Life Lines 100 -- PCR and Crime

About twenty years ago Kary Mullis discovered a technique to clone small amounts of DNA. He called this process polymerase chain reaction or PCR. DNA is our genetic material and indeed the hereditary stuff of all cellular life. The technique uses heat to separate the two strands of DNA and some fancy regions are attached, which allows primers that recognize them to begin copying the separated strands. The process takes a few hours to generate a million-fold increase in DNA of any given type.

The PCR technique is very useful for geneticists who wanted to study the isolated genes or fragments bearing genes from any organism. They can use the technique to study the DNA of long dead organisms from humans ancestors to extinct quaggas and dodos. But most striking has been the application of PCR studies to crime.

DNA is found in cells. We are composed of tens of trillions of cells. Every time we lick a stamp or envelope we leave some cells with our saliva on the paper and glue. When we drink from a glass, toss a tissue that we used to wipe our nose or mouth, we discard some of our cells. Dandruff flakes, hair, the aerosol of our sneeze can also scatter our DNA about. The man who rapes his victim leaves tell tale evidence on clothing. A criminal’s DNA will be found under the nails of a victim who scratches her assailant. Any cut by a criminal will leave drops of blood as evidence.

The first few years that PCR evidence was used to convict criminals, it was vigorously challenged by defense attorneys, just as early attempts to use fingerprints were resisted until the public and the courts got used to the accuracy of the methods used to establish identity. To the surprise, and disappointment, of district attorneys, the PCR evidence works both ways. Many convicted rapists were released because the crime, and not the criminal was convicted. Shady looking people at the wrong place and time are sometimes railroaded into prison and PCR has been their deliverance. Both the prosecution and the defense benefit from this reversal of fortunes because if the technique is valid the DNA evidence can be very convincing.

Why then did the jury reject the PCR evidence in the Simpson case? Some believe in conspiratorial theories – that Simpson was framed and his blood dribbled here and there by racist enemies. Some still discount the evidence because it is relatively new compared to fingerprints. There is another reason too. Juries consider what is right and wrong differently from professionals who know, teach, and practice the law. I remember, years ago, when Charlie Chaplin was very unpopular for his communist sympathies. He had an alleged affair of many months duration with a “crumpet girl,” whom he had befriended. She became pregnant. She sued for child support. A test for blood groups (the method then in use in some 40 years ago) revealed Chaplin was not the father. He was convicted. Why would a jury convict a person whose sperm did not produce this child? I remember reading a comment of one of the jurors: Chaplin cohabited with her; he could have been the father; he can afford it; it was only luck that someone else was also intimate. Sometimes a moral view prevails over a technical one.