Chapter 15: The Use of Biological Evidence and DNA Databanks to Aid Criminal Investigations
DNA was the key evidence from a young woman who was sexually assaulted by a male unknown to her. This sexual assault was similar in several elements to two other rapes that had occurred in the neighborhood. The woman was able to give a description of her assailant to law enforcement but could offer few other details about the incident. The police conducted a standard investigation and developed a suspect after consulting a number of their regular informants. The person who was the focus of the police investigation was a homeless male who roughly fit the description provided by the victim. After several hours of interrogation by detectives, the suspect confessed to all of the sexual assaults, although his version of the crime differed in some details from the actual assault. As part of their investigation, the police collected known blood and hair samples from the suspect for comparison to the biological materials in the sex assault evidence kits collected in all three cases. Laboratory scientists conducted DNA testing on vaginal swabs in the three cases. All three sperm-rich samples yielded identical DNA profiles-there was indeed a serial rapist in that city. The evidentiary profiles generated, however, did not match the person who had confessed to the crimes. Initially, detectives were sure that some error was made during the lab analysis because of the confession. The profiles generated in these cases were entered into the local and national DNA offender databases, but no match was found initially. Several months later, a sample collected from a convicted offender matched the profile generated in those three cases. The person identified by his DNA ‘hit’ pleaded guilty to all three crimes. Certainly without DNA analysis, an innocent person who confessed for some reason to crimes he did not commit would have gone to jail for many years. At the same time a dangerous sexual predator would have been released into the community. The DNA data bank provided investigative information that would not have been possible only a few years earlier.
1: Explain the impact of DNA analysis in this case. How would the outcome have changed without this evidence? |
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| 2: How has the DNA data bank changed the landscape of forensic casework? |
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A woman was stabbed to death in a parking garage. Based on the bloodstain patterns and other evidence at the scene it was apparent that the killer had been injured during the incident. When a suspect was finally identified 25 years after the crime through a search of the automated fingerprint identification system database, the prosecutor wanted additional corroboration of this individual as the perpetrator of the murder. A few aged blood samples that had been stored in less than optimal conditions, some of which had been treated with numerous chemical reagents, were available for DNA testing. Although many other stains were originally recovered, years of serological testing reduced the number and size of material available for DNA profiling. However, DNA was extracted from some of the remaining stains and PCR analysis was conducted. STR profiles were generated from many of these samples. Although some of the bloodstains showed the presence of more than one DNA profile, the suspect was included as a possible donor to those stains. Significantly, a 13-locus match to the suspect was obtained from one of the bloodstains on a handkerchief found near the victim. The accused was convicted 29 years after the murder, and these DNA profiles were key evidence presented during the trial.
3: Is this an example of low-copy-number DNA? Why? |
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| 4: What precautions must forensic nurses take in collecting and preserving biological evidence? |
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