How Does DNA Help To Solve Crimes?
Forensic science,
alternatively known as legal medicine in many countries, aims to help the jury
members to solve both criminal and civil cases. Advancements in the
technologies of analysis of DNA have revolutionized this discipline of science.
DNA or the Deoxyribonucleic acid is universally present in all the living
beings. DNA is also known as the genetic blue print, as it stores information
about an individual’s entire genetic makeup. It determines the physical
features of an individual and various other attributes. This information stored
in the DNA gets passed down from one generation to another. The unique nature
of DNA in each individual (except identical twins) makes it to act as a
powerful tool for law enforcement investigations. Apart from this, it can be
extracted from various sources like blood, teeth, saliva, semen, hair, etc. DNA
extracted from biological specimen acts as a lead to the investigators,
especially in cases where there is no eyewitness (Butler, 2005). It is a very
powerful tool, as the evidence collected form the scene of crime can be used as
evidence even after decades, if it is properly preserved (Hart, 2002). Selected
DNA sequences, called loci, are used to create a crime profile in the crime
laboratory, which is later used for the identification of the suspect.
Currently, DNA evidence is used to resolve criminal cases in two ways. Firstly,
if the culprit is known, the DNA obtained from the crime scene can be matched
with that of the culprit to establish that the culprit is involved/not
involved, in the crime. Secondly, in cases where the suspect is unknown, the
DNA obtained from the scene is matched with offender profiles obtained from the
existing DNA databases. This helps to identify the culprit. This technology
helps to link the evidence obtained from one crime scene with another, so that
the same criminal responsible for both the crimes can be identified (James,
2012).
History of the Evolution of DNA
Technology in Forensic Science
A forensic geneticist aims
to identify the source of the biological specimen with high accuracy. Since the
discovery of the human ABO blood group polymorphism by Landsteiner, it has been
used to solve criminal cases. The main limitation of this system was that it
can be used to prove an exclusion (that the specimen did not come from a
particular individual), but cannot be used to ascertain the exact source of the
material. Serological and protein electrophoretic methods were the only methods
available till 1980s and the diversity in blood groups and polymorphic proteins
were the only markers studied.
The application of DNA in
forensic science began after the discovery of DNA fingerprinting by Alec
Jefferies in 1984 (The History of DNA, n.d.). Detection of hypervariable loci,
known as “minisatellites”, through hybridization of probes, increased the
chances of finding an exact match. During the same period, differential lysis
method, which enriches the sperm concentration in the vaginal fluid or semen mixture,
was developed. This method facilitated the access of investigators to a surplus
of suspect’s DNA, especially in the cases of rape. DNA fingerprinting
technology was helpful in solving many paternity cases and criminal cases for
some years. Later on, the focus shifted to the use of single-locus probes
(SLPs), wherein each probe targeted a single, highly polymorphic, RFLP, making
the interpretation highly specific. Development of PCR (polymerase chain
reaction) methods in 1988 was another break through achievement, in the field
of forensic science. Discovery of PCR technology which enables the rapid
multiplication of a small fragment of DNA, made the analysis of minute
quantities of degraded DNA possible. PCR based methods are currently widely
used for all forensic DNA typing (Jobling and Gill, 2004). The practical
application of DNA as an evidence was first noticed in 1987, when UK forensic
investigators solved the ‘Black Pad’ murders and identified Colin Pitchfork as
a killer. The launch of the national DNA database by FBI in 1998 was another
major achievement (The History of DNA, n.d.).
Advancements in DNA Technology and
Increased Forensic Applications
Since the discovery of
polymorphism in certain areas of DNA (polymorphism is a character of DNA which
means certain areas of DNA take different forms in different individuals), by
Alec Jeffreys, these variable areas were used to differentiate one individual
from another. Forensically important polymorphism is noticed in the
non-expressed or junk DNA. The DNA present in the non-coding region of DNA
exhibits high variability in length, as certain base sequences are repeated
several times within these regions (Lyle, n.d.). Initially, only restriction
fragment length polymorphism (RFLP) technology was used to solve forensic
cases. In this technology, enzymes known as “restriction endonucleases” were
used to cut DNA in a specific pattern. DNA fragments of variable lengths were
generated due to the presence or absence of certain recognition sites in DNA
tested. However, this technology is not widely used currently, as large
quantity of DNA required for testing may not be available or degradation of the
sample due to environmental factors may have rendered the DNA unfit for RFLP
analysis. The newer technological advancements in DNA analysis have proven to
be more efficient in solving cases (Hart, 2002).
STR analysis- Currently, a forensic
scientist generally looks for two kinds of repeats, which are, variable number
tandem repeats (VNTRs) or short tandem repeats (STRs). VNTRs are hundreds of
base-pairs long and repeat along the length of the DNA. The number of repeats
of these sequences differs from one person to another. STRs are very short,
usually three to seven base-pairs long, which repeat in a fragment of DNA as
long as 400 bases. Hence, STRs can be used to analyze very short fragments of
DNA including those obtained from degraded samples (Lyle, n.d.). Federal Bureau
of Investigation (FBI) has identified 13 specific STR loci and the chances of
finding two different individuals (except identical twins) with same 13-loci
DNA profile are very low (one in one billion). These 13 specific STR loci are
universally used by all the forensic laboratories so that a uniform DNA
database can be established across different regions (Hart, 2002).
PCR analysis - Minute quantities of DNA
obtained as biological evidence can be enhanced using PCR as it produces exact
copies of DNA present in the evidence. It does not affect the original sample
during this process. As it has the potency to replicate very minute quantities
of DNA, this technology can be used to multiply degraded samples obtained from
the crime scene. However, care should be taken while collecting and preserving
the sample as this technique can multiply the DNA of unwanted contaminants also
(Hart, 2002).
Mitochondrial DNA analysis
- Evidences
which cannot be analyzed through RFLP or STR are subjected to mitochondrial DNA
analysis. Evidences like hair shaft, bones or teeth lack nucleated cells; hence
DNA obtained from mitochondria is used for analysis as STR, and PCR can
multiply the DNA obtained from the nucleus. The limitation of this technology
is that, all the maternal relatives have identical mitochondrial DNA, hence
making the investigation difficult. However, the other supporting evidences
obtained can help to solve the cases.
Y- Chromosome analysis - Y-chromosome contains many genetic markers which can
be used for forensic purposes. Analysis of these markers targets only the male
suspects and it can prove to be a very valuable technique especially when the
evidence contains a complex mixture containing the contributions from two or
more male individuals. It can be used to trace family relationships amongst
males, as the Y chromosome gets directly transmitted from father to sons.
A cooperative effort of
police and crime laboratories will help to decide which method will suit better
for a case.
Steps Involved In DNA Aided
Investigation
FBI was authorized by
Federal law to maintain a national DNA database, that is, National DNA Index
System (NDIS), which works along with Combined DNA Index System (CODIS)
software. Initially NDIS started with 9 participating states, but later it
extended to almost 50 participating states (The FBI and DNA, 2011).
Collection of DNA samples - In order to establish such
large database containing over 10 million DNA profiles, the first step is to
obtain DNA samples legally. DNA sample can be acquired through any of the four
fundamental legal strategies.
Voluntary - The suspects are asked to
undergo DNA analysis voluntarily. Earlier, blood was drawn, but now-a-days
buccal swab obtained from a tooth brush is used for analysis.
Court order - Depending upon the case,
the court determines whether it is necessary to authorize a law enforcement
agency to collect DNA sample from a particular suspect.
Law - Collection of a DNA sample
from a defined group of individuals, like a group of arrestees or offenders, is
made mandatory by the statute. This authorization is given when it is felt that
the DNA samples of these criminals should be kept in a permanent record, that
is, State or national DNA database.
Abandonment - The suspect himself handovers an item
which contains his DNA sample during crime, unknowingly. For example, the
suspect drinks beer and leaves the glass (Dale, Greenspan, and Orokos, 2006).
Storage
of DNA profiles - The obtained DNA sample is subjected to the DNA analysis
techniques mentioned above to achieve a DNA profile, which is just a series of
numbers. Specific identification markers on the individuals DNA decide the
numbers of the DNA profile (The FBI and DNA, 2011). These DNA profiles of
different offenders are collected in a DNA database which is maintained by
every state of the nation. The DNA databases can be maintained at three
hierarchical levels, that is, local, state and national levels. The profiles
obtained from the local cases are entered into the local databases (LDIS) and
also recorded in the state level database (SDIS). A DNA profile obtained from a
recent scene of crime may or may not match with DNA records obtained from the
state level database. Hence, the state level information is uploaded into NDIS
(national level database) on a weekly basis (The FBI and DNA, 2011).
Use of CODIS software - CODIS is a computer
software which allows the law enforcement crime laboratories at national and
state levels, to compare DNA profiles obtained from the crime scene with that
of the profiles obtained from convicted offenders, electronically. Many
criminals show recidivist nature and have a tendency to get involved in cases
of sexual assault and burglary repeatedly. Hence, there is a high chance of existence
of the DNA profile of such criminal in the database and can be used to match
through CODIS (Hart, 2002). CODIS can be used to link one crime scene with
another and can be helpful in identifying serial offenders. CODIS operates at
three hierarchical levels, which are, local, state and national levels (Hart,
2002). Only criminal justice agencies meant for enforcement of law and
identification of criminals, are authorized to use CODIS according to the
federal law. The information in the database is not associated with any
personal identifiers like name, date of birth, etc, to ensure that perfect
confidentiality is maintained (The FBI and DNA, 2011).
Achievements of DNA Aided
Investigation
The success of DNA aided
investigation can be understood from the fact that a large number of criminal
cases are presently being solved with the help of CODIS. A remarkable example
of the application of DNA aided assistance is the identification of the victims
of the September 11, World
Trade Center
attack, in 2001. It was a massive attack claiming the lives of a very large
number of victims. Only bones or the tissue fragments were the remnants of many
victims of the attack and identification of the victims was a biggest
challenge. Professionals from the National Institutes of Health and other
institutions were assigned the job of identification of victims from the
degraded samples. Profiles were developed using SNPs and mitochondrial DNA and
entered into the database of the victims. By the end of 2005, out of 2792 people
claimed to be dead in the attack, 1585 people could be identified. The remaining cases were abandoned as the
samples were highly degraded. In 2007, this case was reopened with the help of
more sophisticated technology to enable further identification (DNA Forensics,
2009).
The results obtained from
a five-city field study, conducted by the National Institute of Justice in
2008, showed that use of DNA was effective in the investigation of ordinary
property crimes also. Almost twice the number of property crime suspects were
caught when DNA was used as evidence. As the rate of suspects being caught
increases, the rate of crime tends to decrease, and hence, the DNA technology
ultimately proves to be cost effective (DNA and Property Crimes, 2010). Crimes
like burglary, assault, and larceny were not generally associated with DNA
analysis. Under a pilot project conducted by New York City Police Department
(NYPD), officials were trained to identify the items which might contain
biological evidence like cigarette butts, drink glasses etc. These evidences
were subjected to DNA analysis. The aim of this project was to determine the
extent to which these cases can be linked to more serious offenses like rapes
or homicides. This study shows that offenders committing fewer offensive crimes
can be linked to cases of rapes or homicides (in more than 30% of incidences).
The success of this pilot project redefines the methodology of solving serious
crimes (Dale, Greenspan and Orokos, 2006).
The Missing Persons DNA Database, created in Texas using mitochondrial DNA, has helped in
the search of missing persons. This database targets the identification of the
skeletal remains of unidentified human beings, kidnapped children, etc. This
database is managed by FBI and the DNA profiles of the missing persons are
matched with unidentified human remains, for example, those obtained from uncovered
graves, crime victim remains etc.
Two hijackers of American
Airlines Flight 11, involved in the attack of World Trade
Center, were investigated
to be brothers through near-match searching, using DNA. Apart from human crimes
DNA testing has also been used to protect endangered species of animals and to
prevent animal poaching.
Limitations of the use of DNA
Technology in Forensic Science
DNA database can act as an
efficient tool in the course of law enforcement only when it has more data of
the offenders. But many jurisdictions are still in the process of developing
their database. Because of this and several other reasons, many cases remain
unsolved, creating a backlog. This DNA backlog may prevent the prosecution of
the actual culprit or imprisonment of wrongly convicted individuals. This might
increase the chances of crime as the offender is left free. Moreover, as the
DNA database expands, there is an increased demand for the experts who can
analyze DNA and create profiles of a large number of offenders. Space
limitations of the laboratories allow the processing of the samples obtained
from the most serious crimes only.
Overcoming the Technological Hurdles
through New Developments in Molecular Biology
There have been major
developments in the field of forensic science due to the developments in the
field of molecular biology. Gone are the days when investigators used to rely
on only physically observable large evidences. Now, it is possible to analyze
samples obtained from a single strand of hair. Now-a-days, scientists are
motivated to analyze more difficult samples like trace samples, termed touch
DNA. Recent developments in molecular biology help the scientists to face
tougher challenges. The use of mini-STRs, can enhance the efficacy of
detection, especially when the DNA sample obtained is highly degraded. Because
of the small size, the amplification products of mini-STRs have a tendency to
overlap more in comparison to conventional STRs. Hence, reconfiguration of STR
kits into mini STR kits, can promote betterment of the routine analysis. SNPs
can be used as genetic markers to predict the appearance of the culprit in
cases where the suspect is unidentifiable. These markers can be used to
identify the individual’s facial features, height complexion, etc. This makes
the investigation processes more focused. Post mortem analysis can be improved
if genetic variation and its effects on metabolism are also considered. This
will help to solve the cases which are usually considered as suicides or are
categorized under unexplained deaths. The deaths in these cases can be due to
various reasons like poisoning of the victim, making the victim incapable of
performing any action, forcible intoxication of the victim by alcoholic drinks,
etc. Present DNA typing techniques can only identify the source of the sample
but, with the help of expression analysis, it is also possible to identify the
tissue from which the source was obtained. Stains noticed in the crime scenes
can be from both organic and inorganic sources and this technique might help to
screen the samples from the human origin. This will prevent the unnecessary
effort of subjecting all the stains to DNA typing (Budowle and van Daal, 2009).
There is no doubt that use
of DNA in solving cases has significantly affected the speed and accuracy of
investigation of criminal cases. A collective effort of the forensic scientists
and police can help in catching criminals, hence reducing the crime rate. Due
to the current advancements in this area, we might expect to see a group of
highly trained and educated scientists with ultramodern devices meant for DNA
profiling in their hand, at the scene of crime.
References
Butler, J, M. (2005). Forensic DNA
typing : biology, technology, and genetics of STR markers. Amsterdam,
Boston:
Elsevier Academic Press.
Budowle, B and van Daal, A. (2009). Extracting evidence
from forensic DNA analyses: future molecular biology directions. Biotechniques. 46(5), 339-40, 342-50. doi: 10.2144/000113136.
Dale, W,M., Greenspan, O and Orokos, D. (2006). DNA
Forensics: Expanding Uses and Information Sharing. Retrieved April 17, 2013
from http://bjs.gov/index.cfm?ty=pbdetail&iid=774
DNA Forensics. (2009). Retrieved April 17, 2013 from http://www.ornl.gov/sci/techresources/Human_Genome/elsi/forensics.shtml
DNA and Property Crimes. (2010). Retrieved April 17,
2013 from http://www.nij.gov/topics/forensics/evidence/dna/property-crime/
Hart, S, V. (2002). Using DNA to Solve Cold Cases.
Retrieved April 17, 2013 from https://www.ncjrs.gov/pdffiles1/nij/194197.pdf
Jobling, M,A and Gill, P. (2004). Encoded evidence: DNA
in forensic analysis. Nat Rev Genet,
5(10), 739-51.
James, N. (2012). DNA Testing in Criminal Justice:
Background, Current Law, Grants, and Issues. Retrieved April 17, 2013 from http://www.fas.org/sgp/crs/misc/R41800.pdf
Lyle, D, P. (n.d.). Forensics: Fingering Criminals Using
DNA. Retrieved April 17, 2013 from http://www.dummies.com/how-to/content/forensics-fingering-criminals-using-dna.html
The FBI and DNA. (2011). Retrieved April 17, 2013 from http://www.fbi.gov/news/stories/2011/november/dna_112311
The History of DNA. (n.d.). Retrieved April 17, 2013
from http://www.forensicscience.ie/Services/Forensic-Areas/DNA/The-History-of-DNA/
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