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We’ve all heard about DNA testing, but what is it exactly?  DNA is the genetic code for all living organisms and it is made up of four chemicals, Adenine, Thymine, Guanine and Cytosine, abbreviated as A, T, C and G.  These chemicals are like teeth on a zipper, joining two sides of the DNA strand together and forming a “double helix”.  A is joined with T and C is joined with G. 

Most of the DNA strand is sequenced in a standard pattern, shared by the majority of humans; this would the basic directions for building a human body versus a lizard. However, there are a few sections (1/10 of 1% of DNA) called minisatellites that are very unique to individuals.  The unique combinations at these locations are used to match two samples of DNA.  

“Forensic scientists scan 13 DNA regions that vary from person to person and use the data to create a DNA profile of that individual (sometimes called a DNA fingerprint)” (1)

DNA is found in most living cells, including: blood, semen, skin, saliva or hair.  Scientist find DNA markers - created with small pieces of DNA (probes) which will bind to complementary segments (A to T, C to G).  The forensic scientist knows the sequence of the probes, so they can determine the sequence of the DNA sample when the two pieces link together. 

According the Human Genome Project, the following DNA technologies are currently used in forensic investigations:

Restriction Fragment Length Polymorphism (RFLP)

A specific enzyme called a restriction endonuclease, cuts DNA at a specific sequence pattern, a recognition site. The resulting DNA fragments are then analyzed to verify the presence or absence of certain recognition sites.  This was one of the original methods used to match DNA samples, but it required lots of material to get a match.

Polymerase Chain Reaction (PCR) Analysis
Begins with copying original DNA sample and then using degraded samples for analysis.  Minut amounts of DNA (a few skin cells) can result in millions of copies which can be thoroughly tested.  

Short Tandem Repeat (STR) Analysis

Used to evaluate specific regions of DNA called loci.  The FBI “uses a standard set of 13 specific STR regions for CODIS.  CODIS is a software program that operates local, state, and national databases of DNA profiles from convicted offenders, unsolved crime scene evidence, and missing persons. The odds that two individuals will have the same 13-loci DNA profile is about one in one billion.” (1)

Mitochondrial DNA Analysis
Most DNA is taken from the nucleus of a cell, but the mtDNA or Mitochondrial DNA comes from the mitochondrion. This test is used when the samples no longer have good DNA material for testing.  Hair, bones and teeth can be analyzed years after a crime using this technique. 

The fact that criminals are aware of DNA testing makes them even more deceptive and cunning.  Planting DNA or using bleach to destroy trace evidence at a scene is becoming more common place. 

Dr John Schneeberger was accused of raping a patient, Candice Foley.   The rape kit provided an semen sample which was compared against a sample provided by Schneeberger. The samples didn’t match.

In 1995 Ms Foley hired a private detective to obtain a second DNA sample from Schneeberger.  The second sample matched the sample of DNA taken from the crime scene. Schneeberger volunteered for a second blood test.

Lisa Schneeberger came forward with allegations that her husband John had repeatedly raped his step daughter.  More extensive DNA testing was done and finally the state received a match. The doctor saved a patient’s blood and placed it in a tube which he then surgically implanted into his arm. When the blood was drawn, it came for the blood contained in the tube rather than his vein.

1. Human Genome Project: DNA Forensics, November 2007, Retrieved January 2008