Medical Genetics Index	Glossary

	Strengths, Limitations and Controversies of DNA Testing

DNA testing has a number of real advantages over serological methods such as blood typing and HLA analysis for use in forensic investigations: Unsurpassed discriminatory potential: Complete blood group testing allows discrimiation of one person in several thousand and HLA typing one in several million. DNA typing can routinely provide exclusion probabilities on the order of one in billions. Exquisite sensitivity: Standard DNA typing can be conducted with DNA extracted from the roots of a few hairs. In contrast to proteins, DNA can be amplified, and by using polymerase chain reaction methods, even smaller sample sizes are adequate. One important consequence of this great sensitivity is that it allows rather small samples to be split and submitted for testing to more than one laboratory, which can identify laboratory errors more commonly serves to nullify objections that laboratory erros were committed. Application to any body tissue: Complete serologic testing requires blood, but because DNA testing can be conducted with any sample having nucleated cells, it is applicable to such samples as hairs, semen, urine and saliva. DNA is stable in comparison to proteins: In comparision to protein, DNA in quite resistant to degradation by common environmental insults. DNA testing can therefore often be performed on samples that have been exposed to detergents, acids and bases, gasoline, salt, and bacterial contamination. Importantly, DNA is also long-lived in comparison to protein. It does degrade over time, but reliable information can be obtained from samples that are years old. Overall, DNA is remarkably robust as a sample for foresic testing, which, for example, has allowed it to be used on skeletenized remains for identification of soldiers missing in action. DNA typing has often been portrayed in the media and the courtroom as a controversial technology, largely because it has been so characterized by many defense attorneys. When DNA evidence demonstrates that the odds that someone other than your client committed the crime are one in a billion, there is really nowhere else to go but to attack the basic technology of DNA testing. With increased experience and standardization of testing methods, these assaults are heard less frequently. There have been some scientifically legitimate criticisms of DNA testing, based on concerns about allele frequencies in certain populations. These frequencies are used in calculation of calculating probability of identification. The worry was that the chances of a random match may be higher than stated because the database used was inappropriate for the subpopulation of people containing the suspect. For example, the frequency of a specific allele under test may be 4% in Asians instead of 1% as it is in Northern Europeans. However, most experts concluded that such differences in allele frequency have rather little impact on the final diagnosis - it makes little difference whether the probability of innocence or guilt is one in 10 million or one in 100 million. Nevertheless, the basic premise of the argument is valid and has been incorporated into recommendations about how forensic DNA testing be conducted and interpreted.

	Next Topic for DNA Testing: DNA Polymorphisms: The Basis of DNA Typing
	References and Reviews Weedn VW, Roby RK: Forensic DNA testing. Arch Pathol Lab Med 117:486, 1993. [good review of the strengths of DNA testing, but few technical details].

Last updated on March 18, 1996

Send comments via form or email to rbowen@lamar.colostate.edu