appropriate DNA test systems, the uniqueness of any individual on the planet (except an identical twin) is likely to be demonstrable in the near future. In the meantime, the justification for an inference that two identical DNA profiles come from the same person rests on probability calculations that employ principles of population genetics. Such calculations are, of course, subject to uncertainty. When in doubt, we err on the side of conservatism (that is, in favor of the defendant). We also discuss ways of keeping laboratory and other errors to a minimum. We emphasize that DNA analysis, when properly carried out and interpreted, is a very powerful forensic tool.
This committee was asked to update an earlier report, prepared for the National Research Council (NRC) in 1992. There are two principal reasons why such an update is needed. First, forensic science and techniques have progressed rapidly in recent years. Laboratory standards are higher, and new DNA markers are rapidly being introduced. An abundance of new data on DNA markers in different population groups is now available, allowing estimates of the frequencies of those markers in various populations to be made with greater confidence. Second, some of the statements in the first report have been misinterpreted or misapplied in the courts.
This report deals mainly with two subjects:
The first involves the laboratory determination of DNA profiles. DNA can be obtained in substantial amounts and in good condition, as when blood or tissue is obtained from a person, or it can be in limited amounts, degraded, or contaminated, as in some samples from crime scenes. Even with the best laboratory technique, there is intrinsic, unavoidable variability in the measurements; that introduces uncertainty that can be compounded by poor laboratory technique, faulty equipment, or human error. We consider how such uncertainty can be reduced and the risk of error minimized.
The second subject is the interpretation of a finding that the DNA profile of a suspect (or sometimes a victim) matches that of the evidence DNA, usually taken from the crime scene. The match might happen because the two samples are from the same person. Alternatively it might be that the samples are from different persons and that an error has occurred in the gathering of the evidence or in the laboratory. Finally, it might be that the samples are from different people who happen to have the same DNA profile; the probability of that event can be calculated. If the probability is very low, then either the DNA samples are from the same person or a very unlikely coincidence has occurred.
The interpretation of a matching profile involves at least two types of uncertainty. The first arises because the US population is not homogeneous. Rather it consists of different major races (such as black and white), within which there