Criminology’s only scientifically tested technique.
– In 1987, California strip club owner “Jimmy Casino” was shot three times in the back of his head. His alleged murderer was discovered in Hawaii in 2008, thanks to a match of DNA at the slaying site with the murderer’s DNA samples.
– In 1994, former American football great and actor O.J. Simpson was acquitted of killing his ex-wife and a third person, largely due to errors the Los Angeles Police Department made in preparing DNA samples from Simpson and the murder scene.
-Last fall, 8,000 men living near the town of Zwaagwesteinde, The Netherlands, were asked to submit DNA samples to prosecutors investigating the murder of a 16-year-old girl in 1999.
What these cases have in common is the use of DNA to solve crimes, or defend against accusations of crimes. Police and prosecutors, and defense attorneys and prison advocates alike have embraced DNA evidence. The U.S.-based Innocence Project has documented 303 exonerations of prisoners due solely to new DNA evidence. Much of this technology wasn’t used in police work until the early 1990s. And advances in genetic sequencing, which can differentiate between individuals, are still not part of the crime lab yet.
How does DNA forensics work?
About 1/10th of one percent of our DNA differs from another person’s DNA. Since our DNA consists of about 3 billion base pairs, that could be enough to differentiate a person. In fact, the test used on O.J. Simpson’s blood samples, called RFLP (short for restriction fragment length), can make a 1 in 170 million match (that’s the likelihood of two identical strips of DNA). Typical DNA fingerprinting, as this profiling is called, looks at 13 areas (pictured below) where we usually differ from one another. These areas, called markers, are compared to a suspect’s blood or tissue samples. If there’s a good match, there’s a good chance you’ve got your man—or woman.
But there’s a problem. The main technique for analyzing crime scene DNA—polymerase chain reaction—is fantastic for amplifying incredibly minute samples of DNA so we can read the genetic patterns (RFLP requires much more DNA than PCR, and isn’t used much anymore). Unfortunately, PCR is also very, very good at picking up every other piece of DNA that could have contaminated the sample—from a neighbor, a lover, a spouse, the crime scene technician; the list goes on. In the O.J. Simpson trial, the LA Police Department crime lab was shown to have very sloppy preparation procedures, so contamination couldn’t be ruled out.
But, as the Innocence Project has shown, DNA evidence is one of the only forensics tools to have been developed under scientific rigor, which gives it a lot of promise for catching the bad guys—and freeing the good ones.
Sources: Genome.gov, Innocence Project, DNAForesnsics,com, bxscience.edu
Photos: Wikimedia Commons
how dna is used in criminal investigations