Forensic Dna Profiling, Facts on Dna Mistakes


According to Patrick (17), DNA or Deoxyribonucleic Acid can be described as the ingredient in a person’s complete genetic makeup which can be seen as ingredient that every individual cell found human body has. Throughout a person’s lifespan this component (DNA) does not change. Every individual has his/her own DNA which is similar even if it can be found in the various parts of the body which may include; skin tissue, sweat, bone, the root and shaft of hair, ear wax among others (Patrick,18).

This unique character of an individual’s DNA has led to its utilization as an investigative tool in the criminal justice system (David, 25). This is primarily based on the premise which is proved through identical twins which shows that no two people share the same DNA.The reason for this is that every person has his/her own DNA which is totally different from that of another person since each individual has a different arrangement of DNA in the cells of the body. In the criminal arena DNA evidence obtained from the scene of crime can be employed to link a suspect to a crime or eliminate one from suspicion, in a similar manner as is the case of finger prints (Epplen and Lubjuhn, 18).

In circumstances where a victim’s body is missing, matching of his/her DNA left behind with those of his/her relatives can be done to identify the victim (Patrick 58). DNA is also used to link perpetrator/s of various crimes through comparing the DNA profiles developed in different scenes of crimes. DNA profiling makes it possible to connect different crimes to a single perpetrator through comparing DNA evidence left behind at different scenes of crime by a particular individual (Andrei, 42-49).

In Cases of extreme sexual assault as well as those of brutal crimes the suspects can be proved guilty through DNA analysis. This is normally done through the genetic evidence which the injured individual leaves behind at the scene of crime. The genetic evidence therefore can be proved through saliva, blood, skin tissue, hair or even semen recovered from crime scenes (David, 57). In sexual assault crime more often than not their perpetrators leave behind biological evidence in the form of hair, skin tissue, semen, blood or saliva, either on the scene itself or on the body of the victim. With due care being maintained while collecting such evidence (from victim, scene or suspect) notes that DNA comparison can help place the suspect at a scene of crime (Andrei, 61).

In the United States the body which has been established to deal with such cases is the Federal Bureau of Investigations (FBI) which normally uses a combination of DNA index system (CODIS) where the DNA report recovered from scenes of crimes are entered and kept for reference(Simon Easteal et al, 49). This is later matched with other DNA profiles entered into local, state and national data bases with a view of pin-pointing a suspect or linking serial crimes. However it is imperative to note that effective use of DNA evidence demands a thorough collection as well as analysis of eliminating samples to effectively prove the biological evidence of the suspect (David, 95).

It is advisable that a person investigating sexual assault or rape case can obtain elimination sample (blood or saliva) from the relatives of the victim or consensual sex partner inorder to account for every DNA recovered from the victim or at the crime scene.

Role of Custodian of DNA evidence

As noted earlier, mistakes in DNA evidence can arise as early as when the evidence is being identified, collected, transported and/or being stored, hence it is crucial that every victim service provider, whether crime scene technician, nurse examiner, or medical personnel, to be cautious while dealing with it DNA evidence (Andrei,117-119). To avoid flawed results there should be proper identifications of DNA evidence found in the body of the victim or scene of crime, proper collection of the identified biological evidence as well as proper transportation and storage of it to avoid any contamination or degradation (David, 88-170). The whole DNA investigation process requires to be done with proper DNA body fluids or bones which can be kept for several years as well as they can be used after many years to effectively determine the identity in older unresolved or disputed cases (Patrick, 98).

The conduct of a sexual assault or rape victim after the incident is crucial in as far as availability of DNA is concerned. Since the biological evidence of the sexual assault victims can be obtained from their bodies they should be informed not to change clothes, shower or wash any part of their body after the assault. Through this the physicians are able to discover the evidence, through the use of semen which normally remains behind on individuals clothing, bedding or on the victim’s vaginal, rectal or oral areas (David, 95).

Bathing can wash away the offender’s saliva in circumstances where the victim was licked or bitten. Moreover the assailant’s DNA of skin tissue can be collected from beneath the victim’s finger nails if s/he happened to scratch the offender (Ibid, 97). This implies that the collected evidence found inside or on a victim’s body relies with the physician or assault nurse examiner who thoroughly examines the victim as opposed to evidence that can be obtained from the technicians (Andrei, 117-119). In so doing the victim should be treated if injured, tested for STDs in addition to collecting of forensic evidence (Simon et al, 96). David (101), states that Fluids from parts suspected to have come into contact with the assailant such as the vaginal cavity should be drawn using sterile implements. Garments worn at the time of the assault are important items for evidentiary use. It is always advisable that examiners (medical) do take control standard from a victim in the form of blood or saliva (Andrei, 101).

DNA evidence mistakes can be due to the DNA evidence having been contaminated during identification, collection or preservation. Numerous ways exists through which DNA evidence can be contaminated. One method of contaminating DNA evidence is through mixing the DNA of a particular case with foreign DNA. Normally the foreign DNA self contamination can through the mixing of the DNA of the victim and the suspect through the investigator coughing, sneezing, talking while touching his mouth, nose, hair or any other part of his/her body while collecting or conducting particular DNA relevant to the case. It can also be through cross contamination especially through inadvertent mixing of unrelated mixing of DNA’s while analyzing. (Andrei, 108-119).

In addition the contamination of DNA can also be caused by environmental factors such as heat and/or humidity apart from human factors. Under such conditions DNA breaks down chemically or physically. For instance DNA evidence stored in plastics, offers conducive environment for bacteria growth consequently destroying the evidence. Therefore it is imperative that all DNA evidence be thoroughly air-dried, packed in paper and be properly labeled to ensure that chain of custody is well maintained (Andrei, 96).

It is important to note that DNA evidence can be available for use even after a long period of time, if it is properly identified, collected and preserved without being at risk of degradation.

The way DNA testing is done?

Analyses process of DNA evidence in criminal cases is cumbersome and sometime it takes between several weeks and months depending on the amount of pieces of evidence submitted. During this process of analyzing the evidence it is crucial that laboratory personnel work in conjunction with the police department and the prosecutor in resolving the most probative evidence to a particular case (David, 38-39). Once a decision is reached as to the evidence to be tested, laboratory analysis commences.

DNA analysis is a process which involves the assessment of DNA tresses in a number of locations not less than 13 locations (loci) (Ibid, 49). After DNA analysis has been done, the investigator does a thorough comparison of the two profiles thus the DNA profile of a suspect and that from the scene of crime, such as semen wash down. Comparison can exclude or include a suspect. If the suspected is included as per the results of the analysis what should follow is generation of statistics to find how often one is able to find that particular DNA profile in the general population (Ibid, 50).

Omitting this part is a dangerous move since it assists in determining whether the evidence obtained from the scene of crime came from the suspect. Andrei (118), states that “More over DNA evidence found at the scene of crime can be compared with DNA profiles of convicted offenders found in the CODIS database to establish its source”.

DNA analysis is done through various methods. However this technology presently utilizes 3 methods. Whereas all these methods are reliable, however, inadequate quantity of evidence to be analyzed or contaminated evidence can lead to failed or inconclusive results (David, 185). One of the commonly used DNA analysis method is the Polymerace Chain Reaction (PCR). The rationale underlying this method is its ability to have the biological evidence from crime scenes being broken down into million parts via a technique referred to as PCR amplification (Simon et al, 18- 9).The PCR amplification creates sufficient DNA capable of being generated into a DNA profile. With this process, corrupted genetic material can be analyzed (Ibid, 87).

The ability of PCR testing to magnify very small quantities of DNA implies that proper care should be exercised when it comes to identification, collection and preservation of biological evidence to avoid contamination (Ibid, 89). This implies that it is necessary and ethical for investigators and laboratory personnel, to take safety precautions when it comes to handling the DNA and this can be done by the investigators putting on large disposable gloves on their hands, using sterilized instruments and avoiding touching other objects when handling the evidence (David, 55).

Restriction Fragment Length Polymorphism (RFLP) is also another important method used in analysis of the DNA evidence. Simon et al (18- 97) acknowledges that RFLP is suitable where the sample contains 100,000 or more cells with un-degraded or broken down DNA (Simon et al 18- 97). Wrongly accused persons can be excluded by use of RFLP (Ibid 18-97). The third and last method of DNA analysis is the PCR analysis on DNA from the mitochondria of the cell which is used in circumstances where the DNA evidence cannot be obtained by RFLP or PCR nuclear DNA testing. These samples include for instance dried bones, teeth, hair shafts or samples that contain very little or highly degraded nuclear DNA (Simon et al 18- 97). Since this method is time consuming it is only available only in limited number of laboratories.

Facts of mistake in DNA evidence can also occur in interpreting the results of DNA analysis. However, as David (76) notes no matter the method employed in DNA analysis the results are either inclusive or exclusive or inconclusive. (Simon et al 18- 97).

In circumstances where the results of DNA profile of a known individual (victim or suspect) match the DNA profile from the crime scene evidence (David 174) this leads to inclusion which makes it more difficult for the investigator to easily differentiate the DNA samples since the particular individual evidence is also “included” as a potential source of the evidence (Ibid 175). David (176), however states “for the inclusion to be bring out more evidence then the number of DNA locations examined should be upto13 and which should be proved by realistic evidence that shows how commonly is that particular profile found in the general population” because the power of DNA evidence depends on the number of DNA locations tested, this therefore means that all the relevant locations are therefore tested (Ibid 187).

It is important to note that not all inclusive results are valuable to the investigative process. For instance in sexual assault cases a victim’s DNA profile may match with the one obtained in the scene of crime. The result of this could be possible if the suspect used a condom during the assault or no ejaculation process took place during the assault. (David, 78).

This should not be taken to mean that the suspect did not commit the crime only that the suspect was not the source of the substance that was tested. Moreover a suspect should not be construed to be guilty by mere inclusion.

On the other hand exclusion occurs if DNA profile from an individual (victim or suspect) fails to match with DNA profile generated from the crime scene evidence (Epplen and Lubjuhn 18). The consequence is that the marked individual is excluded as being the donor of that particular evidence. Some cases require performance of extra testing so as to establish the source of DNA profile evidence. For instance DNA test of a husband match with the evidence sample can indicate prior consensual act as opposed to sexual assault (incase of semen swabs from vagina of the victim) (David, 67). It is decisive to note that elimination of the suspect does not imply that the suspect is innocent.

Inconclusive results are deemed to occur in circumstances in where DNA testing fails to generate information that can permit an individual to be either included or excluded (Andrei 109-119). This can be caused by poor quality and quantity results of DNA that are obtained from biological evidence (Ibid). It can also be as a result of poor DNA sample which could have been contaminated through the mixture of other DNA samples from several individuals as in semen from a gang rape victim (Andrei, 79). Inclusive or exclusive results cannot be established even if a suspect DNA profile is found in such biological evidence due to availability of DNA from other sources.

A sexual assault involving more than one particular perpetrator, same as prior consensual intercourse, offers profiles for more than one person. In such circumstances it is always difficult to determine which specific types came from which donor. The main purpose of this is to block the suspect from being conclusively excluded as being the potential donor of that particular evidentiary sample DNA, this may greatly affect the conclusive result. This implies that inconclusive results should be interpreted basing on the available evidence of that particular case (David, 67).

As priory indicated contamination and other errors in DNA analysis do occur as was the case at Washington State Patrol Crime Laboratories. In this lab, most of the errors were found to be as a result of sloppy work. A Majority of these errors were caused by cross contamination by microscopic traces of unrelated evidence. In some cases the forensic scientists were found to have accidentally mixed their own DNA with the sample under test. This happened for instance, while the scientist analyst talked while handling a sample, consequently leaving an invisible trace of his/her saliva.

In one incident involving cross contamination a forensic scientist, Lisa Turpen reported to have contaminated one of four vaginal swabs with semen from a positive control sample. In the same laboratory Solomon and Turpen contaminated hair roots with their own DNA. In a similar case it was reported that Solomon had once messed with the reference blood sample with his DNA by contaminating it (Ibid). Many errors were documented running from cross contamination, to self contamination through to testing errors. Also as noted DNA test interpretation is critical. In a similar development George Chan misstated statistical probability of match with suspect. This resulted to the suspect being convicted.

Errors in DNA evidence both in United States and also United Kingdom Laboratories. These errors mostly in the data base have raised concerns from various quarters globally. Mistakes in spelling and duplication have been identified in UK. In a May 2007 report, DNA database unit acknowledged that it did not load 26,200 records to DNA data due to errors. The same year in August the statistical report released by Home office showed 550,000 files with wrongly recorded or mispelt names. Accuracy of DNA data is very crucial so as to include or exclude a suspect. Mistakes in DNA analysed have in the past placed innocent people behind bars while at the same time setting guilty suspects Scot free.

An example to this is the case of Josiah Sutton who was sentenced to 25 years in prison, depicts how errors in DNA analysis can defeat justice. Sutton’s conviction was as a result of rape charge on a woman taken at gun point and then dumped in a Fort Bent County Field. Five days after the rape, Sutton and his friend were arrested after being spotted by the victim near her compound. Saliva and blood samples were obtained from Sutton and his friend to be analysed so as to determine if there was a match between their DNA profiles and those collected from crime scene and the victim.

Shockingly after conviction of Sutton on the evidence based on DNA profiling, Houston Police Department later issued a statement to the effect that upon retesting the evidence, Sutton was excluded as being the potential donor of the evidential matter analyzed.


This therefore implies that the main purpose of criminal justice system is to investigate for the truth. Even though DNA profiling advances this search, through exonerating the innocent and convicting perpetrators, errors and mistakes in DNA profiling, jeopardizes the much sought truth and consequential miscarriage of justice.

Work Cited

DNA Testing Mistakes At The State Patrol Crime Labs (2004). Web.

Judge Releases Inmate after DNA Mistake Uncovered DNA Expert Says Government To Blame (2003). Web.

David Lazer, (2004) DNA And The Criminal Justice System: The Technology Of Justice (Basic Bioethics) MIT Press (MA). Pp 17 – 197.

Andrei Semikhodskii (2007) Dealing With DNA Evidence A Legal Guide Routledge-Cavendish. Pp. 32-119.

Simon Easteal, Neil Mcleod And Ken Reed, (1991) DNA Profiling: Principles, Pitfalls And Potential 1st Edition.. CRC. Pp 18-97.

Patrick J. Lincoln, (1998) Methods In Molecular Biology #98: Forensic DNA Profiling Protocols Humana Press ( 42-173).

J. T. Epplen And T. Lubjuhn (Eds) (2000) DNA Profiling And DNA Fingerprinting. Birkhäuser. P 18.

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