what every prosecutor and aefense attorney should know about dna

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The analysis and understanding of DNA is very complex. It takes years of education and experience to become an expert in analyzing DNA. The development of forensic DNA typing is a relatively new phenomenon and has greatly expanded technology in the field of forensic biology. Understanding the biology of DNA and the processes for analysis is critical. However without the necessary knowledge of prosecutors and defense attorneys to use such evidence in court, DNA is of little value in criminal cases. It is important that these actors in the court systems have a base knowledge of DNA and the standard procedures in the field so that they are able to display the precision of DNA analysis in their arguments and more critically communicate that knowledge to the jurors because they ultimately determine the verdict of the criminal cases. Since DNA typing is a very complex and difficult process to understand, it is evident that prosecutors and defense attorneys will not know as much as an expert DNA analyst. The most important things for prosecutors and DA's to understand and consider when introducing DNA evidence into a case is the evolution of DNA typing, the standard and accepted procedures within the community, the artifacts in genotyping STRs and lastly, newer developments in DNA analysis.


Every individual has their own unique DNA, except in the case of identical twins. According to Butler (2005), surprisingly enough 99.7 percent of human DNA is the same and only .3 percent of our DNA is unique. DNA has a random match probability, the possibility of a profile being identical of another individual from the population, of approximately 1 in 3 trillion (Butler 2005). A DNA profile is unique to each human as well and this profile is what is entered into the FBI's CODIS database, which will be discussed later. An English geneticist Alec Jeffreys first described DNA fingerprinting in 1985 but James Watson and Francis Crick were credited for the discovery of the double-helix model of DNA in 1953 (Butler 2005). The first use of DNA testing in a forensic case was in 1986, when they still used Jeffrey's multi-locus RFLP probes. Mitochondrial DNA was introduced in 1996 and two years later, the FBI launched their nation-wide DNA database in 1998.

Technologies used for DNA analysis has changed drastically since their introduction to the field in the mid-80s. DNA testing once took 6-8 weeks to analyze but today can be performed in a few hours due to the technology advances (Butler 2005). With computer technologies, DNA profiles can now be examined side by side to increase the likelihood of identifying matches. Through research, we have been able to distinguish male profiles from female profiles because males contain an XY gender ID while females contain an XX pattern. Samples with limited amount or highly degraded DNA can be analyzed with the advances of mtDNA testing. Lastly and probably one of the most important advancements of DNA is that the power of discrimination has increased.

This evolution of DNA typing is important for prosecutors and defense attorneys to be aware of because DNA has made such rapid progress in the field since it was first introduced in the courts in 1996. It is likely the field will be seeing much more progress in the years to come. It is also critical that these actors relay this developing phenomenon to the jurors so they are aware of the impact, credibility, and accuracy DNA plays as evidence in criminal cases.


As any community goes, there are standard procedures that individuals within that community follow for the most part. This is no different for the forensic community in which it is vital that DNA standards are understood and followed.

A major change within the forensic DNA community with the success of DNA use in criminal cases since its evolution in the mid-80s was the development of the FBI's CODIS database in 1998. Within the database, 13 STR loci were chosen to serve as the standard and are widely accepted within the community. Various technologies and commercial kits have developed in recent years to identify these specific loci since time and energy is essential from an investigative standpoint. CODIS has been a very successful program and in 2003, the U.S. entered their one millionth DNA profile of convicted offenders in their database and currently hold over 7.2 million DNA profiles (Gabriel, Boland, Holt 2010).

Although the United States is a few years behind the United Kingdom and Europe in utilizing a national DNA database, collaboration among the countries have increased in their abilities to solve international crimes but this has not gone without its drawbacks. Due to the U.K. and Europe having utilized their DNA databases longer, they have substantially more DNA profiles collected in their database in comparison with the amount of people they have in their countries. The United Kingdom currently has over 2.5 million convicted felon DNA profiles in their database (Butler 2005). The main issue with the collaboration between countries is the law differences. There are different laws in each country in regards for obtaining a DNA profile and entering it in their database and which STR loci are included (Butler 2005). For example, the European database utilizes 10 main loci, in which eight loci overlap and provide two additional loci that the United States model of 13 STR loci (Gabriel, Boland, Holt 2010).

Another part of the DNA typing community that is crucial in understanding is the commercial kits commonly used in various steps of the DNA process. These processes vary from detecting the presence of DNA back at the crime scene to determining the 13 STR loci used in the CODIS database. Commercial kits have been evolving for decades now and have helped save time, money, and resources than ever before. Dotan et al (2010) explains that commercial kits are also able to calculate a higher discriminatory power and have the ability to detect null alleles, which can potentially result in false negative results or incorrect exclusions of two samples. It is also imperative to be aware of the studies conducted on commercial kits because these provide evidence of their effectiveness (or lack there of) and guide analysts in choosing the most appropriate techniques, which often become accepted among the whole community.

One last area worth mentioning is the importance of accurate sampling collection while in the field. Without research, it would not be understood what are commonly considered 'accurate' techniques for collecting DNA samples from crime scenes. In a personal communication with Larry Barksdale (2010), he describes some of these good practices as including: wearing gloves at all times around the evidence, rubbing the stained area with a moist swab to transfer the DNA evidence from unmovable surfaces, and air-drying all samples before packaging in paper bags.

As was discussed in this section, the standard and accepted procedures from the DNA typing community are important for prosecutors and defense attorneys to consider while in court. The laws regarding the CODIS database may be important to understand in some cases but the uses of commercial kits and accurate collecting methods are essential in each situation. It is not only the prosecutor and defense attorney's responsibilities to prove guilt or innocence but to also point out potential flaws in investigative procedures. For instance, if evidence is not carefully collected, preserved, stored and transported correctly, these court actors can use those deficiencies to strengthen their arguments and raise concerns of possible contamination in the DNA samples collected from the scene, ultimately devaluing the DNA evidence in a case.


It is the sole purpose of the defense to prove the defendant is innocent in court, oftentimes picking apart the inconsistencies in the case. One major concern with DNA is the possibility that errors can be made during the process or that the analyst can interpret the STR analysis wrong. Interpretation of forensic DNA relies on an analyst's professional judgment and expertise and can be brought into question by the defense in a trial (Butler 2005).

During PCR amplification, Butler (2005) describes a number of artifacts can appear and interfere with the interpretation and genotyping of the alleles in a DNA profile. These artifacts consist of stutter products, non-template nucleotide addition, microvariants, tri-allelic patterns, null alleles, and mutations. These artifacts are of much importance for the analyst to consider while interpreting results and also the trial actors to know when the expert DNA analyst explains those results.

For instance, it may be difficult to determine whether a small peak is a real allele or a shutter product of another allele because shutter products are the same size as actual allele PCR products (Dotan et al 2010). This misidentification could impact the interpretation of DNA profiles where two or more individuals may have contributed to the DNA sample. Another potential artifact in STR interpretation is null alleles and allele dropouts in which a DNA template exists for a particular allele but fails to amplify during PCR (Dotan et al 2010). The detection of null alleles is very important because they could potentially result in a false negative or incorrect exclusion of two samples that come from a common source (Butler 2005).

As you can tell, there relies great judgment on the hands of the experts in interpreting DNA results and this leaves substantial amount of questionable testimony that the defense can dispute and downplay the credibility of DNA evidence. However there are studies published on commercial kits and the ability for them to detect such artifacts in genotyping. This is an important aspect that prosecutors should be aware of because commercial companies have been working to overcome these problems and have the studies that show the improvement in detecting such artifacts (Butler 2005).


Lastly, it is important for prosecutors and defense attorneys to know the direction in which DNA evidence is headed. For some time, the DNA community has been struggling with nuclear DNA and not being able to produce a useable DNA profile due to the circumstances of limited samples or the DNA being highly degraded.

Until recently, there have been no solutions to these problems. Although mitochondrial DNA was discovered in the 1960s, it was not used for human identification in court until 1996 (Davis 1998). With the introduction of mtDNA, DNA from low quantity samples and degraded DNA can now be useful in identifying an individual. Evidence as small as a hair shaft can yield mtDNA. Since mtDNA is only inherited from the mother, it has a lower power of discrimination and often needs additional evidence to prove guilt against a person because it is not unique to individuals (James and Nordby 2003).

According to Davis (1998), over 600 papers study the forensic utility of mitochondrial DNA and have proven validation in DNA identification of degraded evidence. It is also a useful tool in identifying old remains and victims in mass disasters where bones are often times old or charred. The downfalls of mtDNA are that there are a limited number of forensic laboratories that do the analysis because of their expensive and time-consuming process (Butler 2005). There are currently no available commercial kits for the entire process of mtDNA sequencing which also makes the standardization of mtDNA difficult.

As proven, the introduction of mitochondrial DNA has helped yield useful information in DNA that had not originally been available until a decade after the evolution of DNA. The analysis of mtDNA has been used in past cases where nuclear DNA could not previously produce a profile and has exonerated some people from prisons for crimes they did not commit. Mitochondrial DNA has also been used to identify remains of past wars and was a primary source of identification in the 9/11 terrorist attacks where many remains were badly charred (James and Nordby 2003). The advance in DNA use that these processes have brought along are important for prosecutors and defense attorneys to keep in mind because it shows that there is more than one option of analyzing DNA and it is important that these actors raise such questions when the analysis and investigative techniques seem incomplete.


It is difficult to determine the extent to which the prosecutor or defense attorney should know about DNA because the evidence varies in each case. It is up to the prosecution to show the DNA evidence against the defendant as substantial and that there is credibility and standardization behind the methods of DNA analysis. While on the other hand, it is up to the defense to dispute the evidence being used against the defendant and argue the methods or techniques of analyzing DNA evidence is not credible enough to determine guilt. This can include anything from poor investigative procedures to potential misinterpretation of DNA typing results from the judgment of the analyst.

As I have discussed, the evolving development of DNA is crucial for actors in the court to understand and convey the importance of scientific and technological advances in the role of forensic evidence. This is important for jurors to understand because they ultimately decide the outcome of a case and it is the actor's jobs to transfer this knowledge to the jurors. Standard and accepted procedures within DNA typing is also an important aspect to understand because without the standardization of methods and the studies to back up the effectiveness of DNA methods and findings, using DNA as evidence would have never stood a chance in court. This also allows the prosecution and defense to point out inconsistencies in the case, further diminishing the arguments of the opposing side or pointing the finger at law enforcement failures. Artifacts in genotyping STRs were another important consideration especially on the defense side because it is one area in which the defense attorney can question the certainty of the analyst and potentially reduce the credibility of DNA being used as evidence in criminal cases. The last area of DNA evidence I focused on as most important are the newer developments of DNA analysis. The main importance of new developments is the ability to have options. There was not always a way to extract DNA from a limited sample or degraded DNA but with the introduction of mitochondrial DNA, it is now possible to analyze smaller samples for DNA. This gives a variety of options for the prosecution and defense to consider when walking through a case.

As I have shown, it is imperative that both prosecutors and defense attorneys know about DNA and how this new phenomenon has and will continue to change the way evidence is used in forensic cases. It is also their jobs to act as safeguards against poor law enforcement procedures and to make sure procedures are followed through accurately so that the DNA evidence has not be contaminated or overlooked in any case. DNA will continue to serve as the foundation for our future in forensic science and criminal investigations.


Barksdale, L. 2010. Personal Communication.

Butler, J.M. 2005. Forensic DNA Typing: Biology, Technology, and Genetics of STR

Markers. 2nd ed. Elsevier Academic Press, Burlington, MA.

Davis, C.L. 1998. Mitochondrial DNA: State of Tennessee v. Paul Ware. CASE


Dotan, K., B. Feldman, B. Goldman, Y. Peri and L. Peleg. 2010. The Single Cell as a

Tool for Genetic Testing: Credibility, Precision, Implication. Journal of Assisted

Reproduction & Genetics. 27(6): 335-341.

Gabriel, M., C. Boland and C. Holt. 2010. Beyond the Cold Hit: Measuring the Impact of

the National DNA Data Bank on Public Safety at the City and County Level.

Journal of Law, Medicine & Ethics. 38(2): 396-411.

James, S.H., J.J. Nordby. 2003. Forensic Science: An Introduction to Scientific and

Investigative Techniques. Pp. 279-314. 2nd ed. CRC Press, Boca Raton, FL.