encoded evidence: dna in forensic analysis

14 Pages·2005·0.46 MB·English

REVIEWS
ENCODED EVIDENCE: DNA IN
FORENSIC ANALYSIS
Mark A.Jobling* and Peter Gill‡
Abstract | Sherlock Holmes said “it has long been an axiom of mine that the little things are
infinitely the most important”, but never imagined that such a little thing, the DNA molecule, could
become perhaps the most powerful single tool in the multifaceted fight against crime. Twenty
years after the development of DNA fingerprinting, forensic DNA analysis is key to the conviction
or exoneration of suspects and the identification of victims of crimes, accidents and disasters,
driving the development of innovative methods in molecular genetics, statistics and the use of
massive intelligence databases.
Forensic science (known in some countries as legal analysing degraded DNA from stored swabs or micro-
DNA FINGERPRINTING
Generation ofa pattern of medicine) is a specialism that aims to help judges and scope slides.Victims ofmass disasters such as air crashes,
bands,by Southern blotting and juries solve legal issues,not only in criminal law but also where physical identification might be impossible,can
hybridization with a multilocus in civil cases.The field has great breadth,crossing the be identified unambiguously in days.However,as pow-
probe,which is highly
boundaries between biology,chemistry,physics and erful as DNA analysis is,it is far from being the sine qua
individual-specific.
mathematics,and including disciplines as varied as nonofforensic casework.DNA evidence must always
FORENSIC GENETICS botany and ballistics,and the analysis offingerprints, be considered within the framework ofother evidence
The application ofgenetics for ear-prints,recorded sound and handwriting.Over the ofmany types,and the role ofthe forensic geneticist is
the resolution oflegal cases. past 20 years,however,one particular biological tool has not to make presumptions ofguilt or innocence,but to
revolutionized forensic investigations — the analysis of provide unbiased information to judge and jury.
PATERNITY TESTING
Determining whether or not a DNA.As all living things contain DNA,and all DNA We concentrate here on the analysis ofhuman DNA,
particular man is the father ofa exhibits variability both among and within species,any including a discussion ofrecent massive forensic cases
child,using genetic analysis.This biological material associated with a legal case carries in following wars and disasters.However,we also describe
generally uses similar autosomal
it information about its source. applications ofnon-human DNA analysis,in particular
markers to individual
identification work. In this review,with the twentieth anniversary appr- the use ofanimal and plant DNA-typing and the field of
oaching ofthe development ofDNA FINGERPRINTING1,2— ‘microbial forensics’,which has expanded as a response
the first molecular genetic forensic technique — we take to the threat ofbioterrorism.Finally,we ask what the
*Department ofGenetics, this opportunity to present an overview ofthe field. future holds for FORENSIC GENETICS,including a considera-
University ofLeicester,
DNA analysis has evolved to become an indispensable tion ofnew technological developments and ethical
University Road,
Leicester LE1 7RH, and routine part ofmodern forensic casework,employ- issues arising from expanding DNA databases.PATERNITY
United Kingdom. ing extremely sensitive PCR-based techniques to analyse TESTING(reviewed in REF.3) forms part ofthe field of
‡Forensic Science Service, biological material.Suspects can be linked to crime forensic genetics and is ofgreat importance in civil and
Trident Court,
scenes,or one crime scene to another,using DNA evi- immigration cases,but owing to space restrictions we
2920 Solihull Parkway,
dence from as little as the saliva on a cigarette butt,skin cannot discuss it here.
Birmingham Business Park,
Birmingham B37 7YN, cells on a steering wheel or pet hairs on clothing.Large
United Kingdom. DNA databases can be rapidly interrogated for matches The evolution of forensic genetics
Correspondence to to DNA profiles found at the scene ofa crime,or even The aim ofthe forensic geneticist is one ofattribution
M.A.J.or P.G.
partial matches to close relatives of a perpetrator. — to identify with as much certainty as possible the ori-
e-mails: [email protected];
[email protected] Undetected ‘cold’cases involving sexual assault can be gin ofa biological sample.The amount ofvariation that
doi:10.1038/nrg1455 solved decades after investigations were begun by is currently accessible in DNA is extremely informative
NATURE REVIEWS |GENETICS VOLUME 5 |OCTOBER 2004 |739
REVIEWS
Timeline |Developments in forensic genetics
First commercial First Y-STR described, and
forensic PCR kit - used in casework – acquittal
First genetic First use of DNA in detecting SNPs at of rape (Germany)135
polymorphism, Locard lays (1960s–1980s) a criminal case the polymorphic Publication of National
human ABO down principle Multilocus DNA (identification of HLA-DQA1 locus First commercial First mass Research Council
blood groups, that ‘every fingerprinting developed double-murderer by dot blot and forensic STR disaster case report ‘The Evaluation
discovered by contact leaves by Jeffreys1,2; followed by Colin Pitchfork, in oligonucleotide profiling kits (Waco, of Forensic DNA
Landsteiner4 a trace’ single-locus profiling (SLP) Leicestershire, UK) hybridization developed Texas)95,121 evidence’137
1900 1915 1920 1960 1984 1986 1987 1988 1991 1992 1993 1995 1996 1997
First antibody test (1920s–1950s) Discovery Discovery and use New York vs. First useful First use of mtDNA UK National DNA DNA profiling from
for blood group and use of other blood of electrophoretic Castro judgment polymorphic in casework (UK) Database established touched objects138
(ABO) introduced groups and serum variants of red blood on admissability human STRs (STR profiles) and single cells139
and used by Lattes proteins (e.g. MNSs cell enzymes (e.g. of DNA evidence characterized134 Publication of demonstrated
system, Rhesus, Lewis, phosphoglucomutase, led to strict National Research
Kell, haptoglobin)132 glyoxylase)133 quality control Council report ‘DNA
technology in
See also REF.6for further general background. HLA, human leukocyte antigen; mtDNA, mitochondrial DNA; forensic science’136
STR, short tandem repeat.
and the degree ofcertainty can be correspondingly high. sequences’between different minisatellite loci allowed
However,it was not always this straightforward. probes to detect many independent minisatellites simul-
taneously, yielding the hypervariable multi-band
Early markers.The evolution offorensic genetics has patterns known as DNA fingerprints2,5.Using only a
been driven by the analysis ofhuman genetic variation, single probe,the match probability was estimated to
beginning more than a century ago with Karl be <3 ×10–11and two probes together gave a value of
Landsteiner’s discovery4 of the human ABO blood <5 ×10–19(REF.2)— so low that the only individuals
group polymorphisms and his early realization that this sharing DNA fingerprints are monozygotic twins.At the
variation was applicable to solving crimes.The TIMELINE same time,a method known as DIFFERENTIAL LYSISwas devel-
summarizes the important developments that have oped5that selectively enriched the sperm concentration in
occurred since that time.It is noteworthy that even a vaginal fluid/semen mixtures,thereby avoiding the prob-
simple genetic system such as ABO can be used to show lem ofthe victim’s DNA (which is in great excess) mask-
conclusively that a sample did notcome from a specific ing the rapist’s.This is the only protocol to have remained
person —to prove an ‘exclusion’.However,showing that unchanged throughout the past 20 years.
the sample actually didcome from another specific per-
son is more difficult and depends on the degree ofvari- Single-locus probes.Although use ofDNA fingerprint-
ation revealed by the typing system.Until the 1980s, ing persisted for some years in paternity testing,crimi-
serological and protein electrophoretic methods were nal casework soon concentrated on the use ofspecific
used to access diversity in blood groups and polymorphic cloned minisatellites — ‘single-locus probes’(SLPs) —
proteins,but the main drawback ofthese markers was that each revealed only a single,highly polymorphic,
that they tended to rapidly degrade or were compromised restriction fragment length polymorphism,therefore
by bacterial enzymes.In addition,they showed relatively simplifying interpretation.Typically,four SLPs were
MATCH PROBABILITY
low variability and informativeness;when eight systems used successively to probe a Southern blot,yielding
The chance oftwo unrelated
people sharing a DNA profile. were used together to analyse a bloodstain,the probability eight hypervariable fragments per individual.
oftwo unrelated people sharing a combination (the It was with SLPs that the first DNA-based criminal
MINISATELLITES MATCH PROBABILITY,Pm) was ~0.01–0.001,but for other investigation was carried out;this case,culminating in
Loci made up ofa number
body fluids,such as semen,not all markers were present the conviction ofColin Pitchfork for a double rape and
(~10–1,000) oftandemly
and so the match probability was greater.Also,contami- homicide in Leicestershire in 1986,encapsulated many
repeated sequences,each
typically 10–100 bp in length. nation of body fluid from one person by that from of the defining characteristics and virtues of DNA
Usually GC-rich and often another (a ‘mixed stain’),such as in rape cases,was diffi- analysis.First,the two killings,spaced three years apart,
hypervariable. cult to resolve because the vaginal cellular component were shown to have been committed by the same indi-
tended to mask the contribution from the sperm. vidual,because SLP profiles (and DNA fingerprints)
DIFFERENTIAL LYSIS
A method to enrich for sperm from the crime scenes matched.Second,a suspect who
DNA in a mixture ofsperm and DNA fingerprinting.The DNA revolution began in 1984 had confessed was excluded because his SLP profile and
epithelial cells by preferentially with the discovery,by Alec Jeffreys in Leicester,UK,of that found on the victims did not match,demonstrat-
lysing the latter using detergent hypervariable loci known as MINISATELLITES1.These were ing the power ofDNA to exonerate innocent people.
and protease,so that sperm
detected by hybridization ofprobes to Southern blots of Third,the first ever ‘mass screen’was organized by the
nuclei can be recovered by
centrifugation. restriction-enzyme-digested genomic DNA.Shared ‘core Forensic Science Service,in which all 500 local men not
740|OCTOBER 2004 |VOLUME 5 www.nature.com/reviews/genetics
REVIEWS
Marker Source of Advantages Disadvantages
types variation
Autosomes STRs Independent Extremely Very degraded
chromosomal high DNA difficult
TPOX assortment; discrimination to type
recombination; power
D3S1358
mutation
D5S818 HUMTH01
HUMFIBRA/ D7S820
D2S1338 FGACSF1PO D8S1179
SNPs Independent Usable on Mostly biallelic,
chromosomal very degraded so relatively low
AMELX
HUMVWA sex-test assortment; DNA discrimination
recombination; power; mixtures
D13S317 mutation - difficult to resolve
D16S539 D18S51D19S433 D21S11 but low rate
Y chromosome Pseudoautosomal X
(one copy per region 1(2.6 Mb) DYS393
M(10ito0csh poenrd creiolln) mSlFlaooBGccleIuuM Cssc OePllDlu)sIS™ combining region Non-recombiningeuchromatin ( 23 Mb) ADDDDDDYCMYYYYYYSSSSSSAE133333ILIY998989-19052sI,eIIx-test STRs Monulytation Msummpsaaiexelltefecuu--ifrlf ieecinsmitya;le Rdpwpiooeistwslhcasrietniiimbvr ;pel eisnaly hapt rlatoioilroipnwinnueg;lation
e
Nucleus liYon- cSbuTosRt hused Presgeiound o2a (u0t.o3s2o Mmbal)Non-r Hetero-chromatin structure problems
mtDNA (several copies Control region
per mitochondrion) (1.1 kb)
SNPs, Mutation High copy Heteroplasmy;
HVS I HVS II
(341 bp) (268 bp) usually only number, low discrimination
in control therefore power; sharing
region good survival within matriline;
in old/damaged possible population
16,569 bp samples structure problems
Blood stain
Figure 1| Sources of human genetic variation used in forensic analysis.Further details of the properties of different loci can
be found in the text. Heteroplasmy describes the presence of two or more different mitochondrial DNA sequences in the same cell,
or individual. FBI CODIS, US Federal Bureau of Investigation Combined DNA Index System; HVS, hypervariable site; Mb,
megabase; mtDNA, mitochondrial DNA; SGM, second generation multiplex; STR, short tandem repeat.
eliminated by a preliminary test for conventional protein PCR-based methods.DNA amplification by PCR pro-
markers were recruited for DNA testing.Pitchfork vided an enormous increase in sensitivity,allowing
showed prescience in realizing the power ofDNA analy- minute amounts ofdegraded DNA to be analysed,and
ISO17025
A global standard,established by sis:he attempted to evade the screen,but his evasion was now forms the basis ofall forensic DNA typing.Early
the International Organization detected,and when his profile was shown to match those PCR-based systems targeted a small number ofSNPs
for Standardization,for the from the crime scenes,he pleaded guilty to the killings. in the HLA-DQA1 GENE7.Although these systems were use-
technical competence of
Methods in forensic genetics must surmount several ful when the SLP technology failed,discriminating
calibration and testing
laboratories (see Online links hurdles before being applied to casework.First,tech- power was low and mixtures were difficult to interpret.
box). niques must be adapted to work on samples that are not Consequently,there was a period when both PCR and
pristine and are often limited in quantity.Second,exten- SLP tests were done in parallel.It was the discovery of
HLA-DQA1 GENE sive validation is required to demonstrate robustness, SHORT TANDEM REPEATS(STRs),discussed in the next sec-
A polymorphic gene within the
to pass the test ofadmissibility in court6(admissibility tion,together with the introduction ofautomated
MHC class II region on
chromosome 6,encoding a criteria and legislation differ between countries and even sequencing technology,that led to the current power-
human leukocyte antigen cell- among states in the United States).Finally,quality man- ful systems for individual identification.Subsequently,
surface protein. agement systems must be implemented once processes the use ofSTRs supplanted both the early PCR and
are introduced into casework:external accreditation of SLP tests worldwide once their advantages ofhigh dis-
SHORT TANDEM REPEAT
A DNA sequence containing a forensic laboratories to internationally recognized stan- criminating power,sensitivity and ability to resolve
variable number (typically ≤50) dards (such as ISO17025) is a prerequisite.Because ofthese simple mixtures were realized.In addition,the time
oftandemly repeated short (2–6 constraints,adoption ofnew technical developments can needed to carry out an analysis was greatly reduced.
bp) sequences,such as (GATA);
n sometimes be slow.For example,SLP technology was still Reduction ofcosts resulting from partial automation
forensic STRs are usually
widely in use long after methods based on the polymerase paved the way for the creation ofnational STR DNA
tetranucleotide repeats,which
show little PCR stutter. chain reaction (PCR) were developed in 1988. databases.
NATURE REVIEWS |GENETICS VOLUME 5 |OCTOBER 2004 |741
REVIEWS
Table 1 |International coordinating bodies in forensic genetics
Organization/subgroup Purpose Web site
International Society of Forensic Genetics International organization promoting scientific http://www.isfg.org/
(ISFG) knowledge in forensic genetics
• DNA Commission • Makes recommendations for use of DNA markers http://www.rechtsmedizin.uni-
• European DNA Profiling (EDNAP) group • Harmonization of European DNA technologies mainz.de/Remedneu/ednap/ednap.htm
• Paternity Testing Workshops
European Network of Forensic Science Mainly represents government institutions; www.enfsi.org/
Institutes (ENFSI) coordinates efforts to develop European DNA
databases
American Academy of Forensic Sciences Academic body for north American forensic http://www.aafs.org/
(AAFS) scientists
Federal Bureau of Investigation (FBI) Responsible for setting standards, training and http://www.fbi.gov/publications.htm
• Scientific Working Group DNA Analysis development of the national DNA database
Methods (SWGDAM)
National Institute of Science and Supports the forensic community by organizing http://www.nist.gov/
Technology (NIST) collaborative proficiency exercises
• STRBase • Database giving characteristics of forensically http://www.cstl.nist.gov/biotech/
useful short tandem repeats (STRs) and SNPs strbase/index.htm
Current methods in human identification is mostly owing to active collaboration between interna-
Human forensic casework is now done using commer- tional groups that are coordinated under various acade-
cially developed autosomal STR multiplexes (single-tube mic and government-sponsored institutions (TABLE 1).
PCR reactions that amplify multiple loci);other sources Recommendations on standard practice,quality issues
ofgenetic variation that find more specialized uses are and collaborative activities are made at a global level.
autosomal SNPs,and markers on the Y chromosome
and mitochondrial DNA (mtDNA) (FIG.1).Differences Autosomal STR profiling.The first widely used multi-
in practice between jurisdictions are considerable owing plex (the ‘quadruplex’8) consisted offour SIMPLE STRS.
to historical,social and legal circumstances.Detailing However,because it had a high match probability of~1
these is beyond the scope ofthis article,so we take a pre- in 10,000,the first criminal cases involving autosomal
dominantly UK perspective here.However,despite these STR profiling were reported in conjunction with SLP
differences,the rapid development and universal accep- profiling.Subsequent addition oftwo highly variable
tance ofnew DNA-based technology in forensic genetics COMPLEX STRSdecreased the match probability to ~1 in
Box 1 |Evaluating the weight of DNA evidence
The evidential weight ofa match between crime stain profile and suspect is quantified by the match probability (Pm);
the chance oftwo unrelated people sharing a profile.For independently inherited loci,Pmis calculated by multiplying
the individual allele frequencies in the profile in question (the ‘product rule’):the greater the number ofloci,and the
greater the heterozygosity ofeach locus,the lower the value ofPm.However,there are a number ofsituations in which
Pm can be substantially increased:
• ifthe profile is partial because ofdegradation,reducing the number ofinformative loci;
• ifa suspect and a perpetrator share many alleles by descent (for example,are brothers);
• ifa suspect and a perpetrator originate from the same subpopulation.
POPULATION STRUCTUREcan cause frequencies ofalleles (and hence profiles) to vary between subpopulations — an issue
that caused great controversy in the application ofSLP profiling90.The debate was resolved by applying guidelines to
ensure match probabilities quoted in court were conservative (that is,favourable to the defendant).Similar conservatism
is now applied to STR profiles91.
POPULATION STRUCTURE Despite the high discriminating power ofvery low Pmvalues,interpretation in the courtroom has not been without
The absence ofrandom mating
controversy92,and this is because ofthe way that DNA evidence is sometimes presented.A wellknown example is the
within a population,leading to
‘prosecutor’s fallacy’,or ‘fallacy ofthe transposed conditional’:suppose a crime is committed in London (population ~7
allele frequency differences
among subpopulations. million) and a crime-scene profile is obtained that has a Pmof10–6.The prosecutor,finding that a defendant matches the
profile,might say:‘The odds are a million to one in favour ofthe defendant being guilty.’But,given the population size,
SIMPLE STRS ~7 people in the city are expected to match the profile,so it can then be argued that the odds are actually 7 to 1 in favour
Short tandem repeat loci ofinnocence;however,this ‘defence fallacy’unrealistically assumes that each ofthe 7 people has equal probability of
composed ofuninterrupted runs
guilt,which is untrue as DNA evidence is not used in isolation.This problem oflogic can be avoided by an approach
ofa single repeat type.
based on a likelihood ratio,using conditional probabilities based on prosecution and defence scenarios:the job ofthe
COMPLEX STRS court,based on an evaluation ofboth DNA and non-DNA evidence,is to decide the ultimate issue ofguilt or innocence,
Short tandem repeat loci given allofthe evidence.Under complex scenarios with many different variables,‘Bayesian networks’93,94— intuitive
containing more than one run of graphical means to display hypotheses regarding the probabilistic relationships between variables — are a powerful aid
repeats that can be ofone or to understanding,although they are not intended to supplant the role ofthe jury.
more repeat type.
742|OCTOBER 2004 |VOLUME 5 www.nature.com/reviews/genetics
REVIEWS
Molecular weight (bp)
80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320
a
Amelogenin D8S1179 D21S11
D18S51
X Y 1314 2930 16 18
D3S1358
HUMVWA D16S539 D2S1338
1617 15 18 1112 1819
d
D19S433 HUMTHO1 HUMFIBRA/FGA yiel
nt
e
c
14 6 9.3 15 18 s
e
or
b Flu
Amelogenin D8S1179 D21S11 D18S51
X Y 1213 15 28 303132.2 1314
c
Amelogenin D8S1179 D21S11 D18S51
X Y 10 12 28 30 13 18
Figure 2| Electropherograms illustrating autosomal STR profiles. a| An electropherogram of the second-generation multiplex
‘SGM Plus’ profile from a male, including X- and Y-specific amelogenin products of 106 and 112 bp, respectively. Most short tandem
repeats (STRs) are heterozygous and the alleles are evenly balanced. Numbers beneath STR peaks indicate allele sizes in repeat
units. The STR profile is displayed in the green, blue and yellow channels of a four-colour fluorescent system, with the red channel
being used for a size marker (not shown). b| A typical mixture from two individuals (green channel only shown). Mixtures can only be
identified if the alleles of the minor component are above the background ‘noise’ in an electropherogram (in practice a ratio of ~1:10)
and can usually be resolved by inspection95. In this example, the contributions are in even proportions — for example, D21S11
shows four alleles where the peaks are approximately equal in height, whereas D18S51 shows two peaks in a 3:1 ratio. The X- and
Y-specific amelogenin peaks are of approximately equal height, indicating that this is a mixture from two males. More complex
mixtures might require interpretative methods based on likelihood ratios96,97, calculations based on peak area98and consideration of
PCR stutter99, and have led to the development of computer programs to deduce potential underlying genotypes100,101. c| A profile
(green channel only shown) obtained by ‘low copy number’ (LCN) testing102, a method used in the United Kingdom when little DNA
(typically <100 pg or <17 diploid genomes) is available. The method uses an increased number of PCR cycles (for example, 34,
rather than the usual 28), and leads to marked heterozygote imbalance at some loci (here, D21S11 and D18S51) because of
stochastic variation in amplification. Extreme imbalance is drop-out (failure to amplify) of an allele (not shown). Drop-out is a
stochastic effect of the limited number of template molecules, and therefore no two amplifications of the same extract will behave
the in the same way. The LCN protocol requires duplicate PCR of an extract and only those alleles that are observed in both PCRs
are reported. (See also BOX 2.)
50 million.This ‘second-generation multiplex’(SGM) used in the United States11,but to improve efficiency,
also included a PCR assay targeted at the XY-homolo- new multiplexes that amplify 16 loci in a single reaction
gous amelogenin genes9,thereby revealing the sex ofa (including amelogenin) have also been introduced12.In
sample donor.It became clear that STRs were more Germany,eight loci are used,including the ACTBP2
sensitive than other methods and allowed unambigu- locus13.International collaborations have recommended
ous assignment ofalleles,making the method suitable core loci to facilitate international data exchange — for
for the development ofdatabases.In 2000,an addi- example,a set ofseven for common use in Europe14.
tional four loci were added to the multiplex,which Detailed information on most forensic STR markers
was renamed SGM Plus10,thereby reducing the match can be found at STRBase(see Online links box) and
probability to less than 10–13.The above account tracks in REF.15,which also describes the technology ofSTR
developments in the United Kingdom;events elsewhere typing.
unfolded differently,and globally there are now a num- The match probabilities obtained with STR multi-
ber ofdifferent systems that nonetheless have many loci plexes are so low that their reciprocals vastly exceed the
in common.The US FBI CODIS (Combined DNA entire human population.However,although DNA
Index System) contains 13 STRs plus the amelogenin sex profiling is often referred to as ‘individual identification’
test (FIG.1),with a match probability lower than that of and assessments can be made whether or not a DNA
the UK system.Typically,two separate multiplexes are profile is unique in an unsampled population16,it would
NATURE REVIEWS |GENETICS VOLUME 5 |OCTOBER 2004 |743
REVIEWS
be a rare case in which everyone on the planet could be account fragment size (accurately measured with
ALLELIC LADDER
An accurate marker used to considered as the pool ofpotential contributors for a respect to internal standard markers and to an ALLELIC
identify alleles at a particular crime-scene sample17.Reporting ofDNA evidence in LADDER19used to identify alleles),ELECTROPHEROGRAMpeak
STR,generated by PCR court takes a conservative approach to the low match height and area and an assessment of HETEROZYGOTE
amplification ofa series of
sequenced alleles from that STR. probabilities and commonly uses likelihood-based BALANCE,and include automated checks to interpret arte-
methods to take account ofthe factors applying to a facts such as PCR STUTTER20.DNA quality from ‘reference
ELECTROPHEROGRAM particular case (BOX 1). samples’(taken from individuals to create databases —
The graphical output of Multiplexes are analysed and typed using automated see next section) is predictably good and makes
electrophoresis devices in STR
sequencing equipment.These are typically multi-chan- automation of typing and interpretation relatively
and sequencing analysis,
nel capillary electrophoresis systems that are used to straightforward.In the UK Forensic Science Service,if
showing fluorescence intensity as
a function ofmolecular weight; detect fluorescently labelled PCR products (FIG.2a)and two expert systems working on different principles
peak at a particular wavelength are combined with robotics and laboratory information agree on a profile it is accepted,but there are regular
(colour) corresponds to a management systems,including bar-coding ofsamples challenges with ‘blind’controls.For casework samples,
specifically labelled molecule
to reduce operator errors.This automation reduces cost preliminary assessment is vital to determine the best
ofa particular size.
and increases throughput.Interpretation (defining method ofprocessing;but automation is more difficult
HETEROZYGOTE BALANCE the alleles in a profile) is more difficult to automate. because DNA quality and quantity are variable and
The proportion ofthe two alleles However,there has been progress in converting tradi- DNA mixtures are often encountered,complicating
ofa heterozygote,expressed as
tional subjective expert opinion into programmable interpretation.Anomalous profiles can also arise for
the area ofthe smaller peak
(heuristic) rules for computer programs (known as biological reasons (such as mutation) and ifmethods
divided by the area ofthe larger
peak in an electropherogram. ‘expert systems’18),generally intended to complement, are used that are sensitive enough to detect single DNA
rather than replace,the human expert.These take into molecules (‘low copy number’),then laboratory-based
contamination ofsingle or multiple alleles is a strong
possibility and interpretation strategies are needed to
deal with this.These situations are discussed in BOX 2.
Box 2 |Dealing with anomalous autosomal STR profiles
STR-based forensic DNA analysis has achieved
As shown in FIGURE 2,several factors can complicate the interpretation ofDNA profiles. worldwide public and professional acceptance as a reli-
These include mixed samples and the availability ofonly small amounts ofDNA,which able means ofindividual identification and has had a
(in the UK) is analysed by ‘low-copy number’(LCN) methods and can lead to allele major impact on criminal justice systems.The increase
drop-out.LCN analysis also increases the probability ofcontamination (additional in sensitivity ofDNA methods has allowed the reopen-
‘foreign’alleles in the profile),despite stringent precautions to prevent it. ing and solving of‘cold’cases and has also led to the
Contamination can be gross and lead to full additional profiles,where typically exoneration ofprisoners (some ofwhom were awaiting
the negative control will be affected;these can be searched against forensic staff execution) convicted through miscarriages ofjustice.
elimination databases and stored in a separate database to allow detection and Most ofthese cases have been in the United States (see
monitoring ofproblems such as contamination ofplasticware during the Innocence Projectweb site in the Online links box),
manufacture103,104.Alternatively,contamination can contribute an extra one or two where post-conviction testing attracts federal funding,
alleles per DNA profile (allele DROP-IN102,104),where the negative control is usually although one case in the United Kingdom has been
unaffected.The probabilities ofa match,ofdrop-out and ofcontamination (based
described21.
on computer simulation analysis ofnegative control data104) can be readily
incorporated into the calculation ofthe likelihood ratio (LR) when the significance
DNA databases ofautosomal STR profiles.As well as
ofthe DNA evidence is being assessed102;calculations are complex but are aided by a
matching a crime-scene profile to that from an appre-
computer program (LoComatioN)105that can also combine the results ofseveral
hended suspect,a match (‘hit’) can be made to a DNA
profiles into a single LR.Here,we no longer think ofa DNA profile either matching
database ofoffender profiles (an ‘intelligence database’),
or not matching a suspect,because the twin effects ofdrop-in and drop-out will
allowing a new suspect to be investigated.The advances
alter the suspect’s apparent profile so that it does not match the crime scene sample,
in automation described above have allowed the estab-
without indicating an exclusion;rather,the probability ofthe evidence is lowered.
lishment ofsuch databases,which are now in use or
Anomalous profiles can arise from causes other than mixtures but can readily be
resolved by careful analysis: under development in many countries.Criteria for
• Mutations in the PCR primer target region can cause allele drop-out or heterozygote including a profile in a database vary among countries
imbalance106,107. (TABLE 2).The largest example,the UK National DNA
Database22,23,contains (at July 25th 2004) ~2.5 million
• An STR can be duplicated and segregated in a normal Mendelian fashion or a somatic
reference profiles (mostly from buccal scrapes) and
STR mutation occurring early in development can lead to a three-peak profile for the STR,
which might vary between tissues;examples are on STRBase (seeOnline links and REF.108). ~200,000 crime-scene profiles.Since 1995,there have
been more than 550,000 matches between reference
• The STR D21S11 can reveal three alleles in trisomy 21 cases (reviewed in REF.109).
profiles and crime-scenes, and more than 30,000
• Discordant results in the amelogenin sex test can be observed in rare individuals,
matches between crime scenes.A new method ofusing
including cases ofsex-reversal (XX males and XY females) and males110–112carrying
the database,‘familial searching’,has recently been intro-
deletions that remove AMELY.
duced.In 2003,a lorry driver was killed in Surrey,UK,
• A true mixed profile can also originate from a single individual.Analysis ofDNA from by a brick thrown through his windscreen.A DNA pro-
people who had undergone successful bone marrow transplants ≥5 years previously113,
file was obtained from the brick,but had no match in
showed a mixed profile in buccal and fingernail samples;in blood,the recipient’s own
the database.A geographically restricted search ofthe
profile had been completely replaced by that ofthe donor,whereas in hair,the recipient’s
database was carried out for potential close relatives of
profile remained unmixed.
the perpetrator, under the assumption that close
744|OCTOBER 2004 |VOLUME 5 www.nature.com/reviews/genetics
REVIEWS
Table 2 |Characteristics of some national DNA databases
Country (Year Reference Crime-scene Suspect to Scene to Entry criteria Entry criteria for Removal criteria
established) profile size sample size scene hits scene hits for suspects convicted offenders
UK 2.5 million 200,000 550,000 30,000 Any recordable Entered as suspect Never removed,
(1995) offence* including suspects
USA 1.52 million 67,000 Figure Figure No suspects entered, Depends on Depends on state
(1994) unavailable unavailable but under revision state law law
Germany 286,840 54,570 13,700 5,500 Offence leading to After court decision After acquittal or 5–10
(1998) >1 yr in prison years after conviction,
if prognosis is good
Austria 64,740 11,460 3,200 1,350 Any recordable Entered as suspect Only after acquittal
(1997) offence*
New Zealand 44,000 8,000 4,000 2,500 No suspects entered A relevant offence Never removed, unless
(1996) (including ≥7 yr in prison) conviction quashed
Switzerland 42,530 7,240 4,840 5,540 Any recordable Entered as suspect After acquittal or 5–30
(2000) offence* years after conviction
France 14,490 1,080 50 70 No suspects entered Sexual assault and 40 years after
(2001) serious crime conviction
Finland 8,170 5,450 2,080 780 Offence leading to Entered as suspect Only after acquittal
(1999) >1 yr in prison
Slovenia 4,820 2,360 370 80 Any recordable Entered as suspect Depends on severity
(1998) offence* of crime
Netherlands 4,260 13,700 2,520 4,260 No suspects entered‡ Offence leading to >4 yr 20–30 years after
(1997) in prison conviction
Sweden 3,980 9,860 2,500 4,750 No suspects entered Offence leading to >2 yr 10 years after release
(2000) in prison from prison
*That leads to a term of imprisonment. ‡Except when the suspect’s DNA is tested for the case. Adapted from REF.140, with additional information from Peter Schneider and
Jill Vintiner (personal communications). See also BOX 3.
relatives are more likely to share alleles than unrelated international.However,although large databases with
people (50% for brothers).This highlighted 150 candi- permissive entry criteria can be powerful,they also raise
dates,leading to the identification ofa suspect whose ethical questions (BOX 3).
profile matched that on the brick.He was convicted of
manslaughter24. Autosomal SNP typing.Compared to STRs,SNPs have
Legal differences between countries make such much lower heterozygosities (a per-SNP maximum of
speculative searches impossible in some jurisdictions 0.5) and so ~50 SNPs are required to approach the low
and can also complicate the international exchange match probability ofan STR profile25(see BOX 1).In
ofdata when crime is itselfbecoming increasingly addition,mixtures are especially difficult to resolve for
Box 3 |DNA databases: ethical issues
DNA databases are seen by some as without fault114,but they have not been without their critics115,116.Any criminal
whose profile is in a database risks detection in further criminal activity — the probability ofidentifying a suspect when a
crime-scene profile is checked against the UK database is >40%23.Culprits are apprehended more quickly and criminals
might be deterred from future offending,but it is unclear whether overall crime rates are reduced.The UK database is
projected to reach 5 million samples22,which is ~10% ofthe population and >30% of10–50-year-old men (those most
likely to offend30).This enormous size is a result ofthe relatively liberal criteria for database entry:most samples are taken
(with or without consent) from individuals arrested for offences that could lead to a prison sentence,whereas in some other
jurisdictions only more serious offences are considered and a specific degree ofconnection (decided by a court) between the
suspect and the offence is necessary before DNA can be sampled (TABLE 2).One justification given for the United Kingdom
practice is that minor criminals might also be perpetrators ofmore serious crimes — one example is the arrest and
subsequent conviction,in 2001,ofa man for a murder committed in 1968,triggered by a match between crime-scene DNA
PCR STUTTER
evidence and a sample taken from the culprit in connection with a motoring offence114.Recent legislative changes allow
A PCR artefact in which,as well
as a band ofthe expected size,an retention ofprofiles from exonerated suspects and from individuals profiled during mass screens (albeit with consent).
additional band is seen which is The use ofany database involves a balance between the rights ofthe individual and the interests ofthe state;this differs
typically one repeat unit smaller, from country to country,and some believe it is tipped too far towards the state in systems like that ofthe UK117.However,the
resulting from slippage synthesis discoverer ofDNA fingerprinting,Alec Jeffreys118,has argued for the UK database to be extended to the entire population,
errors by the PCRpolymerase
under the auspices ofan independent authority,arguing that the current database is discriminatory because some groups in
the population are probably overrepresented among suspects.James Watson119has called for a global database to fight crime
DROP-IN
Addition of(typically) one or and terrorism — an enormously costly and complex endeavour,raising serious issues in ethics and law117.
two alleles to a DNA profile, Retention ofsamples for possible retesting with future technologies (as is done in the UK) is also considered
owing to contamination. controversial by some,who argue that it could reveal private genetic information.
NATURE REVIEWS |GENETICS VOLUME 5 |OCTOBER 2004 |745
REVIEWS
Box 4 |The challenge of large human identification cases
DNA analysis has a key role in the identification ofvictims ofaccidents,disasters and wars,therefore aiding
emotional closure for bereaved relatives.Reference material often comes from these relatives3— for example,an
approach used early on in the identification ofthe skeletal remains ofmurder victim Karen Price compared STR
alleles with her presumptive parents120.
An early example ofa mass identification case followed the Waco disaster95,121in 1993,in which more than 70 occupants of
a heavily fortified compound in Waco,Texas,perished in a fire following a siege by US law enforcement agencies.About 40
bodies were unidentifiable by conventional means and,ofthese,26 could be identified by quadruplex STR profiling.
Reference samples came from living or dead (but positively identified) relatives.For instance,given the alleles present in
parents,the possible genotypes ofchildren could be sought among profiles ofvictims (taking account ofthe possibility of
STR mutations between the generations),and the weight ofidentification evidence considered on a likelihood basis (BOX 1).
Air crashes,such as the August 1996 Spitzbergen disaster122or the September 1998 Swissair flight 111 air crash123,
present problems because ofthe level ofdamage to bodies from fragmentation and burning.However,given an accurate
list ofpassenger and crew,the site can be considered a ‘closed scene’,where there are no unknown victims;families are
often among the dead,so allele sharing is expected.Reference profiles can be obtained from personal effects ofvictims
such as clothing or toothbrushes123,and samples provided by relatives,leading to complete identification.
Without doubt the largest mass identification cases are those arising from wars and genocides,such as that in the
former Yugoslavia,where >30,000 people went missing;in Bosnia-Herzegovina alone >10,000 bodies required
identification (reviewed in REF.124).On a comparable scale,and with unique challenges,has been the effort to identify the
remains ofthe estimated 2,819 people who died in the World Trade Center terrorist attacks ofSeptember 11th,2001
(reviewed in REF.125).Physical identification was impossible for most victims and mixed profiles were common126.
Technical developments during the identification process have included:
• new computer programs to deal with large and complex kinship calculations,involving reference samples from ~6,000
relatives and ~5,000 personal effects127,128;
• novel DNA extraction procedures129;
• redesigned STR multiplexes (‘miniSTRs’,based on shorter DNA amplicons27) and autosomal SNP multiplexes to allow
analysis ofseverely degraded DNA125.
Despite these efforts,the remains of~1,000 people might never be identified125.
binary markers,although this limitation might be male–female body-fluid mixtures where conventional
overcome by targeting rare tri-allelic SNPs26. methods fail to resolve autosomal profiles,Y-STR typing
The practical advantage ofSNP typing is that DNA can give specific information about the male compo-
template size can in principle be only as large as a pair of nent.Although differential lysis often allows autosomal
specific primers;~50 bp.This is considerably smaller profiling of a rapist,the vasectomized or naturally
than the ~300 bp needed for successful STR profiling azoospermic rapist leaves no sperm;in such cases31,
(although special STR multiplexes have been developed Y-specific profiling is effective,even in the presence of
that use particularly small amplicons27) and makes SNPs a 4,000-fold excess offemale DNA32.In multiple rape
ofinterest for the analysis ofseverely degraded material. it might be possible to gain information about the
The technical challenges ofthe World Trade Center dis- number ofassailants.
aster (see BOX 4) have led to the application offorensic There are 219 known useful STRs on the Y chromo-
SNP typing.The European Network ofForensic Science some33,but a set of9 or 11 loci is commonly typed in
Institutes (ENFSI) and the US FBI Scientific Working casework,and there is a large collaborative quality-con-
Group on DNA Analysis Methods (SWGDAM) (TABLE 1) trolled online population database ofmore than 24,000
working groups are assessing potentially useful multi- 9-locus profiles from 200 populations29,34.Clearly,the
plexes and will make recommendations for global stan- product rule (see BOX 1) for independently segregating
dardization28,although it is hard to imagine that SNP autosomal STRs cannot be applied to markers on the
profiling will replace STR-based systems. non-recombining Y chromosome and haplotype fre-
quencies are instead often determined simply by
Y-chromosomal analysis.Autosomal STR profiles owe counting or by more sophisticated BAYESIANmethods35.
their variability to three processes:independent chro- Excluding STR mutation, all patrilineal relatives
HAPLOTYPE
The combination ofallelic states mosomal reassortment,recombination and mutation. (brothers,father,sons,paternal uncles and so on) ofa
ofa set ofpolymorphic markers On the Y chromosome,mutation alone functions to suspect will share his Y haplotype and this needs to
lying on the same DNA diversify STR HAPLOTYPES.These haplotypes are therefore be considered when assessing the strength ofthe evi-
molecule,such as the Y
less diverse than autosomal profiles (genotypes) con- dence36.Furthermore,the potential association ofsur-
chromosome or mtDNA
taining an equivalent number ofmarkers,leading to rel- names inherited through the paternal line37 with
BAYESIAN atively high average-match probabilities of~0.003 for Y haplotypes has led to suggestions that surname pre-
Statistical method,based on 11 Y-STRs29.However,Y chromosomes have one crucial diction from haplotype might be possible.However,the
Bayes’theorem,that allows forensically useful property:they are confined to males. complexity ofthe relationship38probably precludes this
inferences to be drawn from
As most serious offences are committed by men30,we approach as an absolute determinant,although it might
both the data themselves and
any prior information. expect to find their Y chromosomes at crime scenes;in be powerful ifused in the context ofa Bayesian method
746|OCTOBER 2004 |VOLUME 5 www.nature.com/reviews/genetics
REVIEWS
ofanalysis.On a larger scale,the Y chromosome shows basis ofhighly improbable sequences that are detectable
particularly strong population structure39and the avail- by phylogenetic analysis50.
ability oflocal population databases is essential.These Heteroplasmy can lead to different sequences being
difficulties can complicate the evaluation ofmatch sig- found between hairs or tissues in a single individual,
nificance,but exclusion ofa suspect remains straightfor- and even along the length of a single hair shaft51.
ward.The use ofY-chromosome analysis will increase, Mutation,which distinguishes heteroplasmic types,is
particularly in rape casework,aided by the availability particularly common at some sites (‘hot spots’),but this
ofstandardized commercial Y-STR40and Y-SNP kits can be built into the interpretation using a likelihood
(reviewed in REF.41). ratio approach52.Shared heteroplasmy between two
samples can actually increase the strength ofevidence,
Mitochondrial DNA.mtDNA shares many ofthe the- as was the case in confirming the matrilineal relation-
oretical disadvantages of the Y chromosome:it is ship between the putative Tsar Nicholas II and his
non-recombining,so markers (almost all SNPs,but brother Georgij Romanov53(BOX 5).
including length variation in a run ofC nucleotides) do
not segregate independently,thereby reducing diversity; Putting face and place to a DNA profile
it is uniparentally inherited (through the mother),so all When a profile from a crime scene does not find a ‘hit’
members ofa matriline share a haplotype;and it shows in an intelligence database,any information that can
marked populationstructure42.Furthermore,there is be deduced from the DNA about the donor is useful.
the complication ofheteroplasmy. A basic piece ofinformation,sex,has already been men-
The advantage ofmtDNA lies in its copy number, tioned,but two other areas,population oforigin and
which is between ~200 and 1,700 per cell (reviewed in phenotypic features,have also been investigated and used
REF.43);this means that it has a greater probability of to aid criminal investigations.
survival than nuclear DNA does.Forensic applica-
tions43,44include analysis ofsamples that are old or Deducing population oforigin.Most (~85%;REFS 54,55)
severely damaged,or low in DNA (such as hair shafts), genetic variation is found within human populations.
and include historical criminal cases (see BOX 5). Nonetheless,individuals from different populations are,
The normal practice is to sequence two segments ofthe on average,slightly more different from each other than
CONTROL REGION that are particularly polymorphic, are individuals from the same population,and this
known as hypervariable segments I and II (HVSI, allows sets ofmarkers to be used to predict population
HVSII).SNPs outside the hypervariable segments will oforigin (reviewed in REF.56).Similar methods might be
increase the power ofmtDNA typing45,46. applicable to the analysis ofa crime-scene sample.
Rather than considering the average-match proba- Forensic STR profiles are very variable among
bility (which is high47,at ~0.005–0.025),match sig- individuals and so show low inter-population vari-
nificance is usually evaluated by the ‘counting ance (F ).They are therefore not ideal for predicting
ST
method’— how many times a specific sequence has population oforigin.The ability ofSGM Plus profiles
been observed in a population database48,49,with a to classify individuals into one offive police-defined
correction for sampling error.There has been criti- ‘ethnic groups’has been assessed57,and showed,for
cism ofthe quality ofsome forensic datasets,on the example, that 67% of profiles known to be from
Box 5 |Forensic DNA analysis of historical samples — the case of the Russian royal family
The first DNA-based historical criminal investigation was carried out in 1994 when bones purported to be those ofthe
Russian royal family (the Romanovs),executed by a Bolshevik firing squad in 1918,were analysed using a combination
ofmitochondrial DNA (mtDNA) sequencing,sex typing,STR analysis and PCR cloning130.
The samples were more than 70 years old,yet yielded autosomal STR profiles consistent with the presence ofa family
group and mtDNA sequences matching reference sequences from living matrilineal relatives:the sequence obtained
from Prince Philip,Duke ofEdinburgh,matched those ofthe putative Tsarina and her children,whereas those from the
Duke ofFife and Princess Xenia Cheremeteff-Sfiri matched that ofthe putative Tsar Nicholas II,except for one
discrepant base.After PCR cloning,the putative Tsar’s mtDNA was shown to comprise two different molecules (one of
which matched the living reference samples) and was concluded to be an example ofheteroplasmy,thought at the time
to be rare;this led to speculation about the reliability ofthe results.However,when an independent analysis was carried
out on the remains ofthe Tsar’s brother,Georgij Romanov,by the Armed Forces DNA Identification Laboratory in
Rockville,Maryland53,it was discovered that he shared a heteroplasmy at the same position in the mtDNA molecule;this
effectively dispelled any lingering doubts.After a consideration ofall ofthe DNA and non-DNA (anthropological)
evidence,the Russian authorities pronounced the remains to be those ofthe Romanovs.
The remains ofone ofthe Tsar’s daughters,Anastasia,were absent from the grave,and controversy surrounded the
claim that she escaped execution and survived,under the identity ofAnna Anderson.STR analysis131of20-year-old
CONTROL REGION
Part ofmitochondrial DNA that paraffin wax embedded samples from Anderson was inconsistent with her being a daughter ofthe Tsar and Tsarina.
is non-coding and therefore However,the mtDNA sequences matched those ofCarl Maucher,a putative maternal relative ofa woman named
more able to accumulate Franzisca Schankowska.The mtDNA results were confirmed by an independent group from Penn State University,who
variation than the rest ofthe concurrently analysed hair shafts purported to have come from Anna Anderson131.
molecule.
NATURE REVIEWS |GENETICS VOLUME 5 |OCTOBER 2004 |747
REVIEWS
African-Caribbeans were classified correctly,whereas colour65,for which the Pgene,the product ofwhich is
the remainder were wrongly assigned to other ethnic involved in melanin production,is a candidate66.Two
groups.Despite the misclassification,prediction is amino-acid substitutions in the gene are associated with
useful ifit reduces the number ofsuspect investiga- blue or grey eyes66.A broader association study includ-
tions carried out before the actual perpetrator is ing SNPs in several candidate genes67has identified 61
reached.Policemen are not anthropologists and one SNPs that explain 15% ofthe variation in eye colour in a
problem with interpreting these studies is the over- sample,but probably do not provide useful predictive
simplified way in which populations are defined.The testing.Work on these and other phenotypes will proba-
haploid Y chromosome and mtDNA show strong geo- bly increase in the future.However,the complexity of
graphic differentiation because their small EFFECTIVE these quantitative traits,coupled with variability intro-
POPULATION SIZE(one quarter ofthat ofany autosome) duced by environmental and nutritional differences,
leads to enhanced GENETIC DRIFT. Mating practices means that even ifthe genes influencing them were iden-
might also contribute to inter-population differences. tified there is no guarantee that simple deterministictests
These markers therefore contain information on pop- would emerge.
ulation oforigin,but,owing to ADMIXTURE,can give
misleading results. Non-human species in forensic genetics
Markers with greater power have emerged from stud- Forensic analysis ofanimal DNA has been used both
ies ofadmixed populations for epidemiological purposes when animal material (usually pet hairs) is found at
or for mapping disease genes by LINKAGE DISEQUILIBRIUM. crime scenes,and in investigations ofthe illegal trade in
Autosomal binary or STR loci have been identified endangered species.The best-known example ofthe
that show large allele frequency differences (30–50%) former was the matching,using 10 feline-specific dinu-
EFFECTIVE POPULATION SIZE between parental population groups58,59.Multilocus cleotide STRs,ofcat hairs on a bloodstained leather
The size ofan idealized
genotypes based on such ANCESTRY INFORMATIVE MARKERS jacket with a pet cat,known as Snowball,who lived with
population that has the same
(AIMS) can be analysed using model-based cluster- the suspect in a murder case68.More recently,a com-
properties with respect to
genetic drift as does the actual ing algorithms,yielding individual proportions of mercial kit containing 11 tetranucleotide STRs has been
population in question. ancestry from a number ofpopulations.Although produced for the individual identification ofcats.Work
forensic evaluation has not yet been carried out,tests on canine identification is mostly based on STRs devel-
GENETIC DRIFT
using 175 AIMs are already available commercially oped for parentage testing69,but also includes mtDNA
The stochastic fluctuation of
allele frequencies in a population for forensic applications60;their use will probably profiling70.In a recent case,the conviction ofa man for
owing to chance variations in the increase,although it might be limited in admixed the murder ofa seven-year-old girl in California was
contribution ofeach individual populations. supported by mtDNA analysis of dog hairs that
to the next generation.
matched a pet belonging to the victim.
Phenotypic information.A strong prediction ofpopula- In the endangered species field,species-specific
ADMIXTURE
The formation ofa hybrid tion oforigin might indicate some aspects ofpheno- methods target the gene that encodes cytochrome b of
population through the mixing type,such as skin colour.However,direct genetic tests mtDNA71,72;examples include tests for tiger-bone DNA
oftwo ancestral populations. would be more useful.Many human phenotypes (for in traditional Chinese medicines (all ofwhich proved to
example,stature,facial features and pigmentation) have be cow or pig73),and also for rhinoceros horn74.
LINKAGE DISEQUILIBRIUM
a strong genetic component. As with animal material,plant material can be asso-
MAPPING
Analysing single nucleotide The only relevant trait that has undergone serious ciated with a crime scene and provide vital evidence.
polymorphism alleles in investigation is pigmentation.However,although there When morphology is uninformative,DNA could,in
population-based studies to
are many human genes that when mutated are known principle,offer species identification or a link to a spe-
identify loci that are associated
to cause abnormal pigmentation such as albinism61, cific place.However,in the analysis ofplant DNA there
with a particular disease or
phenotype. only a minority appear to influence ‘normal’variation. is no easy equivalent of the widely studied animal
The best studied is the melanocortin 1 receptor (MC1R) mtDNA sequences (although regions ofthe chloro-
ANCESTRY INFORMATIVE gene,the gene product ofwhich lies in the cell membrane plast genome and the nuclear ribosomal RNA loci
MMAarRkKeErRs Sshowing marked allele ofthe MELANOCYTE.Binding ofα-melanocyte stimulating seem promising) and STRs in most species are poorly
frequency differences between factor to the receptor leads to production ofblack/ characterized.PCR-based fingerprinting methods such
ancestral populations,useful for brown pigments,whereas in the absence ofa signal as RANDOM AMPLIFIED POLYMORPHIC DNA (RAPD) can allow
determining the probable through MC1R,red/yellow pigments predominate.The identification ofplant strains and have been used in
ancestry ofan individual. MC1Rgene has more than 30 known variant alleles the analysis ofmosses in a murder case75,and in civil
involving amino-acid substitutions,three ofwhich are disputes over the identity ofcommercially valuable
MELANOCYTE
The specialized cell type,lying at associated with red hair,fair skin and freckling62,63. cultivars ofstrawberry76and chilli77.A species-specific
the boundary between the Population studies64show that homozygosity or com- PCR assay is available for Cannabis sativa78and the iso-
dermis and epidermis,in which pound heterozygosity for such a variant gives a >90% lation ofa hexanucleotide STR from the same species
the pigment melanin is
probability ofhaving red hair.This test is therefore use- provided a marker with some potential to identify the
synthesized.
ful as an investigative tool in populations such as that of source ofcannabis samples79.
RANDOM AMPLIFIED the United Kingdom where red hair is found at an Microorganisms can be sources ofevidence in situa-
POLYMORPHIC DNA appreciable frequency. tions such as foodstuffcontamination and medical
Polymorphic markers generated
Other candidate pigmentation genes have been negligence cases involving infections,such as HIV trans-
by using short (8–12 bases long)
investigated,but with less success.Linkage analysis has mission80.However,in October 2001,at least five people
primers to amplify random
fragments ofDNA. identified a locus on chromosome 15 that influences eye died in the United States from inhalation anthrax after
748|OCTOBER 2004 |VOLUME 5 www.nature.com/reviews/genetics