The criticisms of DNA evidence

Today, DNA profile has been accepted by many as absolute identification evidence. Townley and Ede (2004) recognised its application to criminal law enforcement as the biggest development of human identification since the discovery of fingerprint in the start of the 20th century. Ebisike (2000) baptised it as the most powerful and most convincing evidential tool in criminal trials.

However the opinions about DNA profile are in dichotomy. DNA profile has been heavily scrutinized by Hall1 who resembles it to evidential “black box” and “black hole,” and Farrington2 who argues that DNA stands for “Do Not Accept.” It is the myth that DNA evidence is infallible which has led to such criticisms. It has also been reported that most cases in which DNA evidence is presented end in conviction.

The collection and preservation of evidence are quite sensitive procedures which entail great dangers. DNA evidence may be contaminated if appropriate care is not taken. Therefore, the prosecution must be able to specify the chain of custody of a DNA sample which produced a DNA profile. Furthermore, the sample cannot produce an accurate profile if it has been subject to degradation. The significance of a match after the profiling process is also an area where lawyers have challenged DNA evidence. There may be unexplained discrepancies between two profiles, or inaccurate databases chosen.

The presentation of DNA evidence in the courtroom has been a difficult task, both for the prosecution and the judge to present and for the Jury to understand. It has been argued that the most contentious debate in forensic science involves the use of statistics to estimate the rarity of a given DNA profile4. Furthermore, its introductory term as DNA “Fingerprint” is misleading for the reason that it has less discriminatory power than the dermal fingerprint5. Moreover, while presenting DNA evidence in court, the expert might adopt false logic in his interpretation, known as “the prosecutor’s fallacy.”6 This has been the reason for barrister Mahendra to argue that DNA evidence baffles judge and Jury7.

The increasing police power to retain DNA samples has become today ever greater since the establishment of the National DNA Database in 1995. In some occasions the admissibility of DNA evidence was challenged either because it was retained unlawfully or because it was in conflict with civil liberties.

This work will review the main criticisms of DNA evidence and by considering the current development of science and law it will try to quantify its power as an criminal identification tool. For this purpose, the work will be divided in three parts. The first will analyse the dangers that entail the collection and preservation of DNA evidence, the second will argue in respect with the police powers to retain DNA samples and the third part will critically analyse the presentation of DNA evidence in court.

Part 1 “From the crime scene to the forensic lab” 1.1 Collection and preservation of DNA evidence The first step in the long procedure, which will lead to DNA evidence, is the collection of a sample from the crime scene. The sample must be taken from items that the offender had either touched or been in contact with, which contain cells with his unique genetic information. Under current testing methods it is possible to obtain DNA profiles from cigarette ends, clothing, stamps, or from soft drink cans, which contain only a few cells8.

Appropriate care is essential to be taken by the first officer attending in protecting and securing the crime scene, in order to ensure the integrity of the sample, until its arrival to the analysing laboratory. In addition to that, there is always a danger that the sample may be contaminated and henceforth rendered inadmissible as evidence. This means that foreign biological material could be transmitted to the sample after the commission of the crime9.

Contamination may occur from human interference, such us actions of the victim, the witnesses and police officers that were in the crime scene. It can also occur from natural factors like the weather, insect activity and fire10. Contamination may affect the sample by interfering in the analytical procedures and produce an inconclusive result11. On the one hand, it seems very possible for physical evidence to be contaminated from human interference, taking into account the number of agents involved in attending the crime scene. A representative scenario can be that of a police officer transporting a suspect to the police station. Contamination may occur from the car or the police officer to the suspect.

Furthermore, police officers may come to the extreme case of deliberately planting DNA samples at the crime scene. This may happen if their superior is pressing them to close a case. A more extreme situation is where the police officers plant evidence after they have been bribed. Although, this hypothesis is of minimal probability, the word “bribery” still exists in the pages of the oxford dictionary. And even though such an action from a police officer appears to be of minimal probability, nevertheless it cannot be excluded if, for example, a police officer is envying a suspect

Such an example could be found from the following case from the United States. Although the recently elected governor of California Arnold Schwarzenegger refused to grant clemency to Kevin Cooper, who was given the death penalty for the brutal murders of four people, the federal appeals court ruled more evidence must be examined, before Cooper will be given the lethal injection. The defence maintained the position that the traces of blood found on the victims T-shirt, were planted by corrupt detectives. Thus, Cooper’s lawyers have asked from the court to order for the blood to be tested for the chemical EDTA which is commonly used by crime investigators to preserve evidence12.

In order to minimise the danger of contamination from human interference, the Prosecution must be able to establish the chain of custody, which is the changes in possession of the evidence from the time it was gathered until it was presented to the court13. The prosecution must be able to demonstrate the integrity of, not the DNA only, but of all forensic items and casework material, and must also be able to ensure that the chain of custody remains sound14. The prosecution must show a prima facie case that the primary DNA evidence material collected from the crime scene is original and, having been submitted to forensic investigation, has not been tampered in any step of the procedure.

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