A man entered a 43-year-old woman’s apartment through a back door that she had left open to get some air and sexually assaulted her. A year passed with no suspects. Now, DNA links Christopher Nelson to the crime. He has been denied bail.
Decades of CSI-style TV shows have led the American public to believe that DNA evidence is unimpeachable. Indeed, it provides law enforcement with its best means of tracking individuals to certain crimes. DNA links Nelson to the crime and the woman he assaulted also picked him out of a lineup.
Understanding DNA Evidence in Rape Cases
You have probably heard of the term ‘rape kit.’ The rape kit contains a literal kit and checklist of instructions and procedures for preserving genetic evidence left behind during a rape. Those who are survivors of sexual assaults are advised to keep from bathing, changing their clothing, or even using the bathroom before having a rape kit done. After the examination has been conducted, the evidence is stored.
For forensic investigators, DNA evidence can be collected in various ways. The first is small-sample DNA from skin cells left behind at a crime scene. This is known as touch DNA and trace DNA. It is considered more controversial to use in forensic investigations and is often used by defense attorneys to create reasonable doubt. This DNA can be transferred from a handshake or a brief encounter with an object. These tests look for markers in small samples of DNA left behind by alleged criminals.
DNA can also be recovered from semen and this is often what is tested in rape cases. For instance, let’s say that semen is recovered from the victim’s skin or clothing. A test will first be done to ensure that the substance is semen. Once that has been established, the organic material is dissolved in a solution in order to extract it from whatever material on which it was found. The technicians will use a microscope to search out sperm cells. The DNA is extracted from the sperm. A test is run to determine if a sample recovered from the suspect matches the DNA found at the scene using a method known as restriction fragment length polymorphisms (RFLP). If the enzymes applied to sample cut the DNA into the same number of lengths as DNA recovered from the suspect, odds are that the DNA is a match.
However, there is some chance that the DNA recovered from the crime scene and the DNA recovered from the suspect react the same way to the enzymes. Expert witnesses then testify that the chance of this DNA belonging to someone other than the suspect is something along the lines of one in eleven million.
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Charged with a serious crime in Chicago? Criminal defense attorney David Freidberg can help protect you from overzealous or hamfisted prosecutions, reduce your charges, or exonerate you completely. Call us at (312) 560-7100 to set up and appointment today.