Gene Watch Page
"Promoters of forensic DNA testing have, from the beginning, claimed that DNA tests are virtually infallible....
"In this article, I will argue that this shared assumption is wrong. Although generally quite reliable (particularly in comparison with other forms of evidence often used in criminal trials), DNA tests are not now and have never been infallible. Errors in DNA testing occur regularly. DNA evidence has caused false incriminations and false convictions, and will continue to do so. Although DNA tests incriminate the correct person in the great majority of cases, the risk of false incrimination is high enough to deserve serious consideration in debates about expansion of DNA databases....
"This article will discuss major ways in which false incriminations can occur in forensic DNA testing, including coincidental DNA profile matches between different people, inadvertent or accidental transfer of cellular material or DNA from one item to another, errors in identification or labeling of samples, misinterpretation of test results, and intentional planting of biological evidence. It will also discuss ways in which the secrecy that currently surrounds the content and operation of government databases makes these issues difficult to study and assess. It will conclude by calling for greater openness and transparency of governmental operations in this domain and a public program of research that will allow the risks discussed here to be better understood.A coincidental match between different people who happen to share the same DNA profile is one way a false incrimination can occur. To understand the likelihood of a coincidental match, it is important to understand what a DNA profile is and how DNA profiles are compared. Forensic laboratories typically "type" samples using commercial test kits that can detect genetic characteristics (called alleles) at various loci (locations) on the human genome. The test kits used in the United States generally examine the 13 STR loci selected by the FBI for CODIS, the national DNA database.10 Some of the newer test kits also examine two additional STR loci.
"At each STR locus, there are a number of different alleles (generally between 6 and 18) that a person might have. Each person inherits two of these alleles, one from each parent. Numbers are used to identify the alleles and the pair of alleles at a particular locus constitutes a genotype. Hence, one person can have a genotype (for a locus called D3S1358) of "14, 15;" while another person has the genotype "16, 17." The complete set of alleles detected at all loci for a given sample is called a DNA profile. When describing DNA profiles, people sometimes mention the number of loci they encompass.
"In cases I have reviewed over the past few years, evidentiary samples from crime scenes often produce incomplete or partial DNA profiles. Limited quantities of DNA, degradation of the sample, or the presence of inhibitors (contaminants) can make it impossible to determine the genotype at every locus. In some instances the test yields no information about the genotype at a particular locus; in some instances one of the two alleles at a locus will "drop out" (become undetectable). Because partial profiles contain fewer genetic markers (alleles) than complete profiles, they are more likely to match someone by chance (see endnote 1). The probability of a coincidental match is higher for a partial profile than for a full profile."