The resulting DNA profile for a sample, which is a combination of individual STR genotypes, is compared to other samples. In the case of a forensic investigation, these other samples would include known reference samples such as the victim or suspects that are compared to the crime scene evidence. With paternity investigations, a child's genotype would be compared to his or her mother's and the alleged father(s) under investigation (Chapter 23). If there is not a match between the questioned sample and the known sample, then the samples may be considered to have originated from different sources (see D.N.A. Box 1.2). The term used for failure to match between two DNA profiles is 'exclusion.'
If a match or 'inclusion' results, then a comparison of the DNA profile is made to a population database, which is a collection of DNA profiles obtained from unrelated individuals of a particular ethnic group (Chapter 20). For example, due to genetic variation between the groups, African-Americans and Caucasians have different population databases for comparison purposes.
The Innocence Project: using DNA evidence to exonerate wrongfully convicted individuals
Forensic DNA testing can play a role in protecting the innocent as well as implicating the guilty. In recent years, the use of DNA evidence to free people from prison has been highly publicized and has altered some perceptions of the criminal justice system. For example, capital punishment in Illinois was put on hold by the governor after learning of several inmates being exonerated by post-conviction DNA testing.
As of May 2004, a total of 143 people including some 'death row' inmates previously incarcerated for crimes they did not commit have been released from prison thanks to the power of modern forensic DNA typing technologies. Many of these wrongfully convicted individuals were found 'guilty' prior to the development of DNA typing methods in the mid-1980s based on faulty eyewitness accounts or circumstantial evidence. Fortunately for the 143 so far exonerated by post-conviction DNA testing some evidence was preserved in police lockers that after many years could be used for DNA testing. The results successfully excluded them as the perpetrator of the crimes for which they were falsely convicted and imprisoned.
Defense attorneys Barry Scheck and Peter Neufeld launched the Innocence Project in 1992 at the Benjamin N. Cardozo School of Law in New York City. This non-profit legal clinic promotes cases where evidence is available for post-conviction DNA testing and can help demonstrate innocence. The Innocence Project has grown to include an Innocence Network of more than 40 law schools and other organizations around the United States and Australia. Law students and staff carefully evaluate thousands of requests for DNA testing to prove prisoners' innocence. In spite of careful screening, when post-conviction testing is conducted, DNA test results more often than not further implicate the defendant. However, the fact that truly innocent people have been behind bars for a decade or more has promoted legislation in a number of states and also at the federal level to fund post-conviction DNA testing. The increased use of DNA analysis for this purpose will surely impact the future of the criminal justice system.
Finally a case report or paternity test result is generated. This report typically includes the random match probability for the match in question (see example in D.N.A. Box 1.3). This random match probability is the chance that a randomly selected individual from a population will have an identical STR profile or combination of genotypes at the DNA markers tested (Chapter 21).
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