Deoxyribonucleic acid, or DNA, is a molecule that encodes the genetic information in all living organisms. Its chemical structure was elucidated in 1954. More than 30 years later, samples of human DNA began to be used in the criminal justice system, primarily in cases of rape or murder. The evidence has been the subject of extensive scrutiny by lawyers, judges, and the scientific community. It is now admissible in all jurisdictions, but there are many types of forensic DNA analysis, and still more are being developed. Questions of admissibility arise as advancing methods of analysis and novel applications of established methods are introduced.1
This reference guide addresses technical issues that are important when considering the admissibility of and weight to be accorded analyses of DNA, and it identifies legal issues whose resolution requires scientific information. The goal is to present the essential background information and to provide a framework for resolving the possible disagreements among scientists or technicians who testify about the results and import of forensic DNA comparisons.
Section I provides a short history of DNA evidence and outlines the types of scientific expertise that go into the analysis of DNA samples.
Section II provides an overview of the scientific principles behind DNA typing. It describes the structure of DNA and how this molecule differs from person to person. These are basic facts of molecular biology. The section also defines the more important scientific terms and explains at a general level how DNA differences are detected. These are matters of analytical chemistry and laboratory procedure. Finally, the section indicates how it is shown that these differences permit individuals to be identified. This is accomplished with the methods of probability and statistics.
Section III considers issues of sample quantity and quality as well as laboratory performance. It outlines the types of information that a laboratory should produce to establish that it can analyze DNA reliably and that it has adhered to established laboratory protocols.
Section IV examines issues in the interpretation of laboratory results. To assist the courts in understanding the extent to which the results incriminate the defendant, it enumerates the hypotheses that need to be considered before concluding that the defendant is the source of the crime scene samples, and it explores the
1. For a discussion of other forensic identification techniques, see Paul C. Giannelli et al., Reference Guide on Forensic Identification Expertise, in this manual. See also David H. Kaye et al., The New Wigmore, A Treatise on Evidence: Expert Evidence (2d ed. 2011).