DNA profiling is the determination of a DNA profile for legal and investigative purposes. DNA analysis methods have changed countless times over the years as technology changes and allows for more information to be determined with less starting material. Modern DNA analysis is based on the statistical calculation of the rarity of the produced profile within a population.
While most well known as a tool in forensic investigations, DNA profiling can also be used for non-forensic purposes such as paternity testing and human genealogy research.
History
The methods for producing a DNA profile were developed by Alec Jeffreys and his team in 1985. Jefferys discovered that an unknown sample of DNA such as blood, hair, saliva, or semen could be analyzed and a unique DNA pattern/profile could be developed. A year after his discovery, Jefferys was asked to use his new found DNA analysis to convict a man that police believed was responsible for 2 rape murders. Jefferys proved that the man was innocent using DNA from the crime scene.
When DNA analysis was first discovered, a process called Restriction Fragment Length Polymorphism (RFLP) was used to analyze DNA. However, RFLP was an inefficient process due to the fact that it used up large amounts of DNA which couldn't always be obtained from a crime scene. Modern day technology has evolved beyond RFLP. Short Tandem Repeat (STR) analysis is the modern day equivalent of RFLP. Not only does STR analysis use less of a sample to analyze DNA, but it also is a part of a larger process called Polymerase Chain Reaction (PCR). PCR is a process that can be used to quickly reproduce up to a billion copies of a singular segment of DNA.
Methods
Retired methods
RFLP analysis
The first true method of DNA profiling was restriction fragment length polymorphism analysis. The first use of RFLP analysis in forensic casework was in 1985 in the United Kingdom. This type of analysis used variable number tandem repeats (VNTRs) to distinguish between individuals. VNTRs are common throughout the genome and consist of the same DNA sequence repeated again and again. Different individuals can have a different number of repeats at a specific location in the genome. For example, person A could have four while person B could have 5 repeats. The differences were visualized through a process called gel electrophoresis. Smaller fragments would travel farther through the gel than larger fragments separating them out. These differences were used to distinguish between individuals and when multiple VNTR sites were run together, RFLP analysis has a high degree of individualizing power.
The process of RFLP analysis was extremely time consuming and due to the length of the repeats used, between 9 and 100 base pairs, amplification methods such as the polymerase chain reaction could not be used. This limited RFLP to samples that already had a larger quantity of DNA available to start with and did not perform well with degraded samples. RFLP analysis was the primary type of analysis performed in most forensic laboratories before finally being retired and replaced by newer methods. It was fully abandoned by the FBI in 2000 and replaced with STR analysis.
DQ alpha testing
Developed in 1991, DQ alpha testing was the first forensic DNA technique that utilized the polymerase chain reaction. This technique allowed for the use of far fewer cells than RFLP analysis making it more useful for crime scenes that did not have the large amounts of DNA material that was previously required. The DQ alpha 1 locus (or location) was also polymorphic and had multiple different alleles that could be used to limit the pool of individuals that could have produced that result and increasing the probability of exclusion.
The DQ alpha locus was combined with other loci in a commercially available kit called Polymarker in 1993. Polymarker was a precursor to modern multiplexing kits and allowed multiple different loci to be examined with one product. While more sensitive than RFLP analysis, Polymarker did not contain the same discriminatory power as the older RFLP testing. By 1995, scientists attempted to return to a VNTR based analysis combined with PCR technology called amplified fragment length polymorphisms (AmpFLP).