DNA Fingerprinting How Accurate is it

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Deoxyribonucleic acid, or DNA, is the genetic material that is contained within the cells of living organisms and it is considered to be the building blocks for a person's genetic makeup. Each cell in the human body that has a nucleus contains exactly the same DNA as all the other cells and each person's DNA is unique to that person, with the exception of identical twins. This means that the DNA that comes from a person's blood, skin cells, semen or saliva is all exactly the same.

DNA fingerprinting, also known as DNA typing, is the process of taking DNA samples from a crime scene and attempting to match it to samples taken from possible suspects. The DNA that is collected can come from a variety of difference sources; it can come from blood, semen, skin cells, saliva, and a variety of other bodily fluids and tissues. DNA can be a powerful tool in forensic science because of the fact that it is unique to the person that it came from. However, when a sample is not handled carefully or results are not interpreted properly the same powerful tool can set guilty people free or condemn innocent people. The question that must be answered then is should DNA evidence be the backbone or merely the starting point of an investigation?

The fact that DNA is now being used to solve so many crimes there became a need for a way to store the information from the samples collected from crime scenes and criminals. The Combined DNA Index System (CODIS) is the center of the national DNA database that has been established and funded by the Federal Bureau of Investigation (FBI). CODIS allows laboratories from all over the country to share and compare DNA data collected from crime scenes and suspects in order to match criminals to their crimes. CODIS includes the National DNA Index System (NDIS), which contains the data for the entire country, the State DNA Index System (SDIS), which contains the data for each state, and the Local DNA Index System (LDIS), which contains information for a particular city.

Uses of DNA in Court Cases

Many people play a vital role in the collection and processing of DNA samples. It begins with the first responders to the crime scene, followed by the technicians who collect the samples and then concludes with the laboratory technicians that process and interpret the results. The first responders must every effort possible not to contaminate any possible sources of DNA before they can be collected. This requires that the technicians be familiar with where the DNA may exist at the crime scene therefore reducing the chance that this evidence becomes contaminated or damaged.

The DNA that is collected from a crime scene must first be amplified so that a sufficient amount of DNA is available to test and analyze. The DNA must first be removed from the cells that it is contained in and this is done through a chemical process that lyses the cell and releases the DNA. Once the DNA has been released from the cell it can then be amplified which is done through a process called Polymerase Chain Reaction (PCR). During PCR the DNA is heated to a temperature of 90-95°C which allows the strands of DNA to separate from each other and once the strands are separated they can then be duplicated creating two identical strands of DNA. This process is a chain reaction because each time it is repeated the amount of available DNA is doubled. Each cycles take anywhere from 2-5 minutes and when repeated 25-30 times can increase the amount of DNA over 1 million fold. This means that a very small amount of DNA collected from a sample can be tested.

Once the samples have been amplified there are prepared for matching by using a test kit that can detect characteristics called alleles at different locations, called loci, on the genome. These alleles are present in what are called short tandem repeats (STRs). STRs are repeated portions of DNA that appear in close relation to each other. These repeating portions of DNA can allow differentiation of DNA from one person to another. Currently laboratories in the United States use kits that identify 13 loci that have been selected by the FBI for CODIS. Each locus can have a number of different alleles and the pair of alleles at one locus constitutes each person's genotype at the loci. Once each set of alleles is detected at each locus it is now called a DNA profile. This profile can then be used to match samples taken from the crime scene to the suspects or to CODIS in an attempt to find the perpetrator of the crime in question.

Accuracy of DNA Fingerprinting

There are many factors that come into play in the validity of the forensic DNA fingerprinting. These can include coincidental matches between DNA profiles, contamination of the DNA evidence through accidental transfer of cellular material, errors in labeling or classification of samples, misinterpretation of the results, either accidental or intentional, and planting of biological evidence.

The coincidental matches can come from the fact that degradation of the DNA, limited quantities of DNA, or contamination of the samples can cause a result of incomplete or partial DNA profile. A full profile match would match all 13 of the loci where a partial match while a partial match would have fewer alleles. When a profile is incomplete or partial it can produce fewer markers to match it can be more likely produce a match by chance. That is, as the number of available markers goes down the probability of false positive increases. Another factor that can increase the likelihood of false positive is the relation of the people that the samples are taken from. What this means is that two people who are related to each other have a higher probability of producing a partial match and how closely they are related can increase this likelihood. The closer the family relation is the higher the probability of a partial match. The probabilities are as follows, 1 in 14 billion for a first cousin; 1 in 1.4 billion for a nephew, niece, aunt or uncle; 1 in 38 million for a parent or child and 1 in 81 thousand for a sibling. 7

A larger risk of a false positive is possible when the DNA sample used has an incomplete profile. When an incomplete profile is put through CODIS the probability of getting a match is increased which may result in an innocent person being accused of a crime they did not commit. This was the case in Chicago in 2004 when a 6 locus profile was run through the state database after a burglary. This partial profile returned a match to a Chicago woman. The woman was arrested but then had to be released when she provided an alibi, there was no way she could have committed the crime because she was in prison serving time for another crime. 7

Contamination of the samples is the most common factor that can affect the accuracy of the DNA profile. Contamination can come from many sources and can occur at different points along the sample collection and preparation timeline. The first time that contamination can occur is during the collection process. Carelessness during the sample collection can introduce unwanted DNA into the sample therefore making it impossible to distinguish the DNA from the unwanted source from the DNA evidence. If someone sneezes or coughs on the sample, if the person collecting the samples touches their face or other body part and then touches the area that might DNA to be tested it essentially ruins the sample. Other ways that the sample can become tainted is through the use of dirty or contaminated collection tools, using gloves that are not clean, or by putting the samples into plastic rather than paper bags.

The second opportunity for sample contamination can occur in the laboratory where the sample processing takes place. In the laboratory the sample is susceptible to the same types of contamination as when the samples are being collected as evidence and great care must be taken by the laboratory technicians to prevent contamination from occurring in the laboratory. The only exception to this is that in the laboratory the samples can be mislabeled. If the laboratory technicians are careless and mislabel

Recourse for a Wrongful Conviction

There are, of course, some people that can be set free by the use of DNA testing. Before DNA fingerprinting became the widely used forensic technique cases relied heavily on eyewitness testimony and the testimony of the victims themselves. However witnesses can sometimes be so traumatized or the crimes can happen so quickly that the wrong person can be accused and the testimony can be so convincing that innocent people can be convicted of crimes.

There is now a way that people who have been wrongfully convicted can prove their innocence. The Innocence Project is a non-profit organization that takes requests from inmates that believe that there may be a chance that DNA evidence would exist such that they can be exonerated for their crimes. To date the Innocence Project has been able to help get ***** innocent people freed from prison.

While this new freedom is a joyous time for the wrongly accused and convicted it can be a very confusing time for the victims. Even though the victims have wrongly identified the perpetrators of their crimes they spend years believing that the person that was convicted was actually the person that committed the crime against them. It can be a very painful and confusing time for the victims and they sometimes have a hard time believing that they were responsible for an innocent person spending years, sometimes decades in prison. In other cases some victims have a very hard time believing that the person they have spent years believing committed this crime against them is not actually the person that committed the crime.

Ethical Evaluations

As for the Innocence Project to perform a utilitarian analysis you would have to weigh the good of freeing innocent people from a life behind bars to the pain and fear that the victims feel when learning that these people are set free. While no person should have to feel pain and fear, the good of freeing innocent people is far greater.



Many people rely on the accuracy of the DNA evidence that is presented in court cases and expect this evidence to be infallible. The victims of the crimes are relying on this DNA to be able to find the persons responsible for the crimes so that they can be convicted. On the other hand the innocent people accused of crimes are relying on this DNA evidence to prove their innocence and set them free. But in both cases the people involved are relying on the fact that this evidence is accurate and does not contain any errors, errors that can introduced through human error or errors that are innate in the statics of DNA "matches" between the samples taken. The fact that these errors are present and may not be avoidable leads to the conclusion that DNA evidence should not be the only evidence that is used in cases but rather a starting point for an investigation.