Investigating Secondary DNA Transfer

Investigating Secondary DNA Transfer

DNA evidence has largely been viewed as the ‘gold standard’ of forensic science, offering a seemingly solid means of linking individuals to crime scenes and, in more recent years, exonerating those wrongfully convicted. Whereas successful DNA analysis previously required a visible biological contribution, for instance a drop of blood, new advances in DNA technology have allowed for profiles to be produced from just a few dozen cells (you may have heard the term ‘Touch DNA’ be used to describe this). But as DNA technology has advanced, the improved sensitivity of DNA analysis techniques has become something of a double-edged sword, with concerns being raised over DNA analysis being too sensitive.

Imagine a scenario. A man and a woman are having an innocuous conversation. Some physical contact happens, perhaps the touching of hands or the brush of a cheek. The woman later experiences a sexual assault and an investigation ensues, an investigation in which DNA evidence is likely to play a pivotal role. During a sexual assault investigation, it is likely that swabs may be taken of a suspect’s clothing and genitals, specifically aiming to detect any of the victim’s DNA. This may particularly be the case if no semen has been detected, necessitating any other means of establishing whether or not sexual contact may have occurred.

But is it possible for a person’s DNA to be inadvertently transferred to the clothing or body of another person through innocent contact, only to later wrongfully incriminate that person? Research recently published in Science & Justice aimed to provide some insight into this question.

The aim of the study was to determine the frequency and amount of DNA transferred from a female to a male’s underwear and genitals following a non-intimate social contact situation. Using a staged scenario in which a male and female are interacting, the male participant was asked to touch the female’s face for 2 minutes and then hold her hands for 3 minutes whilst maintaining a conversation. This exchange provided the opportunity for the direct transfer of DNA from female to male. Following this exchange, the male participant went to the bathroom to simulate urination, offering the opportunity for secondary transfer of the female’s DNA to the male’s underwear and genitals. Other trials also introduced a 6-hour delay between the social contact and bathroom visit. Swabs were then taken of the man’s underwear and penis. In separate experimental trials, the same swabs were taken from male participants immediately following unprotected sexual intercourse to act as a comparison.

Following SGM Plus DNA profiling (routine at the time of the research), female DNA was found on the waistband of the underwear on only 5 occasions out of 30, on the penis in 4 out of 30 samples, and just once on the front panel of the underwear. In no other instances was female DNA detected. Unsurprisingly, this was even lower in trials implementing a 6-hour delay. In comparison to swabs taken from a male following sexual intercourse, transferred female DNA was detected in all samples and in larger amounts. So although the research demonstrated the possibility of the transfer of the female’s DNA to the male’s underwear and genitals (obviously somewhat incriminating if this occurred during a sexual assault investigation), the frequency and level of occurrence was much lower than if sexual intercourse had actually occurred.

The concept of secondary DNA transfer is not novel, and it has been known for some time that it is possible for DNA to be transferred through everyday contact. In fact the very idea of secondary DNA transfer was first described in literature almost two decades ago (Oorschot & Jones, 1997). The aforementioned research follows previous studies conducted investigating similar scenarios but reaching somewhat different conclusions.

Research published last year by the University of Indianapolis conducted their own DNA transfer study in which participants were asked to shake hands for two minutes before one of the participants handled a knife. The study aimed to determine whether this social interaction and handling of the object was sufficient to allow DNA from one individual to be passed to the knife via secondary transfer, without that person coming into any direct contact with the knife itself. Subsequent analysis of the knives showed that in 85% of cases DNA detected on the knife belonged to the participant who had not handled the object, and in one-fifth of the samples they were even the main or only contributor of DNA found on the weapon. This study essentially implies it is possible for someone to be linked to a crime scene via secondary transfer of their DNA to a murder weapon or victim, for instance.

Conversely, research published back in 1997 also conducted similar research, but this time not supporting the idea that secondary DNA transfer can provide misleading results (Ladd et al, 1997). Participants were instructed to shake hands for varying lengths of time before handling an everyday object, such as a coffee mug. The research concluded that a complete DNA profile of the secondary participant (who had not directly handled the object) was never detected. So although various studies have been carried out, although using different experimental conditions, results are to an extent contradictory.

These studies discussed have obvious limitations. The scenarios staged are far from realistic – the average person does not shake someone’s hand for two minutes before handling an incriminating object, which is then immediately swabbed for DNA by investigators. Nor does the research take into account factors that might affect DNA transfer and persistence.

It is worth noting that these concerns are not confined to the research lab. In 2010, former cab driver David Butler found himself imprisoned, accused of murdering 46-year-old Anne Marie Foy. The evidence against him? His DNA allegedly found under the fingernails of the victim. Butler had previously offered up a DNA sample years before during the investigation of a burglary and, although the DNA profile obtained from the victim’s body was merely a poor quality partial match, this was seemingly sufficient to land Butler in prison on remand for nearly eight months. However the DNA evidence was later called into question when it was suggested that Butler, who had a skin condition causing him to shed more skin cells than the average person, could easily have transferred his own DNA to a person or money which was then transferred to the victim via secondary transfer.

Cases such as this highlight the need for further investigation. Although recent research has provided a good starting point for investigating secondary DNA transfer through non-intimate contact, as DNA analysis techniques improve and achieve greater sensitivity, there will be an increased need to extend research. Further studies examining new DNA profiling techniques, different scenarios and the effects of possible affecting factors will be necessary in ensuring secondary DNA transfer in situations of everyday social contact will not be mistakenly interpreted in a criminal investigation.

 

References

Van Oorschot, R. A. Jones, M. K. DNA Fingerprints from Fingerprints. Nature. 387(1997), 767.

BBC News. DNA test jailed innocent man for murder. [online] Available: http://www.bbc.co.uk/news/science-environment-19412819

Cale, C. M. et al. Could Secondary DNA Transfer Falsely Place Someone at the Scene of a Crime? J Forensic Sci. 61(2016) pp. 196-203.

Jones, S. et al. DNA transfer through nonintimate social contact. Sci Justice. 56(2016), pp. 90-95.

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Forensic Failures: Three Men, Three Hairs, Three Wrongful Convictions

Forensic Failures: Three Men, Three Hairs, Three Wrongful Convictions

In the summer of 1978, 63-year-old taxi driver John McCormick was robbed and shot on his own doorstep in his Washington, D.C home. His wife, roused from sleep by the cries of her husband pleading for his life, ran to his aid to briefly glimpse the gunman, a man with a stocking mask shielding his face. McCormick died as a result of the attack, shot by a .32 calibre handgun. A police officer and his dog later found the stocking mask nearby, and from this a number of hairs were recovered.

santae

Santae Tribble (Source: The Innocence Project)

Suspicion soon fell on 17-year-old Santae Tribble, implicated by an informant, Bobby Jean Phillips, who claimed Tribble had recently sold a .32 calibre handgun to her roommate. A weapon that, incidentally, was never successfully linked to the gun used to kill McCormick. Tribble was pulled in for questioning. Hairs recovered from the stocking used by the killer were compared to samples collected from the suspect and, according to an FBI analyst, “matched in all microscopic characteristics”. At trial, the prosecution went on to declare that there was perhaps a “one chance in ten million” that the hair belonged to someone else. Seemingly compelling “statistics” and apparently sufficient in helping the jury reach a verdict. Despite having testified that he was in Maryland at the time of the attack, an alibi supported by half a dozen witnesses, in January 1980 Santae Tribble was found guilty of murder and sentenced to 20 years to life in prison.

Meanwhile, as Tribble was adjusting to prison life, another man’s world was about to be turned upside down thanks to a single hair. In February 1981, a 27-year-old woman was bound, raped and robbed in her apartment, briefly glimpsing her attacker in the dim light. The distraught victim helped police construct a composite sketch of her attacker; a young, cleanly shaven African American man. Unfortunately for 18-year-old Kirk Odom in the following weeks, a passing police officer decided that Odom resembled the composite sketch, passing on his suspicions to the detective leading the case. As a result of this Odom was presented to the victim as part of a somewhat dubiously organised line-up, with Odom standing on a box in order to match the height of the other men in the line-up, who were actually police officers. The victim identified Odom as her attacker.

kirk

Kirk Odom (Source: The Innocence Project)

During the trial, FBI Agent Myron Scholberg testified that the “Negroid hair” found on the victim’s nightgown was microscopically similar to a hair sample taken from Odom, “meaning the samples were indistinguishable”. He declared this to be “a very rare phenomenon”, and in the thousands of occurrences in which he had compared hair samples, on only eight or ten occasions had he failed to distinguish the hairs of two different people. Understandably, this implied a very substantial likelihood that the hair did in fact belong to Odom, and after only a few hours deliberating he was found guilty and sentenced to 20 to 66 years in prison.

In June that same year, a worryingly similar scenario ensued after 21-year-old student Catherine Schilling was found raped and murdered in Rock Creek Park, Washington. She was discovered naked and shot five times in the head. A somewhat questionable police informant, Gerald Mack Smith, relayed to police that he had been drinking in the park with a man who had admitted to killing the woman after attempting to rob her. That man was allegedly Donald Eugene Gates. It is worth noting that, not only was Smith paid by police for his information, but at the time he had two prior felony convictions and had recently been indicted for a third felony, which was conveniently dismissed after he aided the police in incriminating Gates. This information was not relayed to the defence team.

Donald Gates - Washington Post

Donald Gates (Source: The Washington Post)

So Donald Gates, aged 30 at the time, was arrested and charged with murder, not only based on the testimony of Gerald Smith, but also largely on the comparison of hair samples. FBI forensic analyst Michael Malone testified in this case, establishing that hairs recovered from the scene were “microscopically indistinguishable” from a hair sample taken from Gates. The expert witness claimed that in his many years of hair comparison work, there were perhaps two in ten thousand cases where hairs from two people could not be distinguished. Once again, seemingly convincing statistics to a lay jury. In September 1982, Gates was found guilty and sentenced to 20 years to life in prison.

Forensic Hair analysis

What do these three cases have in common? All men were wrongfully convicted for crimes they did not commit. And all convictions were largely based on flawed expert witness testimony relating to hair analysis.

The potential evidential value of human hair stems from its prevalence at crime scenes and the ease with which it can be imperceptibly transferred. It is fairly resilient and can persist at a crime scene for many years after the incident took place, all-in-all making hair a potentially vital piece of evidence well worthy of examination.

What does this examination involve? Forensic hair analysis is a type of ‘comparison analysis’ – that is an examination that is typically based on individual or class characteristics rather than incorporating numerical analyses. It generally involves the study of microscopic characteristics relating to whether the hair is human or animal, the colour, length, diameter, likely racial group, area of bodily origin, phase of growth, and so on.

Hair 2 - Wikimedia

Microscope image of hair sample

The examination may include an even closer scrutiny of patterns and intricacies along the shaft of the hair. The human hair can be described as being composed of three distinct regions: the medulla, the cortex and the cuticle. The medulla refers to the inner core of the shaft, the cortex as the surrounding material, and the cuticle the outermost protective layer. The cuticle is composed of a scale-like complexion, which can be particularly beneficial in distinguishing between animal hairs and human hairs. Cuticle characteristics will be noted in the comparison of hair samples, particularly the thickness of the cuticle, the colour, and the general shape of the “scales”. The cortex is the portion of the hair containing pigment granules which give hair its colour, obviously vital in the comparison of different samples. These pigment granules vary between racial groups, thus are pivotal in establishing whether a hair is likely Caucasian or Negroid in origin, for instance. The medulla in the centre of the hair shaft will also be scrutinised to determine whether the core is continuous, interrupted, fragmentary or absent.

This is a limited list of the features focussed on during forensic hair analysis. In reality there is a myriad of other features, both natural such as those above and artificial such as dyeing and damage, that can be used in studying the similarities and differences between hair samples. So it would seem there is an abundance of information with which to reach a conclusion. However the problem lies in that these are all class characteristics whose description is based on the individual analyst’s subjective observations.

Obviously the ideal situation would involve some form of statistical analysis to determine the reliability of comparisons, but the vast number of variable factors in hair comparison makes statistical analysis tremendously complex if not impossible. This in itself does not render such analyses worthless in criminal proceedings, provided accurate testimony is given and caveats made clear. In the aforementioned cases, and potentially others, expert witness testimony strongly implied that exact matches between hair samples had been made, and in some instances fictitious statistics were even expressed to the jury. When such evidence is presented by a seemingly distinguished scientist who may be an expert in their field, naturally members of a lay jury with little or no knowledge of the topic are unlikely to challenge this evidence. The FBI’s own guidelines on forensic hair analysis conclude that “the science of microscopic hair examination can never result in an identification” but can “provide a strong basis for an association”, but unfortunately this ethos was not always carried into the courtroom.

Collectively, Tribble, Odom and Gates served nearly 80 years in prison. All three men were eventually released and exonerated by DNA testing and the actual perpetrators identified. However by the time this was achieved, perpetrators had passed away or were able to evade conviction due to expiration of the statute of limitations. The wrongfully convicted men were awarded millions of dollars in compensation, but hardly a prize worthy of losing so many years.

As a result of these incidents, and numerous others, ongoing investigations are aiming to identify further miscarriages of justice in the United States, initially largely powered by the Innocence Project, a non-profit organisation which aims to exonerate wrongly convicted people through DNA testing. Furthermore, efforts are underway to focus on how forensic science standards can be strengthened to avoid future errors, particularly in relation to forensic ‘comparison analyses’.

 

References

Federal Bureau of Investigation. Forensic hair comparison: background information for interpretation. [online] Available: https://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2009/review/2009_04_review02.htm

Innocence Project. Donald Eugene Gates. [online] Available: http://www.innocenceproject.org/cases-false-imprisonment/donald-eugene-gates

National Registry of Exonerations. Kirk Odom. [online] Available: https://www.law.umich.edu/special/exoneration/pages/casedetail.aspx?caseid=3943

Taupin, J. M. Forensic hair morphology comparison – a dying art or junk science? Sci & Justice. 44 (2004), pp. 95-100.

The Washington Post. Santae Tribble cleared in 1978 murder based on DNA hair test. [online] Available: https://www.washingtonpost.com/local/crime/dc-judge-exonerates-santae-tribble-of-1978-murder-based-on-dna-hair-test/2012/12/14/da71ce00-d02c-11e1-b630-190a983a2e0d_story.html