DNA Profiling Research Paper

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The use of DNA for identification purposes has often been the subject of controversy. This is particularly true in the UK at present with the imminent implementation of legislation to destroy a significant number of physical samples and to remove the related profiles from the national DNA database. This research paper covers the basic biology behind the use of DNA profiling and its use in the criminal justice system in the UK, particularly in terms of the relevant legislation and the provision of services by private laboratories. These issues are fundamental to the understanding of the problems that have always existed in terms of DNA interpretation, particularly in terms of mixed and partial profiles, plus emerging issues that have arisen with advances in research and technology.

Fundamentals Of DNA Profiling In Forensic Science

DNA is a complex chemical compound comprised of relatively simple building blocks generally found within the nucleus of cells. The genome is the entirety of this cellular DNA, and encodes all the genetic information that governs an organism’s structure and function, and is unique to the organism, except in genetically identical individuals. During sexual reproduction, the information determining the physical characteristics is inherited, half from each parent. Small sections of the molecule contain specific variations in the genetic code that can be statistically evaluated to assist in the process of individualization and therefore identification, most notably in human subjects. Analysis of the variable sections of DNA is frequently employed to determine paternity and to resolve immigration disputes, but public perception is focussed on its application as an aid to police investigative processes, to identify victims of crime and, by the transfer, exchange, and persistence of traces of DNA, the offenders responsible for crime.

The use of DNA identification methodologies has revolutionized crime scene investigation and the presence of forensic science in the courtroom. Historically, evidence of identity was limited to direct eye witness or, since the late 1800s, photographic recognition. Fingerprint identification has been in common use in the UK since the establishment of the Fingerprint Bureau in 1901, with the first conviction employing this technology being for a murder in 1905. Consequently, public confidence in the veracity of “dactyl” fingerprints and the legislation governing its utility has had considerable time to develop, whereas the revolution of DNA identification is still a relatively recent phenomenon and still presents legal and ethical issues that have yet to be fully resolved.


Deoxyribonucleic acid is a macromolecule containing the genetic instructions for the replication and function of all known living organisms (excepting a small number of viruses). Specific segments known as genes carry this genetic information and encode for proteins, for example, are known as genes. The presence of DNA in living material was originally recognized in 1869 by a Swiss physician, Friedrich Miescher. By 1937, the chemical composition of DNA had been identified and x-ray diffraction had demonstrated that DNA had a regular structure and the link between DNA and hereditary characteristics had been postulated. In 1953 Francis Crick and James Watson proposed the double-helix model of DNA structure. Two strands joined together by four distinct nitrogenous bases form a compact right-handed helix, thus allowing for a large amount of information to be encoded in a relatively small space. Crick and Watson’s DNA breakthrough helix discovery was enabled through the work of two other scientists: Maurice Wilkins, who pioneered X-ray crystallography, and Rosalind Franklin who refined the technique for work with DNA. Together they identified the four bases which link the two strands of the helix; adenine, cytosine, thymine, and guanine (A, C, T, and G). These nitrogenous bases are attached to a ribose sugar on the phosphate-sugar backbone of each DNA strand.

Each molecule of DNA includes a pattern of these bases bonded together. Adenine forms hydrogen bonds with thymine while cytosine similarly engages with guanine. This base pairing holds the two strands of the double helix in close proximity to each other. It is the sequence of these base pairs within genes that is the genetic “blueprint” for the organism, encoding sequential instructions for amino acids, which are the building blocks of proteins. The information encoded in DNA is held in compact structures called chromosomes, of which humans have a normal complement of 46: 23 of maternal and 23 of paternal origin. Humans share almost all of their DNA sequence with other humans and much of the sequence information with other organisms. Only about 1 % is specific to humans when compared to a near relative such as a chimpanzee, and only about one tenth of that 1 % of DNA differs from one human to the next (with the exception of identical twins). However, this small fraction of the DNA still comprises some three million base pairs.

When analyzing person-specific DNA variations for forensic purposes, the sections of DNA examined are called short tandem repeats (STRs). As the name suggests, these are short sequences of DNA normally just four bases long repeated as adjacent blocks. It is predominantly the number of repeats of the core sequence that varies within individuals (e.g., GATA, GATA, GATA). STRs are, fortuitously, found in the noncoding regions of the genome (i.e., not within the genes) but the position (locus) of these on the chromosomes does not usually change from one person to the next. For crime scene investigation in the UK, a DNA profile is produced by measuring the physical length of the DNA at ten STR loci simultaneously, and the DNA analyzer displays the results as a series of peaks on a graph known as an electropherogram (EPG). The information carried therein is statistically very powerful, and the probability of a match being from someone other than the suspect and unrelated to them is given a numerical value. Assessments of match probabilities are made with reference to STR profiles separated into different racial groups in order that the most conservative figure is given.

DNA in the form of linear chromosomes exists within the nucleus of every nucleated cell. Several highly specialized cell types lose their nuclei and their DNA, for example, red blood cells are packed with hemoglobin while white blood cells retain their DNA, but in general DNA recovered from a white blood cell will be identical to DNA from any other cell type or tissue from the same individual. The nucleus is not the only source of DNA; cells contain tiny sausage-shaped organelles known as mitochondrion. These are the powerhouses of the cell specifically designed to make energy. Each mitochondrion has between one and ten copies of circular DNA that code for proteins required for the energy production and each cell may have hundreds of mitochondrion depending on its function (e.g., mitochondrion are found in large numbers in muscle cells). Hence in adverse conditions when the genomic DNA is too severely degraded to be useful, it is sometimes still possible to get some useful genetic information from the mitochondrion. The amount of genetic variation is far lower than genomic DNA but mtDNA can be very useful in identifying family members. The mitochondrion are inherited through the maternal line as at fertilization it is only the sperm’s nucleus that is injected into the egg, not the mitochondrion.

While all nucleated cells contain DNA, not all nuclear DNA is contained within cells. Like many of the elements that go to make up an organism, the DNA is recycled. Dead cells are harvested and the DNA broken down. In this process, long stretches of DNA can be found in the plasma and recently also discovered in sweat. This discovery has farreaching implications for the evidential value of “touch” DNA, particularly in terms of secondary and tertiary transfer between individuals and touched items (Quinones and Daniel 2012).

Development Of Technology

The first paper reporting the use of DNA in a criminal context was published in 1985 by Alec Jeffreys, Professor of Genetics at Leicester University (Jeffreys 1985a).

Jeffreys was researching inherited variation in human DNA and he demonstrated how a DNA profile could be used to resolve issues of identity and kinship. Its initial use in legislative practice was to demonstrate that a child was the legal offspring of two individuals already granted asylum in the UK (Jeffreys 1985b).

DNA technology was the subject of research and development for the purposes of criminal investigation throughout the late 1980s and early 1990s primarily by the Forensic Science Service in the UK (FSS), previously known as the Home Office Forensic Service Laboratory (HOFSL). The FSS was the major provider and innovator in forensic science in the UK until its closure in March 2012. The multi-locus probe was introduced to routine casework in 1987 but required a large amount of sample material to produce a profile, although a full profile could give a likelihood ration of one in a million. This was followed by the single-locus probe in 1989 which allowed smaller samples to be tested and could produce statistics of one in 20–30 million.

However, both techniques were limited by the quantity of starting material required and regularly failed due to the poor quality and small amounts of DNA recovered from crime scenes. A major breakthrough came about with the development of the polymerase chain reaction (PCR) by American biochemist and Nobel Prize winner Kary Mullis. This enabled the biochemical copying of small amounts of DNA. This meant that sufficient target sequences could be amplified so as to be easily detectable by DNA analysis. During the PCR process the sample of DNA is treated with cycles of heating and cooling that denature the DNA and divide it into two separate strands, and a DNA primer is then used to anneal to these strands. Primers are short pieces of DNA containing sequences complementary to the target regions. The annealed primers together with a TAQ polymerase (an enzyme which enables a chain reaction to happen) allows two new copies of the sequence under investigation to be produced, one from each strand. This process of denaturing and rejoining was repeated again and again in a cyclic manner allowing amplifications of DNA even from samples with a relatively limited amount of starting material.

The flexibility of this method meant that only the targeted sections are copied and simultaneously those copies are marked by the addition of fluorescent tags. The fluorescent tags mean that the copied sections of DNA are visible when illuminated by laser light. In the UK, most DNA samples are subject to 28 cycles of PCR, doubling the amount of DNA available for analysis at each cycle.

The first use of PCR was in the detection of Human Leucocyte Antigen DQ alpha (HLA), introduced by the FSS in 1991, thereby allowing the analysis of much smaller stains. However, PCR has now become a fundamental part of all current profiling methodologies. The STR Quad system was introduced in 1994 which looked at four regions of DNA followed by, in 1995, Second Generation Multiplex (SGM) which gave a profile of six areas of an individual’s DNA plus the sex indicator area, giving an average discrimination potential of 1:50 million. The SGM technique facilitated the introduction of a computerized database in 1995. SMGPlus®, used since 1999, is the main technique in use in the UK and looks at ten areas plus the sex indicator area and increases the discrimination potential to 1:1,000 million. However, many countries are moving to 16 or 17 loci multiplexes which the UK plans to do by 2014.

Use In The Criminal Justice System

In the UK, a Royal Commission was set up in 1993 to look at the opportunities that new DNA technologies might be able to offer the criminal justice system. The development of rapid, automated testing and the use of digital tools enabled interrogable databases to be quickly compiled. However, in order to deploy the technology, the police required the power to take DNA samples from those involved in criminal offences. The ability to take samples was restrictions in the Police and Criminal Evidence Act. PACE 1984 had been specific about the consent and authority required before a sample could be taken. Samples were categorized as “intimate” or “non-intimate” and regulations covered who could and could not take samples. The Criminal Justice and Public Order Act 1994 redefined intimate and non-intimate samples: mouth swabs were redefined as non-intimate and could be obtained without the person’s consent. Though a suspect could refuse to open his mouth, it was then permissible to pluck head hairs with roots from which DNA could be obtained. Consent was still required to obtain blood and still required a qualified practitioner to take it. The Police Reform Act (2002) changed the regulations concerning the taking of samples: a police constable could now take non-intimate samples or could delegate this power to a “designated person” such as a civilian forensic officer. It also created the requirement for all new police officers to supply DNA samples to the Police Elimination Database.

In 1995, the evolution of DNA STR profiling technology and the subsequent change in legislation meant that the Home Office was in a position to create a database. The technology was simplified and automated, and the world’s first criminal intelligence database. This was launched in April of that year: the UK National Criminal Intelligence DNA Database (NDNAD) (www.genome.wellcome.ac. uk/doc_wtd020879.html). Scotland and Northern Ireland have databases separate to that in England and Wales but all three are intersearchable.

First Use In A Criminal Investigation

In 1983 Lynda Mann was found raped and murdered in a deserted footpath in Leicestershire. Conventional grouping tests on semen samples from the body suggested that her killer was a person with blood type A and an enzyme type shared by approximately 10 % of males in the general population, but with no further evidence, the case remained unsolved. In 1986 the murder of another girl, also in Leicestershire, was linked by police through modus operandi. Police held a prime suspect, Richard Buckland, who confessed to the second murder but not the first. Jeffreys, in conjunction with the FSS, using extraction methods which enabled DNA from semen to be separated from DNA from vaginal cells, demonstrated that the murders were committed by the same person and that that person was not Buckland. Leicestershire Constabulary and the FSS began an investigation in which 5,000 local men were asked to volunteer blood or saliva samples, but after 6 months, no matches had been found. Later one of those men was heard bragging that he had been paid £200 to give a sample on behalf of Colin Pitchfork. Pitchfork was arrested in September 1987 and samples taken from him matched those of the double killer. Pitchfork admitted the murders and was convicted in 1988, becoming the first man to be convicted on DNA evidence, with Buckland being the first person to be proved innocent by DNA profiling. It was also the first time that the mass DNA screening of a population had been undertaken, a process that has been carried out on numerous occasions since. Even in cases where no suspect has been identified through this process, it has been beneficial in quickly eliminating a large number of individuals as being the donor of a profile believed to be crime related.

Persistence Of DNA And Use In Historic Cases

Trace amounts of DNA can be recovered from bones as much as 5,500 years old, with opportunities within the forensic world for the identification of the victims and perpetrators of crime. In 1992, DNA testing gave compelling evidence linking remains recovered in Brazil in 1985 with Dr Joseph Mengele who died in 1979 by comparing a sample taken from the femur of the skeleton with samples taken from his widow and his son, which indicated full parental inclusion. In 1991, skeletal remains found in a shallow grave in Yekaterinburg were identified by Russian authorities as those of Tsar Nicholas II, the Tsarina Alexandra with three of their children. Remains discovered in a nearby smaller grave in 2007 were identified for the remaining two children using mtDNA, in part using samples from the Duke of Edinburgh who shares the same maternal link. Improvements in technology have meant that DNA is a frequently employed technique in resolving “cold cases,” such as in the recent conviction in the UK of David Burgess for the murder of Yolande Waddington in 1966.


In April 2007, responsibility for the NDNAD was transferred from the Home Office to the National Policing Improvement Agency (NPIA). The NPIA published the following statistics for the databases of England and Wales, Scotland, and Northern Ireland combined as at January 2012.

DNA Profiling Research Paper

In the USA, the Combined DNA Index System (CODIS) is the central database for DNA profiles created by federal, state, and local crime laboratories and funded by the Federal Bureau of Investigation (FBI). Originally holding only the profiles of sex offenders, it has been extended to include a much wider range of offenses and encompasses:

  • The Convicted Offender Index
  • The Arrestee Index
  • The Forensic Index (crime scene profiles)
  • The Missing/Unidentified Persons Index
  • The Missing Persons Reference Index

All 50 states have legislation governing the collection, storage, and retention of DNA and it is state law rather than federal law which governs which crimes qualify for CODIS. CODIS databases exist at local, state, and federal level and separate laboratories can retain or share information as they choose. However, CODIS is the largest database in the world.

Interpol also operates DNA Gateway, inaugurated in 2002, containing more than 117,000 profiles submitted by 61 member countries. Police from any of the 190 countries belonging to Interpol can access information (https://www.interpol.int/How-we-work/Forensics/DNA).


In the UK, legislation regarding the collection, storage, and use of data held on the NDNAD has developed over time, and no single piece of legislation covers every aspect legislative amendments have been made to old laws, and case law originating from judges’ rulings has redefined the application of the legislation.

The Doheny and Adams ruling (1997) addressed the way in which DNA evidence should be presented in court. An expert could no longer give an opinion on whether a crime stain came from a suspect but had to explain its probability. In 2000, the Lashley judgement in the Appeal Court ruled that DNA evidence alone was insufficient to bring a conviction and supporting evidence was also required. However, this can be as limited as geographical proximity to the offence; living in or having visited the region where a crime scene stain is matched can be enough. Furthermore, in 2000, challenges to convictions in two cases, R v. Wier (murder) and R v. “D” (rape), sparked further reform. PACE (1984) required samples to be destroyed after acquittal or discontinuance; Wier and “D” were identified using unlawfully held DNA samples. The convictions were appealed, but the Lords found that it would have been against the cause of justice for the convictions to be set aside, and the Criminal Justice and Police Act 2001 amended PACE so that now all DNA data collected from persons arrested for an offence could be kept, whether found guilty or not guilty. The Criminal Justice Act (CJA) 2003 further extended this so that data could be logged from anyone arrested for an offence, irrespective of whether they were eventually charged. Within a short space of time, the database doubled in size, but the problem of holding records of innocent people was created. Samples and profiles could only be destroyed by application to the Chief Constable of the arresting force. In addition, the holding of samples for the prevention or detection of crime is exempt from the Human Tissue Act (2004), brought about in part as a response to the discovery of the retention of the organs of children without consent by the Alder Hey Children’s Hospital.

Recent legislative changes will have a tangible impact on DNA identifications. The Nuffield Council on Bioethics published a report in September 2007 on “The forensic use of bioinformation: ethical issues” which recommended that proposals to extend police powers even further to include the taking of DNA for minor offences such as littering should not be implemented. In addition there has been a growing perception among civil liberties groups that the retention of samples on the NDNAD of individuals who were never convicted of an offence infringed the civil liberties of those whose DNA profiles were stored. The Appeals of “S” and Marper particularly apply: both were arrested in 2001 in separate incidents, but both cases were dropped. On application to the Chief Constable of South Yorkshire, both were refused the right to have their samples destroyed. Between 2002 and 2004, they were refused a judicial review of the decision, and their appeal was rejected first by the Court of Appeal and then by the House of Lords. However, in 2008, the European Court of Human Rights found in their favor, stating that the indefinite retention of profiles on a database interferes with a right to a private life and is particularly important for minors. Following consultation, this led to further amendments to PACE (1984) being promoted in the Crime and Security Act 2010 which passed into law but which has not been enacted to date. The coalition government elected in 2010 has further revised provisions for the rights of the individual in the Protection of Freedoms Act 2012, with far-reaching consequences for the NDNAD. The terms of the act require that approximately one million records of people on the database in England and Wales, and the copies held elsewhere, must be removed. The law does not require the removal of records of adults who have been convicted or have accepted a caution from the police, and people arrested for (but not convicted of) a serious offence can have their records retained for 3 years in the first instance, or a further two if there is the approval of a court (http://www.genewatch.org/sub-539488).

To date it has been possible to carry out a familial search on the UK NDNAD. On arrest, two buccal swabs are routinely taken, one to be kept as a backup in case the first sample fails to yield a profile. The “A” sample is processed; the remaining “B” sample is stored. When a crime scene sample has not given an immediate match on the database, it has been possible to look for previously loaded profiles that show similarities. Geographical factors and known information about the suspect, such as age, are taken into account to narrow the number of near matches. Once a manageable number of matches are obtained, it is possible to profile the “B” sample looking only at Y-STRs (DNA information obtained only from the Y chromosome and so only paternally inherited). This information allows the formulation of family trees indicating the presence of a male relative who fits the criteria for the offender but who has never been arrested for a recordable offence. The first successful prosecution relying on this procedure was in 2004 when Craig Harman was convicted of manslaughter for throwing a brick from a bridge which killed a lorry driver. Harman had left his blood on the brick (having injured his hand before taking it) but did not at that point have a police record. Forensic experts at the FSS found a profile with similar characteristics using the new techniques of familial searching through a relative whose profile was on the database, and as a result, the police traced Harman. Familial searching has always been limited to the most serious of cases and requires approval from the DNA ACPO lead. However, the application of familial searching in this way will no longer be available to police forces as following the Protection of Freedoms Act 2012, the UK has decided to destroy all “B” samples (over six million samples), even if taken from convicted offenders.

For further details, please refer to:

  • Police and Criminal Evidence Act (PACE) 1984
  • Criminal Justice and Public Order Act 1994
  • Criminal Evidence Act 1997
  • Criminal Justice and Police Act (CJPA) 2001
  • Police Reform Act 2002
  • Criminal Justice Act (CJA) 2003
  • Serious Organised Crime and Police Act 2005
  • Policing and Crime Act 2009
  • Crime and Security Act 2010
  • Protection of Freedoms Act 2012

Some rights of the suspect have been set aside when arrested or detained under certain sections of the Terrorism Act 2000, and specific rights apply to suspects under the age of majority, i.e., 18 (http://www.legislation.gov.uk/).

It is interesting to note that the Association of Chief Police Officers (ACPO) recently announced, before the actual implementation of the Protection of Freedoms Act 2012, a new operation to capture the DNA of individuals whose profiles are not currently held on the database. Using powers under the Crime and Security Act 2010, which became law last year, the aim of Operation “Nutmeg” is to gather DNA profiles from criminals who were convicted before 1995 (when the database was launched). Initially the operation will target 11,993 criminals convicted of serious offences such as murder, manslaughter, and rape over the past 40 years. The success of the initiative is impossible to guess but there is obviously scope for further sampling, plus potential implications for the removal or retention of the samples currently targeted for destruction.

Provision Of Services

The Forensic Science Service pioneered the use of the DNA database. Originally the Home Office Forensic Science Laboratory, it became an executive agency of the Home Office in 1991 but in 2005 changed its status from executive agency to a government-owned company. Demand for DNA services exceeded all expectations particularly after the launch of the NDNADB and large backlogs of samples gave private companies an opportunity to join the market by providing assistance. Police forces paid the FSS for their services but the increasing use of competitive tendering resulted in a loss of market share and the FSS ceased to be financially viable. It closed in March 2012 and services are now provided by a number of private companies such as LGC Forensics and Cellmark, who contract their services to the various constabularies in England and Wales. Private companies are able to invest in additional resources if it will have a beneficial effect on their profit margins and this, together with the competition in the forensic market, has led to some advantages such as reduced costs and turn round times, with a standard sample result being delivered in about 3 days. Forensic services in Scotland are provided by the Scottish Police Services Authority and in Northern Ireland by Forensic Science Northern Ireland. Each of these service providers will have separate provisions for the testing of different types of sample, and each must demonstrate the highest standards for preventing cross-contamination. Environmental testing is routinely carried out to monitor background levels and to inform processes to address any potential issues. It is also a requirement of the UK Accreditation Service (UKAS) and essential in order to maintain ISO 17025 accreditation, an assessment of internal standards, issued by the International Standards Organization and applied by UKAS in the UK.


The NDNAD holds samples from three sources: personal samples from those arrested or charged, crime scene samples, and voluntary personal samples. Prior to 2004, these were non-evidential Criminal Justice (CJ) samples and required a confirmatory sample on rearrest. Since 2004, samples are taken under the provision of the Police and Criminal Evidence Act (PACE) and can be used for evidential purposes. Previously the taking of a confirmatory sample could entail a delay of up to 2 weeks during which time the suspect had the opportunity to abscond or to undertake multiple crimes, knowing their arrest was imminent.

There are three possible samples that can be taken from arrested persons for search against and inclusion on the database: blood, buccal scrapes (from the epithelial lining on the inside of the cheek against the buccal muscles), and pulled head hairs. Blood samples used to be taken from major crime suspects due to the likelihood of obtaining a usable profile, but improvements in technology have meant that this is no longer necessary and buccal scrapes are by far the most commonly taken. Pulled head hairs are an option if a suspect refuses to give a buccal sample voluntarily.

From a crime scene, there are many sources from which to obtain a DNA profile, with varying degrees of evidential value depending on the circumstances.

Blood is commonly found particularly at major crime scenes. The red blood cells contain the protein hemoglobin which bonds with oxygen to carry it around the body and there are about 4.5–5 million red blood cells per microliter (one thousandth of a milliliter) of blood, but they have no nuclear or mitochondrion DNA. It is therefore only the white blood cells, of which there are only 5–10,000 per microliter of blood, which can be analyzed for a DNA profile. In addition to the obvious opportunities such as blood left by suspects or victim’s blood on weapons, the analysis of blood patterns (BPA) and subsequent profiling can be a vital tool in reconstructing a sequence of events.

Saliva does not contain DNA at point of production but epithelial cells from the inside of the cheek regularly slough off and are deposited in the saliva in sputum and on items coming into contact with the mouth, such as cigarette butts, drinking vessels, masks, gags, and on licked stamps and envelopes. Food is also an option but as saliva contains digestive enzymes, the material is often broken down, making odontology a potentially favorable evidential option.

Hairs with roots are a good source of DNA but dead hairs that have fallen out (telegen hairs) usually only contain mitochondrion DNA. The source of a hair may be identified by examining it in section. Determining the source may add value to an investigation, for example, finding head hairs on the boot of a suspect from a victim who has been kicked in the head may indicate the individual responsible for a fatal injury. Chest hairs on a particular knife in a multiple-stabbing incident with more than one suspect may indicate which weapon caused wounds to the chest and thereby which suspect is responsible for the chest injuries.

Sexual assaults are often resolved by finding an exchange of material between victim and suspect. Therefore a large number of samples are taken from victims in order to find traces from the suspect, either at a police station, hospital, or rape suite. These include internal and external vaginal and anal swabs plus samples from the mouth and any other pertinent areas, such as where the victim has been licked or bitten. Urine samples are also taken for toxicology but urine may be examined for DNA as some semen may be washed away with the urine during collection. On arrest, a suspect will also have numerous samples taken in order to find traces from the victim, including swabs from the glans and shaft of the penis. Exhibits are also collected from a crime scene to identify DNA from all parties involved. The ability to split DNA between semen and vaginal material is vital in the investigation of sexual assault using DNA evidence (the cellular and seminal fractions). In addition, a victim of rape is asked for permission to take a DNA sample from a child produced as a result in order to identify the offender, which is achieved by removing the mother’s profile from that of the child, leaving the profile of the child’s father. This can also be achieved after a sufficient period of development with the products of conception after a miscarriage or abortion.

Semen is only produced by post-pubescent men with approximately one million spermatozoa per ejaculate. Vasectomized and naturally azoospermic males have a vastly reduced number of spermatozoa per ejaculate but it is often still possible to obtain a profile.

Footwear and clothing are often analyzed for “wearer” DNA as well as trace materials. This may come from cells sloughed off by close contact with the item or from sweat, as with “touch” DNA. Other items that have come into contact with biological material can also yield a DNA profile. One of the suspects in the Canary Wharf IRA bombings in November 1992 was identified from nasal debris found on a piece of green tissue paper recovered from a van containing a bomb that had failed to explode.

Trace amounts of DNA can be found in urine. They are hard to identify as they are contained only in discarded cellular material washed from the walls of the urinary tract and are diluted by other waste products. Similarly, very fresh fecal material may also yield trace amounts of DNA in mucal secretions on the outside of the stool.

DNA is also used for identification of the deceased. Venous blood is preferable but not always possible, particularly if the victim has bled profusely. Where a corpse is badly burned or heavily decomposed, samples may be sought in deep muscle tissue where nuclear DNA has not been exposed to degradation, or if there is none remaining, DNA may be obtained from bones or teeth.

In all circumstances, a protocol for the collection of samples must be strictly followed to ensure that there is no cross-contamination. Officers collecting samples will wear protective clothing, and once obtained, samples are sealed in sterile containers before processing. Crime scene samples and arrestee samples are processed separately, often at a completely different laboratory site.

Success Rates

It is assumed that subject samples should be almost 100 % successful in yielding a DNA profile.

Statistics for crime scene samples show approximate success rates for DNA profiles as follows:

DNA Profiling Research Paper

Processing And Analysis Of Samples

Arrestee samples are usually buccal scrapes, one from the inside of each cheek. The two samples, “A” and “B,” are kept separately with the “B” sample being stored in the event that initial testing fails. Crime scene samples then go to a casework laboratory. Since the samples come in many forms, each sample will be treated differently to extract the DNA, and some will require presumptive testing, to locate the DNA for profiling and to establish whether a crime scene stain is from saliva, blood, or semen.

Stains and samples are then chemically treated to extract the DNA either manually or robotically and a process of quantification follows. A fresh sample will almost always provide an adequate amount of DNA. The smallest standard starting template permissible is a 1 ng (one millionth of a gram per milliliter) sample. This is subjected to the standard 28 cycles of PCR which provides sufficient amplified DNA to produce a profile which can be converted into a numerical code. If sufficient information from the profile is obtained, then this is sent to the NDNAD where it can be digitally compared to the millions of subject and crime stain profiles held there. Should there be no match, the profile from the crime scene sample will be held on the database indefinitely in case a match should appear in the future, either person to person, person to crime stain, or crime stain to crime stain. However, should the database indicate a match with a profile held on the database, then the information is passed on to the investigating force and a warrant for the arrest of that person is issued. Provided there is other supporting

High-Sensitivity Profiling

In some cases, the possibility of extracting DNA from crime scene samples will be reduced if the sample is old, degraded, or otherwise small in quantity. These can be subjected to more specialized techniques depending on the severity of the crime. All high-sensitivity work is performed in ultra-sterile conditions.

The FSS developed low copy number (LCN) to deal with samples containing insufficient good-quality DNA for standard profiling. The sample is subjected to 34 cycles of PCR to give more copies from which to draw a profile, but there are inherent difficulties. During 2007, a review was set up to examine the standards of science used in the analysis of LCN DNA, but before the review was complete, a challenge was made in the Northern Ireland case of R v. Hoey, a defendant charged with various offences in relation to the Omagh bombing in 1998. The case against him derived chiefly from DNA evidence using the LCN procedure, but the judge was not satisfied as to the integrity of the process and the prosecution case failed. This case was cited in the appeals of Reed, Reed, and Garmson in 2009 where it was found that in a very high proportion of profiles obtained using the LCN procedure, the profiles were not capable of robust and reliable interpretation.

DNASenCE (sensitive capillary electrophoresis) devised by LGC Forensics removes the impurities which interfere with the PCR process. The resulting profile is enhanced by a factor of 13. If necessary, the extracted material is loaded onto the capillary tube in greater concentration. This can enhance the resulting profile by a factor of 62. Other laboratories have devised similar enhancement procedures. The major advantage of post-PCR cleanup and enhancement methodologies is that the original sample remains for resampling using alternative techniques, whereas 34-cycle LCN uses up all the original material. The profile can then be searched against the NDNAD as with a standard SGM + sample (Gross et al. 2009).

In addition to DNA SenCE profiling, other methodologies have been developed, for example, Minifiler STR analysis which has been particularly useful in “cold-case” investigations. The eight mini-STRs examined are smaller versions of eight of the regions looked at in standard SGM + profiling. The smaller size means they are more robust and less prone to degradation but resulting profiles are still compatible with database searching. Y-STR profiling is also of benefit in sexual assault cases where there is semen present but no sperm or where the victim’s profile is likely to mask the profile of the offender, for example, scrapings from a victim’s fingernails.

Promega’s Powerplex 16 and Applied Biosystems’ Identifiler both examine the same ten SGM + sites plus the sex determination site, as well as an additional five “smaller” sites in common and one more which differs in each case. These processes are useful where a sample is degraded and where the new, smaller sites are likely still to be extant.

Next Generation Multiplexes

Next Generation Multiplexes (NGMs) examine an increased number of sites comprising those presently used for SGM + profiling plus additional sites. There is a directive from the European Network of Forensic Science Institutes (ENFSI) urging member states using DNA profiling for forensic science purposes to standardize their methodologies and profile storage across the European Union by applying a European standard set of alleles to improve cross-border compatibility. The aspiration is that this will encourage more cross-border searching. Several commercial companies have developed NGM kits that meet the ENFSI recommendation and in the UK the decision on which Next Generation Multiplex (NGM) kit will be adopted rests with the National Policing Improvements Agency. Once this has been decided, all subject and crime scene samples will be processed using an NGM and the NDNADB will be gradually “upgraded.” The potential advantages of this will be a harmonization of approach across Europe as well as greater discrimination in individual matches. The major disadvantage is that the profiles will of necessity be more complex and therefore much more difficult to interpret, particularly in the case of mixtures.

Contrary to the impression given in “CSI”-type media, even a standard crime scene sample can still take 48 h to process, with further time taken for interpretation, depending on the complexity of the result. Under exceptional circumstances, this time can be reduced, but only with greatly increased demands on equipment time and personnel therefore incurring greatly increased costs. Obviously, the more complex the profiling method used, the longer it will take to interpret the findings.

Mixtures And Partial Profiles

Partial profiles and mixtures of profiles are particularly difficult to interpret. A partial profile occurs when there is insufficient good-quality DNA to produce a full profile. The match probability of such a profile can be significantly reduced to the point where it is not possible to reach any conclusion as to evidential or even intelligence value. In order to calculate the probability of a partial match, reference databases are used to estimate the proportion of the STR profile in the corresponding populations. The rarity of certain characteristics is also taken into account.

A notable case using LCN profiling involving a mixture of profiles was that of R. v. Broughton (2010), jailed for 2 years and 8 months in 1999 in connection with violent animal rights activism when police found a firebomb in his car. He was arrested again in 2007 and remanded in custody after incendiary devices were found in Oxford University colleges. A jury cleared him of possessing explosives but failed to reach a verdict on other charges and so there was a retrial in 2009. He was subsequently convicted and sentenced to 10 years in prison for conspiracy to commit arson. However, in 2010, this conviction was overturned on the basis that the DNA evidence had been unreliable. In the retrial, the uncertainty regarding the reliability of the DNA evidence hinged on differing interpretations of the results of processing which yielded the DNA profiles of more than one person on a matchstick which formed part of the incendiary device. The defense maintained that Broughton’s DNA could not be present, and both prosecution and defense teams assembled impressive teams of DNA experts and statisticians using different software packages to calculate likelihood ratios. There was however fundamental disagreement on the question of how likely Broughton was to be included in the mixture or even how many profiles the mixture collected from the matchstick contained. Despite these differences of opinion, the jury returned a guilty verdict.

Mixed Profile Resolution

The components of a DNA profile are represented by a series of peaks that are measured and given a numerical value. A single-source profile will have two peaks at each locus, one from each parent.

The more contributors to the mix, the more difficult interpretation becomes.

Without being able to subtract known profiles, such as from a victim or others known to have had the potential to contribute, the mixture has too many variables to interpret with certainty.


Each laboratory will set its own interpretation guidelines, giving a peak height at which a numerical value can be “read,” but these are subject to variables including the nature of the offence and the source material. In addition, STR profiles can show “stutter,” a term applied to peaks in a profile caused by the stochastic effects of PCR (Butler and Hill 2006). It is not certain why stutters occur, but they happen during the PCR process and show as a small peak usually one repeat smaller than the main band. Although a mostly clean, single-source profile may show stutter, this is easy to absorb in the overall interpretation, but the small bands often align with common alleles and so can be impossible to discriminate from true small peaks genuinely present from another profile as part of a mixture. Two peaks from the same source should be approximately the same size, but sometimes unidentifiable problems in profiling can lead to peak imbalance dramatically changing the appearance of a profile even though there is no background interference. In addition, stochastic effects can also occur. Referred to as “drop-in” and “dropout,” drop-in is probably caused by contamination from an individual, the laboratory, or consumables. Dropout may be caused by a number of factors. There may be a problem with the primer leading to failure in amplification or the allele may be much larger than the usual size at a particular locus and is not seen with the others. DNA testing laboratories are continually attempting to adjust in order to cope with these difficulties but too little is known to be able to accommodate all eventualities successfully (Gill et al. 2000).

The Limitations Of The System

Even with the incredible advances in technology, crime scene DNA profiling can be used to conclusively exclude a person from an inquiry, but even though it may provide compelling evidence of association, it is not proof of identity. Interpretation, particularly of complex mixtures, can be affected by many variables from interpretation guidelines differing between forensic providers to personal bias. In addition, it is estimated that the addition of partial profiles to the NDNADB may mean that approximately 0.1 % of matches are adventitious, that is, occurring by chance (Werrett 1997). The current sensitivity of profiling means that approximately 75 % of crime scene profiles generated are mixtures. Dr. Itiel Dror has recently expanded his studies on bias in fingerprint experts being dependent on external emotive factors to DNA-reporting officers (Dror 2012). In addition any process involving human beings is subject to human error, accidental or deliberate, and the pressure to perform efficiently in a commercial market place places additional pressures on forensic operators. Finally, despite extensive research in many different areas from collection to extraction to profiling, not enough is known about the propensity of individuals to leave their DNA and how that DNA is subsequently transferred between objects and individuals. While the Next Generation Multiplexes will increase the discriminatory power of profiling, the inherent increase in sensitivity will lead to more background DNA becoming part of a profile, so mixtures will be even more common, and the increase in the number of sites will make these mixtures even more difficult to interpret accurately and will take longer, with inevitable repercussions. Despite public perception, DNA profiling is never the complete answer to solving crime but continues to assist police forces as an aid to the criminal justice process.


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