogate analyst testimony. Your lawyer can demand the testifying analyst be the person who actually performed the analysis.
Chain of Custody Challenges
Chain of custody is frequently the most effective avenue for challenging DNA evidence in Georgia. Under O.C.G.A. Section 24-9-901, the prosecution must authenticate physical evidence by accounting for every person who handled the sample from collection through analysis. Georgia courts apply a reasonable assurance standard rather than requiring absolute certainty, but gaps in documentation create substantial opportunities for defense challenge. Your attorney’s priority is to obtain the full chain of custody log, identify every transfer point, and investigate whether proper storage conditions were maintained throughout. Samples stored at improper temperatures, left unsealed, transported without refrigeration, or transferred without documentation present the strongest chain of custody challenges. A successful chain of custody challenge may result in exclusion of the DNA evidence entirely or, at minimum, provide the jury with reason to question its reliability.
Mixture Interpretation and Probabilistic Genotyping
DNA mixture interpretation occurs when a sample contains genetic material from more than one contributor, common in sexual assault cases, shared living spaces, and items handled by multiple people. The number of contributors, the ratio of their contributions, and the presence of degradation all affect the reliability of the interpretation. Georgia laboratories use probabilistic genotyping software to calculate likelihood ratios for mixtures, replacing older combined probability of inclusion methods that have been criticized as unreliable. Effective defense requires your attorney to scrutinize the number of assumed contributors, the software validation studies, the analyst’s proficiency testing results, and whether the laboratory followed its own standard operating procedures. The statistical weight assigned to a mixture result can vary by orders of magnitude depending on the assumptions applied, and defense experts can demonstrate how different reasonable assumptions produce dramatically different results.
Touch DNA and Secondary Transfer
Touch DNA involves analysis of extremely small quantities of genetic material deposited through casual contact with surfaces. This evidence presents heightened reliability concerns because small samples are more susceptible to contamination, allele dropout, and stochastic effects that can produce unreliable profiles. DNA transfer theory is critical for defense strategy because DNA can move from person to object to object without direct contact between the person and the final surface. A defendant’s DNA on a weapon may have arrived through an intermediary object or person rather than through handling the weapon. Defense experts can testify about published transfer studies showing that DNA moves readily through handshakes, shared surfaces, laundry, and other indirect pathways. This testimony undermines the prosecution’s inference that DNA presence equals physical contact or proves the defendant handled the specific item at the time of the offense.
Crime Laboratory Errors and GBI Forensic Sciences
Georgia crime laboratories must maintain accreditation through organizations such as the American National Standards Institute National Accreditation Board and follow validated protocols. The Georgia Bureau of Investigation Division of Forensic Sciences operates the primary state crime laboratory system, and private laboratories must also meet accreditation standards. Your attorney’s practical approach is to verify current accreditation status, request audit reports and corrective action records, and investigate whether the specific analyst has a history of errors or proficiency test failures. Laboratory errors including sample switches, contamination events, and reporting mistakes have occurred in Georgia and nationally. Prior incidents at the specific laboratory or by the specific analyst are discoverable under Brady v. Maryland, 373 U.S. 83 (1963), and relevant to both admissibility and the weight the jury should assign to the evidence.
Post-Conviction DNA Testing
Georgia law under O.C.G.A. Section 9-14-42 provides a mechanism for post-conviction DNA testing when the testing was not available at the time of trial or when new testing methods could produce results that are materially more discriminating than prior results. The petitioner must show that a reasonable probability exists that the verdict would have been different if favorable DNA results had been obtained. The Georgia DNA database, maintained under O.C.G.A. Sections 35-3-154 through 35-3-165, collects DNA from individuals convicted of felonies and certain misdemeanors, and matches against the database have both identified perpetrators and exonerated the wrongfully convicted. Your attorney pursuing post-conviction relief should identify all biological evidence collected in the case, determine whether it was preserved under the state’s evidence retention obligations, and evaluate whether current testing technology could produce results unavailable at the time of conviction.
Challenging DNA Evidence
Effective DNA defense requires early retention of a qualified forensic DNA expert to review the laboratory’s complete case file, including electropherograms, mixture interpretation worksheets, analyst notes, and quality control records. A well-prepared defense attorney will file discovery requests specifically targeting these materials under O.C.G.A. Section 17-16-4 because they are not always included in standard discovery packages. Expert review may reveal interpretation errors, protocol deviations, contamination indicators, or alternative explanations for the DNA results. When the DNA evidence is scientifically sound, defense strategy shifts from challenging the methodology to challenging what the DNA proves: presence of DNA does not establish identity as the perpetrator, timing of deposit, or criminal intent.