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Amazing Advances in Forensic Science Part 1: DNA
Written by Vernon Geberth
Forensics has made great strides over the last 30 years, providing law enforcement with an assortment of exciting tools. The forensic sciences include the disciplines of pathology, toxicology, serology and forensic DNA analysis, psychiatry, psychology, entomology, odontology, computer and digital science, archeology, anthropology, geology, etc.
Criminalistics is the application of various sciences to answer questions relating to the examination and comparison of biological, trace and impression evidence, such as fingerprint analysis, footwear, tire impressions and toolmark evidence, as well as drug analysis, ballistics and firearm examinations.
However, the greatest advancement in the forensic sciences is the application of DNA technology to the criminal justice process. DNA typing is a powerful tool because proper analysis and collection can convict the guilty and exonerate the innocent. Procedural improvements have made the collection of DNA evidence more efficient and reliable, and advances in science allow forensic scientists to identify DNA samples from hair, bone, and ever smaller amounts of blood and other body fluids.
The Start: Blood Groups
Before DNA technology, all we had to work with was the ABO blood group system, which was discovered by Austrian scientist Karl Landsteiner in 1901. It wasn’t until the late 1970s that serologists began to look at biochemical markers. Forensic scientists were examining enzymes found on the red cell membrane. These PGMs (Phosphoglucomutase) or genetic markers were protein enzymes which were found throughout the entire body.
PGM-1 was also found in semen, which increased its value in forensic serology because of two alleles. The two alleles, designated “1” and “2,” give the phenotypes PGM-1, PGM-2 and PGM 2-1. This discovery of three phenotypes, which provided additional genetic information about the blood or sperm recovered from a crime scene, was an exciting forensic advancement in the early 1980s.
This progress took on speed with the scientific analysis of Deoxyribonucleic Acid (DNA). It was learned that certain areas of DNA vary quite dramatically from one individual to another, and that these polymorphic regions are so unique to each individual that this technology could be used as a forensic tool.
It wasn’t until 1986 that the first forensic application of DNA typing was performed by Dr. Alec Jeffries in England. He was able to match the DNA of a suspect to the biological materials recovered from the bodies of three lust murder victims. The first use of DNA typing in U.S. courts was the Tommy Lee Andrews case involving a series of rapes in Orange County, Fla., in November 1987. This was followed by the serial murder case in Virginia of Timothy Wilson Spencer. Spencer was the first Appellate Division ruling on DNA and the first execution based on DNA.
The Court Disposition
People v. Castro was the first case that seriously challenged a DNA profile’s admissibility. The New York Supreme Court exhaustively examined numerous issues relating to the admissibility of DNA evidence. A number of pre-trial hearings was required to determine whether the testing laboratory’s methodology was substantially in accord with scientific standards and produced reliable results for jury consideration.
The court ruled that the DNA tests could be used to show that blood on Castro’s watch was not his own. But the DNA tests could not be used to show that the blood was that of his victims. The defendant, however, was found guilty.
DNA Testing Today
Since then, millions of forensic DNA tests have been conducted in the United States and around the world. In a major advancement, the analysis of DNA has evolved from a laborious process, taking weeks or even months, to a procedure that can be completed in a matter of days.
The DNA molecule can establish the link between evidential DNA and the possible suspect’s DNA. It can identify whether the DNA in question is human or non-human, and it can be used to establish the sex of the specimen.
The RFLP technology of the 1980s, which was the Ford Model T of DNA analysis, involved the process of identifying the polymorphic regions that are unique to each individual. These Variable Number of Tandem Repeats (VNTR) contained fairly large repeat units with allele sizes thousands of base pairs long.
The PCR Breakthrough
In 1993, Dr. Kary Mullis received a Nobel Prize for his work during the 1980s that resulted in the invention of the Polymerase Chain Reaction (PCR). This mimics the cell’s ability to replicate DNA, enabling scientists to take small samples of DNA and essentially copy them a millionfold.
All the steps in PCR are similar to basic steps in RFLP: extraction, amplification and detection. Additional research identified much smaller VNTRs, which were only a few base pairs long. This, coupled with PCR, was the advent of STR technology.
PCR and DNA
PCR amplification allows the production of many copies of the region of DNA interest. PCR works like a molecular Xerox machine. Millions of copies of a particular sequence of DNA can be made in about three hours in a thermal cycler. This is great for Forensic DNA where there is usually very little DNA to start with.
Initially, DNA samples that were small or degraded were beyond the reach of DNA-typing techniques. Now, saliva found on the back of a licked postage stamp or an envelope can provide enough genetic material to conduct a sophisticated DNA test. In a “cold case” DNA ruse, police sent the suspect a letter from a mock law firm with an invitation to join in a bogus class-action suit.
The suspect replied to the letter, providing a DNA sample by licking and mailing the enclosed envelope. The DNA found in the saliva on the envelope matched a sample taken from the victim’s body. This couldn’t have been done without the benefit of STR/PCR technology.
The Mitochondrial DNA (mtDNA) genome has been completely sequenced and is 16,569 base pairs in length. Mitochondria contain their own DNA. Every cell in the human body contains hundreds of mitochondria, which are the power plants of cells. There are many more copies of mtDNA than nuclear DNA present in a cell.
The advantage of mtDNA typing over nuclear DNA is the added sensitivity in cases where nuclear mtDNA allows for the examination of bone fragments, hair without root, teeth and other biological evidence that may be limited.
The amount of mtDNA isolated from such specimens may be very small, so DNA extraction is followed by PCR amplification. This allows the production of many copies of the region of interest in the mtDNA. After amplification is complete, the mtDNA is sequenced using conventional sequencing methods.
Short Tandem Repeats
The STR class of polymorphisms has become the backbone of modern forensic testing. Short Tandem Repeats (STR) loci are polymorphic genetic markers that are well distributed throughout the human genome. The advantage of STR technology is that the small size of STR loci improves the chance of obtaining a result. The interpretation of STR types is simplified through the use of computers, which analyze the sample. DNA profiling uses high throughput instrumentation equipped with detectors.
Fluorescent detectors identify 13 different loci that can be analyzed simultaneously.
These 13 loci form the basis of the national DNA network CODIS (Combined DNA Index System), which is administrated by the FBI and is installed in forensic laboratories nationwide.
DNA technology is constantly evolving though new applications and innovations. Forensic scientists are combining advances in miniaturization and microchip technologies with well established techniques of forensic DNA analysis. The fusion of these technologies could revolutionize DNA typing. New methods of DNA technologies have provided for the analysis of previously unsuitable case work samples.
The Amelogenin Gene
The amelogenin gene (AAAGTG) is used to identify the sex of an origin of a sample.
The standard CODIS set includes amelogenin markers for X and Y chromosomes. If both X and Y are in a sample, it is a male. If there is no Y marker, it’s a female.
Y-STR DNA allows for the typing of a portion of the Y chromosome. It detects male DNA only. Small amounts of male DNA can be typed successfully. Mixtures of male DNA (multiple rapists or a rapist with a consensual partner) can be resolved, and mixtures of male and female DNA may be resolved.
To make new cells, an existing cell divides itself in two. But first it copies its DNA so the new cells will each have a complete set of genetic instructions. Sometimes cells make mistakes during the copying process, kind of like typos or mutations. These typos lead to variations in the DNA sequence at particular locations called Single Nucleotide Polymorphisms (SNPs).
SNPs are being used to perform DNA profiling of the Y chromosome. In addition to the 13 CODIS loci, a number of laboratories have developed multiplexes of SNPs so that male DNA can be individually typed.
The Touch DNA method was named for the fact that it analyzes skin cells left behind when assailants touch victims, weapons or anything else at a crime scene. Humans shed tens of thousands of skin cells each day. These cells are transferred to every surface our skin contacts, i.e., gun grips, eating utensils, steering wheels, etc. If a perpetrator deposits a sufficient number of skin cells on an item at the scene, there may be Touch DNA.
Touch DNA is not Low Copy Number (LCN) DNA. LCN DNA profiling allows a very small amount of DNA to be analyzed—as little as five to 20 cells. The small amount of starting DNA in LCN samples requires many more cycles of amplification.
DNA technology has become so advanced through the extreme sensitivity of techniques like PCR that DNA from epithelial cells present in saliva can be swabbed from the surfaces of the oral cavity of suspects. This has become the method of choice in screening a number of suspects in an investigation because the samples can be easily and quickly analyzed. During the BTK investigation, Kansas detectives collected more than 4,000 buccal cell samples to compare with their suspect evidence DNA.
Ancestry informative markers are being used by certain DNA firms to help people trace their genographic roots. This technology also has the potential to identify the four main continental population groups, such as sub-Saharan African, East Asian, Indo-European and Native American. It could be utilized to allow investigators to concentrate on the specific physical characteristics of the donor of DNA evidence.
CODIS is the core of the national DNA database. Established by the FBI, it was specifically developed to enable public forensic DNA laboratories to create searchable DNA databases of authorized DNA profiles. The CODIS Unit manages the Combined DNA Index System (CODIS) and the National DNA Index System (NDIS).
This unit is responsible for developing, providing and supporting the CODIS program for federal, state and local crime laboratories in the United States, as well as selected international law enforcement crime laboratories, to foster the exchange and comparison of forensic DNA evidence from violent crime investigations.
CODIS software enables state, local and national law enforcement crime laboratories to compare DNA profiles electronically, thereby linking serial crimes and identifying suspects by matching DNA profiles from crime scenes with profiles from convicted offenders.
CODIS uses two indexes to generate investigative leads for crimes in which biological evidence is recovered from a crime scene. The convicted offender index contains DNA profiles of individuals convicted of certain crimes, ranging from certain misdemeanors to sexual assault and murder.
Each state has different “qualifying offenses” for which convicted persons must submit a biological sample for inclusion in the DNA database. The forensic index contains DNA profiles obtained from crime scene evidence, such as semen, saliva or blood. CODIS uses computer software to automatically search across these indexes for a potential match.
The success of CODIS is demonstrated by the thousands of matches that have linked serial cases to each other and cases that have been solved by matching crime scene evidence to known convicted offenders.
Vernon Geberth, M.S., M.P.S., is a homicide and forensic consultant and the former commander of NYPD’s Bronx Homicide. He can be reached at www.practicalhomicide.com.
Published in Law and Order, Jun 2010
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