meta-analysis of the secondary transfer of dna · 5 1. introduction deoxyribonucleic acid (dna) is...
TRANSCRIPT
i
META-ANALYSISOFTHESECONDARYTRANSFEROFDNA
By
TaraDUNHILL
Athesissubmittedinfulfilmentoftherequirementsforthedegreeof
MasterofForensicScience(ProfessionalPractice)
in
TheSchoolofVeterinaryandLifeSciences
MurdochUniversity
Supervisor:Mr.BrendanChapman
Semester1,2018
ii
DECLERATION
Ideclarethatthismanuscriptdoesnotcontainanymaterialsubmittedpreviouslyforthe
award of any other degree or diploma at any university or other tertiary institution.
Furthermore, to thebestofmyknowledge, itdoesnot containanymaterialpreviously
publishedorwrittenbyanotherindividual,exceptwhereduereferenceshasbeenmadein
thetext.Finally, Ideclarethatallreportedexperimentationsperformedinthisresearch
were carried out bymyself, except that any contribution by others,withwhom I have
workedisexplicitlyacknowledged.
Signed:TaraDunhill
iii
ACKNOWLEDGEMENTS
I would like to acknowledge firstlyMr. Brendan Chapman for his ongoing support and
mentorshipforthisdissertation.Secondly,Iwouldliketoacknowledgeallmyfamilyand
friends for their support and encouragement despite being over the other side of the
country,inparticularJessicaGeorgakisandAshleySymons.
iv
TABLEOFCONTENTS
TITLEPAGE………………………………………………………………………………………………………………………i
DECLERATION………………………………………………………………………………………………………………….ii
ACKNOWLEDGEMENTS…………………………………………………………………………………………………..iii
PARTONE
LITERATUREREVIEW…………………………………………………………………………………………………1-47
PARTTWO
MANUSCRIPT……………………………………………………………………………………………………………49-79
v
THISPAGEHASBEENLEFTINTENTIONALLYBLANK
1
PARTONE
LITERATUREREVIEW
META-ANALYSISOFTHESECONDARYTRANSFEROFDNA
2
ABSTRACT
Deoxyribonucleicacid(DNA)iscomplexmoleculepresentinnearlyeverycellofthehuman
bodythatcontainsallofanindividual’sgeneticmaterial.TraceDNAhasallowedDNAtobe
detectedoneverydayobjectssuchasdoorhandles,tablesandchairs.Currently,theidea
ofthesecondarytransferofDNAhasarisen,involvingthetransferofgeneticprofilesfrom
anindividualthroughavectorandthentofinalindividualorobject.Thishasbecomeare-
occurringterminthecourtsystem,asindividualsstandingtrialusethisargumentforthe
presence of their DNA. It is therefore hypothesized that through the exploration of
literatureandcompletionofameta-analysisthatanappropriateandconciseguidewillbe
produced to determine the chances of a secondary transfer event occurring andwhat
conditions are required, to aid Biologists in expert testimony. Database searcheswere
conductedtogainresourcesinthetopic,followedbyanumberofscreenstodetermine
the suitability of articles for the meta-analysis. With the resultant 38 articles, data
extractionwasadditionallycarried.However,duetothelackofquantitativevariablesand
results,ameta-analysiswasnotabletobeconducted.It issuggestedthatinthefuture,
studiespublishtheirresultsinamorescientificallyrigorousmannerorallowtheaccessof
rawresultsforexternalinterpretationpurposes.
3
TABLEOFCONTENTS
ABSTRACT…………………………………………………………………………………………………………………….2LISTOFABBREVIATIONS……………………………………………………………………………………………….41. INTRODUCTION………………………………………………………………………………………………………5
1.1 THETRANSFEROFDNA…………………………………………………………………………………..61.2 META-ANALYSIS……………………………………………………………………………………………..8
2. LITERATUREREVIEW………………………………………………………………………………...............112.1 OBJECTASVECTOR……………………………………………………………………………………….12
2.1.1 LAUNDERING……………………………………………………............................162.1.2 FINGERPRINTBRUSHES………………………………………………………….......17
2.2 INDIVIDUALASVECTOR…………………………………………………………………………........182.3 INDIRECTFINDINGSOFSECONDARYTRANSFER…………………………………………....212.4 SHEDDERSTATUS………………………………………………………………………….................252.5 OTHERFORMSOFDNAINVOLVEDINSECONDARYTRANSFER……………...........26
2.5.1 TRANSFEROFSPERMATOZA………………………………………..................262.5.2 TRANSFEROFSALIVA…………………………………………………………..........272.5.3 TRANSFEROFBLOOD……………………………………………...……...............29
2.6 TERTIARYTRANSFEROFDNAANDBEYOND……………………………….....................302.7 CASESTUDIES…………………………………………………………………………………...............32
3. PERFORMANCEOFMETA-ANALYSIS……………………………………………………..................333.1METHOD……………………………………………………………………………………....................33 3.1.1SCREENINGOFARTICLES…………………………………………….....................34 3.1.2DATAEXTRACTION………………………………………………………….................353.2RESULTSANDDISCUSSION…………………………………………………………....................36
4. LIMITATIONSOFTHESECONDARYTRANSFEROFDNA…………………………..................384.1CONTROLLEDNATUREOFTHESTUDIES………………………………….........................384.2TIMEBETWEENTAKINGSAMPLES……………………………………………......................394.3REFERENCESAMPLES………………………………………………………………......................40
5. LIMITATIONSOFMETA-ANALYSIS………………………………………………………...................405.1NEWAPPLICATIONTOTHISFIELD………………………………………………....................405.2TYPEOFRESULTSPUBLISHED………………………………………………………..................41
6. THEFUTUREFORTHESECONDARYTRANSFEROFDNA……………………......................417. CONCLUSION………………………………………………………………………………….......................418. REFERENCES…………………………………………………………………………................................43
4
LISTOFABBREVIATIONS
DNA=DeoxyribonucleicAcid
ng=Nanograms
ng/µL=Nanogramspermicroliter(concentration)
pg=Pictograms
STR=Shorttandemrepeat
µL=Microliter
UV=Ultraviolet
5
1. INTRODUCTION
Deoxyribonucleicacid(DNA)iscomplexmoleculepresentinnearlyeverycellofthehuman
bodythatpossessallofanindividual’sgeneticmaterial.1Eachindividualhasapproximately
threebillionbasepairswhichisessentialforthecharacterizationofeachindividual.1Of
thesethreebillionbasepairs,itisthoughtthatapproximatelyonly10%containsgenetically
relevant information.1 The other 90% contain sequences that are highly repetitive and
polymorphic,meaningthattheyarealsohighlyvariable.1Onetypeofthesepolymorphic
sequencesisknownasshorttandemrepeats(STRs),arepeatedsequenceofDNAthatis
usually aroundone to six basepairs in length.2 STRsdiffer in termsof their sequences
betweensitesalthoughwithinasitethenumberofrepeatsoftheuniquesequencediffers
withseriesthatcanbeupto100nucleotidesinlength.2ThesepolymorphicareasandSTRs
are of great interest in the field of forensic science, as they are variable between
individuals.
InForensicScience,STRtyping isusedto infer individualizedsequencingofDNAwhere
todaykitssuchasPowerPlex21andGlobalFilerareusedwithinforensic laboratories in
Australia. Previously kits such as ProfilerPlus have been used, a kit that tests for ten
individual STR loci, although the more STR loci that are tested for, the greater the
discriminationvalue.1ThePowerPlex21kit,asthenamesuggests,testsfor21STRlociand
theGlobalFilerkittestsfor24STRlocibutcurrentlyisonlyusedinonestateofAustralia.
AllSTRlocichosenfortestingwithinkitsaremostlytetranucleotiderepeats(fourbasepairs
inlength)andarecodominant,meaningthattheyareinheritedjustlikeanyothergene,
onefromthepaternalsideandanotherfromthematernalside.1STRsarethereforeable
tobeusedforfamilialsearchingduetothiscodominantinheritancepattern.
6
Sinceitsfirstuseinthecourtsystemsin1986,DNAhasbeenahighlyregardedpieceof
evidence, something that has been very hard to argue against. AsDNAbecomesmore
understood,scientistsareabletogainagreater insight intowhohasbeenpresentata
crimescene.IncreasedsensitivityofkitsallowsforensicscientiststogainDNAprofilesfrom
the smallest of biological substances. Increased sensitivity of kits and a greater
understanding overall, has allowed claims such as the secondary transfer of DNA. An
increasedsensitivityofkitsallowsmultipleindividualstobewithinamixtureandagreater
understandingallowsthepossibilityofthesecondarytransferofDNA.Thismeansthatno
longerdoesanindividualdeposittheirDNAontoanobject,itistransferredthroughthat
object(vector)toafinalobject.Thisisbecomingincreasinglycommonwithincourtanda
need for a concise guide to determine the possibility of secondary transfer has now
emerged.AproposedmethodtoproduceaconciseguidetothesecondarytransferofDNA
isthroughameta-analysis,ananalysisthatpoolsresultsfrommultiplestudiestogetherto
increaseitsstrengthandsamplesize.3Theaimofthisliteraturereviewwastoproducea
combined review into the secondary transfer of DNA in addition to drawing multiple
studies together to attempt to provide an analysis of the literature. It is therefore
hypothesizedthatthroughtheexplorationofliteratureandcompletionofameta-analysis
that an appropriate and concise guidewill beproduced todetermine the chancesof a
secondarytransfereventoccurringandwhatconditionsarerequired,toaidBiologistsin
experttestimony.
1.1THETRANSFEROFDNA
TraceDNAhasbeen regardedasa largephenomenonover thepast coupleofdecades
purelyduetoitssignificanceinthefieldofforensicscience.Ithasallowedthedetectionof
7
individualsDNAoneverydayobjectssuchasdoorhandles,phones,tablesandchairsjust
basedonasingletouch.TraceDNAisdefinedascellsofftheskinsurfacethathavebeen
carriedontoanothersurfacethroughphysicalcontact.4Thismeantthatfingerprintswere
nolongertheonlywaytoidentifyanindividualatacrimescene,collectingDNAprovided
anewavenueforidentificationbetweenindividuals.TheconceptoftraceDNAwasvery
limitedwhenitwasfirstdiscoveredbutastechnologyhasdeveloped,understandingofit
hasevolved.TraceDNAwas firstexplored ingreatdepthbyvanOorschotandJones in
1997,wheretheyfoundthattheycouldobtainDNAprofilesfromswabbingobjectsthat
wereregularlyhandledbyaparticularindividualandthoseobjectsthatwerepre-cleaned.5
Even though traceDNA isnowrelativelywellunderstood, thereare still somediffering
opinionsastowhereitoriginatesfrom.6-9 ItwasfirstthoughtthattraceDNAoriginated
fromtheoutermost layerof theepidermis,butmore recently studieshave shown that
there isactually littleDNA leftby the time thekeratinocytes (epidermal cell) reach the
epidermis.9Besidesthis, it isnotknownastowheretouchDNAoriginatesfrombutwe
havenowwell-understoodthetechnologiesusedtotestit.AtthebeginningsoftouchDNA
approximately1ngofDNAwasneededtogainafullprofilebutcurrentlyapproximately
only100pg isneeded.10,11Duetothis increase insensitivityof thetechnology,concepts
suchassecondaryandtertiarytransferofDNAhavenowdeveloped.
ThesecondarytransferofDNAinvolvesthetransferofgeneticprofilesfromanindividual
throughavectorandthentoanotherindividualorobject.12Itisdefinedasthetransferof
DNA from one person or object to another via an intermediate person or object, not
involvingadirectlinkbetweenanindividualandthetargetsurface.11,13,14Thefirstinstance
8
ofthesecondarytransferofDNAoccurredinastudyconductedbyvanOorschotandJones
in1996intotheprimarytransferofDNA,fromanindividualtoanobject.5Theyswabbed
regularly used objects of individuals and all had provided genetic profiles that were
consistentwiththeowneroftheobject,howeveronetelephonedisplayedageneticprofile
ofaseconduser.5ThiswasthefirstinstancewherethesecondarytransferofDNAwasseen
tooccur.Further,itoccurredthroughanindirectmannerasitwasnotspecificallytested
for,promptingadditionalresearchintothearea.
With continued and more developed research into certain areas also comes with an
increase in knowledge. Even though we are still trying to understand how secondary
transferoccurs,theconceptsoftertiaryandevenquaternarytransferhavearisen.Aslike
secondarytransferofDNA,tertiarytransferinvolvesthetransferofgeneticprofilesfrom
one person or object to another through two intermediary steps involving a vector.15
Quaternary transfer involvesanadditional transferevents, resulting in four transfersof
DNAintotal.Thesephenomenon’shavebeenindicatedtooccurinnumerouspublished
papers,buthavenotyetbeenconclusivelyshowntooccurconsistently.16-20
1.2META-ANALYSIS
InordertocombinestudiesthatexplorethesecondarytransferofDNA,anappropriate
andeffectiveinvestigationmustbecarriedout.Thechosenmethodofcarryingthisoutwas
ameta-analysis.Ameta-analysis is a systematic review following certain criteriawhere
results are pooled and analysed quantitatively.3 They aim to summarise results of
numerousstudies,effectivelyincreasingasamplesizeandthusthepowerofthestudy.3,21
Rareoutcomesofastudymayalsobeovercomethroughtheuseofameta-analysisasthe
9
samplesizeofstudiesisincreasedandthereforearareoutcomebecomesanoutlier.21To
carry out a correctmeta-analysis, it should be done as a part of a systematic review,
includingfeaturessuchasacomprehensive,reproduciblesearchandtransparentselection
criteria.21Therearetwomaintypesofreviews,systematicandnarrative.Narrativereviews
exploremorebroader topicsandqualitatively summariseevidence,whereassystematic
reviewsfocusmoredeeplyonprimarystudieswhoexploreclinicalquestions.21
Meta-analyseswerefirstknownbackinthe17thcenturywhenastronomersthoughtthat
thecombinationofdatamightbepreferredtoindividualchoice.22Inthemedicalfield,the
firstcombinationofdata fromnumerousstudiesoccurred in1904,butnotas regularly
usedinthefieldasitwasineducationandpsychology.21,23Thetermmeta-analysiswasfirst
devisedin1976byapsychologistnamedGeneGlass,andnowhasbecomeoneofthehighly
citedpublicationtypeandwhoseuseisstillincreasing.24,25
Recently,meta-analysesaremainlyusedinthemedicalfieldpurelyduethelargeamount
ofinformationthatisgatheredthroughstudies.3Theyaimtoprovidereaderswiththemost
currentresearchpublishedinaparticulararea.3In2007,asearchof“meta-analysis”would
yield1,473articles.3Meta-analyseshavealsobeenusedintheagriculturefieldwiththe
firststudycarriedoutbyFisherintheearly1900’sinvolvingtheprobableandrealconcerns
thatfertilizereffectswillvarybyyearandlocation.26Theyhavealsobeenusedinother
fieldssuchaspublicpolicy,clinicalpractice,psychologyandeducationalresearch.
Twoofthemainpurposesastowhymeta-analysesarecarriedoutaretosummariseresults
of several studies in addition toovercoming small sample sizes in studies.21 By pooling
10
theseresultstogether,statisticalpowerofthesestudiescanbeincreasedwhenotherwise
impossible.21Inthemedicalfieldinparticular,performingameta-analysiscanoftenhelp
to reduce “negative” results, helping in providing effective treatmentswithout delay.21
Althoughperformedindirectly,meta-analysismayalsoevenindicateoflackofevidencein
particularareas,highlightingtheneedtoperformmorestudies.21
Although meta-analyses have shown to be a powerful tool, misleading results can be
createdwhenconditionsarenotmetcorrectly.3AsstatedbyWalkeretal.,thereisbelieved
tobefourcritical issuesthatneedtobeaddressedwhencarryingoutameta-analysis.3
These include: heterogeneity of results, availability of information, identification &
selection of studies and the analysis of data.3 For example, during the selection and
identificationofstudies,itmustbeensuredthatbiasinanywaydoesnotcomeintoplay.
Inregardtotheavailabilityofinformation,somestudiestendtoonlyincludesummariesof
theirresults,notrawdata.3Thiscanlimitthetypeofanalysesandconclusionsthatcanbe
madewithameta-analysis.3
Therearetwotypesofmodelsthatareusedtoaccomplishmeta-analyses,randomeffect
orfixedeffectmodels.21Thefixedeffectmodelassumesthatthe‘true’effectsarefixed
whereasa randomeffectmodel assumes thatonlya randomsample is included.21 The
randomeffectmodelismorepreferredforameta-analysisasitbetterreflectsrealityas
studiesbarelyevermimiceachother.21Thisisthereforewhyarandomeffectmodelwill
be used for themeta-analysis carried out in this reviewwhere itwill be attempted to
summariseresultsfromstudiesofthesecondarytransferofDNA.
11
2. LITERATUREREVIEW
RegardinganytransfereventofDNA,thewayinwhichitisperformedisrelativelysimilar
acrosstheboard.Theremustalwaysbeaprimarytransfereventinvolvingthemovement
of genetic profiles most commonly from an individual to either an object or another
individual.Thentheremustbeanothertransferevent,hencethetermsecondarytransfer,
wherethatsamegeneticprofilewillbetransferredeithertoanindividual(comingfroman
object)oranobject(comingfromanindividual).Itcannotbesaiddefiantlyastowhichtype
ofmethodwouldbemorecommoninreal-lifesituationsasbothareasfeasibleaseach
other.Intermsofanindividualbeingavector,thismayoccurwhentwoindividualscome
intocontactwitheachother,eitherthroughahandshakeormoreintimatecontact,and
thenthesecondindividualdepositsboththeirownDNAinadditiontothefirstindividuals
onanobjectperhapsatthesceneofacrime.Theothermethodmaymeanthatthetwo
individualsnevercomeintocontactwitheachother.Inthiscaseitmaybemorefeasibleto
callthesecondindividualavictimtoaphysicalcrime.Inthiscasethefirstindividualmay
have touched an object in a public place and the victim had also touched this object
afterwards.DNAtestsonthevictimmayshowtheDNAprofileofthisfirstindividualplacing
thematthesceneofthecrime.
Inthiscase, itcouldbehypothesizedthatthemorecommonscenariothatwouldoccur
would be that of an individual as the vector. Often in the course of an investigation,
individualswillbelinkedwiththecrimebasedontheirDNAbeingatthescene.Inthiscase,
itcouldbearguedthatsecondarytransferwasthecausefortheirDNAbeingpresentatthe
crime scene especially if they had never visited that area. Below studies using both
12
instancesasvectorswillbeexploredandreviewedtogainagreaterunderstandingasto
howthismechanismoccursanditslikelihood.
2.1 OBJECTASVECTOR
When the secondary transfer of DNA is put forth as a possible explanation, it ismore
sensibletohaveoccurredthroughanobjectratherthananindividual.Thisscenarioatleast
isbelievedtobemorereasonableinhowitoccurs,asthechancesofthetwoindividuals
coming intodirectcontactwitheachother ishighlyunlikely.Oneofthefirststudythat
performedsecondaryDNAtransferthroughanobjectwasperformedin2012.Theauthor’s
usedacasescenariotobasetheirexperimentsoff.Thefirstexperimentfollowedhands
comingincontactwithakid’stoyorasingletandthenthatobjectcomingincontactwith
a laboratory coat, both actions occurring for one minute and immediately after each
other.27 The results showed that in 19 out of 20 repeats, DNA from the donor was
observed.27Thisstudyhoweverdoesn’tmimicreal-lifenearlyasmuchasthecasestudy
wouldhave.Actionswereperformedimmediatelyaftereachotherwithnotimedelayand
theindividualwhoparticipatedinthesecondtransfereventwaswearinggloves,therefore
notcreatingtheissueofadditionalDNApresent inthetransferevent.This increasethe
transferpercentagelikelihoodasthedonor’sDNAistheonlycellularmaterialpresent.
ThefindingsinGorayetal.arealsoinaccordancewithastudyconductedbyLehmannet
al.thefollowingyearwheretheyadditionallyobservedsecondarytransferoccurring.27,28
Lehmannetal.additionallyusesglassasavectorobject,howeverunderwentatransfer
eventontothesametypeofmaterialasthevector.28Similartothestudyconductedby
Gorayetal,thisstudydoesnottakeintoconsiderationofbackgroundDNAthatmaybe
13
presentthroughtransfereventstherefore increasingthepercentageofcellularmaterial
transferred.Incontrast,Verdonetal.conductedanexperimentthesameyearthatused
sevendifferenttypesofsubstratesasthevectororsecondarysubstrateandusedeither
plastic or cotton as the opposite substrate.29 The differing types of substrates paired
againstoneanotheryieldedthesameresultofthatinLehmannetal.,wheresecondary
transfer had shown to occur. However, within the study there were differences seen
betweentheporosityofthesubstrates.Whenthetransferwentfromanon-poroustoa
poroussubstrate,thetransferofDNAwashigher.29
Auniqueapproachwasusedby Frenchet al.whereUltraviolet (UV)powderwasused
throughthetransferexperiment.Thisapproachcanbeusedformultipletypesofmaterial
thatmaybetransferredthroughanindividual’shand.Threeparticipantssataroundatable
drinking from a bottle with their own three glasses for 30 minutes where only one
individualstartedwiththeUVpowder.30Therewasnodirectcontactbetweenindividuals
butthestudyshowedtheindirecttransferoftheUVpowdertoallindividuals.30Obviously,
UVpowderdoesnotmimictheactionsofDNApreciselybutitmaybeusedasabeginning
point in order to understand the mechanisms behind the transfer especially within
networksofindividuals.
GorayandvanOorschotperformedaverysimilarexperimenttoFrenchetal.thefollowing
yearwherethesame“uncontrolled”conditionswereused.Herehowevertheydidnotuse
UVpowderbuttestedfortheDNAitself.Intheareaimmediatelysurroundingparticipants
from the chairs and table, no other DNA than that of the relevant participant was
detected.31 In27%of the table samplesand then33%of thechair sampleshadaDNA
14
profilesfromparticipantsthatdidnotcomeintocontactwiththesurfaceindicatingthat
theseprofilesmighthavebeentransferredfromthejug.31Afurther58%oftablesamples
and42%ofchairsampleshadDNAprofilesfromunknownindividuals,possiblyduetothe
secondary transfer of DNA.31 These results support the findings of French et al.where
secondarytransferdoesoccurthroughaseeminglynormalsocialsituation.Onedownside
tothestudyconductedbyGorayandvanOorschotwasthattheyhadcleanedmostareas
involved in the experiment beforehand meaning that background DNA would not be
present. In this case transferpercentagesmaybeuntruly inflatedwhich is the sameof
previousstudies.28-30
In2015, researches seemed to realise the idea that secondaryDNA transfermayoccur
withinthelaboratorysothereseemstobeaninfluxofthestudiesconductedwithobjects
frequentlyused in laboratoriesactingas thevector.Margiottaetal.usedglovesas the
vectorfortransferwhere16usedgloveswereexaminedforDNApresent.Inhalfthecases
tested for, they found amixture of DNA relating to both the test samples and alleles
belonging to an unknown individual different to that of the volunteer.32 In addition, in
another15%ofthecasesjustallelesfromanotherunknownindividualwerefound,unable
tobeattributedtothevolunteer.32Thiscouldbeattributedtosecondarytransferbythe
volunteerontotheirglovesfromanunknownsourcealthoughauthorsdidpointoutsome
oftheirnegativecontrolsfromsupposedly“clean”glovesdidyieldallelespresent.32
Szkutaetal.continuedonfromotherstudiesthatfocusedontheamountofDNAthatis
leftonvectorsafter the transfereventhasoccurred.Theydid thisby theconductinga
secondary transfer event of DNA between two “exhibits” using tools such as forceps,
15
scissorsandglovesasthevector.33Itwasshownthatsecondarytransfercouldoccurfrom
exhibit to exhibit byDNA-free vectorsbut themain findingwas still having a sufficient
quantityofDNApresentonthevectortocontinuetoproducesecondarytransfertoother
objects.33ThesefindingsareparticularlymoreprevalentforbloodratherthantouchDNA
during the transfer event which seems to be due to the higher concentration of DNA
present,additionallyshowninnumerousotherstudies.28,34-36Herethoughthisstudyused
DNA-freevectorswhichagaininreal-lifescenariosisveryunlikelytooccur.Asmentioned
previously,thismayincreasethetransferpercentagehencegivingfalseindicationsofthe
secondarytransferofDNA.
Fonneløpetal.investigatedthetransferofDNAfromanobjecttoindividualandthenback
to an object, with this idea coming fromMeredith Kerchermurder casewhichwill be
furtherexploredbelow.Theyusedthreevolunteerswhowereknowntobegoodshedders,
thosethatdepositafullDNAprofile15minutesafterhand-washing.17Thefirstindividual
pickedupanobjectandhandleditfor30secondsthenplaceditback.17Asecondindividual
thenhandledthissameobjectandthensubsequentlyputpressureonapieceofbench
paperorfabricwithpressurecausingatertiarytransferevent.17Substratesincludedinthis
studywereplasticconicaltubes,metaldoorhandle,disposablegloves,fabricandbench
papers.17Results showed for the first transferevent,83%of theDNAwashighenough
quality for thedetectionof a full profile and for the second transfer event, that figure
droppedto53%whichsawthoseprofilesuptothestandardofcasereportinganddatabase
searches.17
16
In2017,Buckinghametal. followedon from their study in thepreviousyearandused
cottoncoveredglassplates(porous)ratherthanjustglassplatesthemselves(non-porous)
toinvestigatetheresultingyieldandprofileofDNA.Theyagainusedthesamemethodof
theknifeasthetransfervectorwiththefourindividualsalltouchingitatonestagethen
placingtheirhandonthecottonplate.3740handprintswerecollectedwithsomeresults
producingevidenceofsecondaryandtertiarytransferofDNAalthoughtheproportionof
thistothedepositor’sDNAwaslessthan10%.37Therewereadditionallysomeunknown
sources of DNA found in swabs from cotton plates.37 In comparison between both of
Buckingham et al.’s studies show that the transfer of DNA both directly and indirectly
appearstobedependentonthesurfaceinwhichtheDNAisrecoveredfromaswellashow
theDNAisrecovered.37
2.1.1LAUNDERING
A laundrymachine for thetransferofDNA isa relativelywellcoveredtopic in termsof
spermatozoabutthisstudyconductedbyKamphausenetal.in2015wasoneofthefirst
testingfortouchDNA.15individualsrubbednecksagainstacloth(primarytransfer)for5
secondswithmediumpressure.35Witha totalof46 independentwashescompletedby
handormachinewith/withoutdetergentsecondarytransferoftouchDNAwasdeemed
almostimpossibleforreliableSTRanalysis.35
In 2018, another studywas conducted into themechanismsof the transferofDNAvia
laundry.Eightunwornsockswerewashedinanormalloadofhouseholdlaundryoffour
families(2perwash)andthensubsequentlyair-dried.20Resultsshowedthatoutofthe32
samplesthatwerecollectedafterlaunderingonly7ofthem(22%)yieldedaresulthigher
17
thantheminimalthresholdforcasework(0.06ng/µL).20Fourofthosesamplematcheda
profileofafemalefromthehouseholdwhichwaseitherasinglesourceormajorprofile
indicative of the transfer ofDNAoccurring but unknown as to either direct or indirect
transfer.20
2.1.2FINGERPRINTBRUSHES
MoststudiesconductedexploringthesecondarytransferofDNAinvolveavectorthatis
mostlikelyanobjectsuchasaglassbeakerortube,butthereareobjectsthatareusedin
theprocessingofcrimescenesthatmaycauseforexploration.Fingerprintbrushesareused
byForensicScientistsandPoliceOfficerstodeveloplatentfingerprintsatthesceneofa
crime.Oftentheseindividualsdonotusedisposablebrushesbutmorecommonlyreusable
brusheswithoutthinkingofbiologicalcontaminationissuesthatmayarise.VanOorschot
etal.wasthefirsttotesttheextenttowhichDNAtransfermayoccurusingsquirrel-hair
fingerprintbrushes.38Outof26samplesfrompre-usedbrushesbrushedonpaper,only2
resultedinprofileswhichwerepartial,bothbrusheswereusedincasework.38Another73
squirrel-hair fingerprint brusheswere in current use by staff andwere brushed over 5
sheetsofplasticwith4beingcleanand1(the3rdinsequence)presentingafreshdeposited
handprint.38TheresultsshowedthatverylimitedpickupandtransferofDNAoccurredby
thebrusheshoweverwhenchangesweremadetotheDNAanalysis,significantpickupand
transferofDNAwasseen.38
ThepreviousstudyhadconfirmedthatthetransferofDNAcanoccurthroughfingerprint
brushesatcrimescenesbutnotsufficientlytoshowthatitoccursconsistently.Asecond
studywasconductedbyProffetal.wheretheydeterminedthatDNAcontaminationwith
18
fingerprintbrusheswasquitecommon,where85%ofthebrushestheytestedresultedin
eitherfullorpartialprofilespresent.39Intestingbrushesforthepotentialforsecondary
transfer,theauthorsused16brushesagainstanacetatesheetwithcarbonblackpowder
with25%ofsurfaceshadDNAdetectedwithonefullprofile.39Aswiththepreviousstudy,
this onealso founda limited riskof the secondary transferofDNA through fingerprint
brushes.38,39
Even though these are the only two studies that have been explored in the secondary
transfer of touch DNA, there have been studies performed with other biological
substances.Thesewillbereviewinlatersections.Fromjustthesetwostudies,itshowsthe
needtobemoreawareofcontaminationandtransfereventsoccurring.Onesetofauthors
event suggest thatdecontaminationprocedures shouldbeput intoplace regarding the
continuous use of fingerprint brushes between crime scenes.39 In today’s crime scene
examination, this has already been put into place where either disposable fingerprint
brushesorde-contaminatedfingerprintbrushesareused.
2.2INDIVIDUALASVECTOR
Although this scenario seems like themore common scenario to occur in a “real-life”
situation,itseemstobetheleastresearchedofthemall.Thefirststudythatdidusean
individualasthevectorforthetransferofDNAwasconductedbyLoweetal.in2003.The
authorsusedpairingsofgoodandpoorshedderswhoshookhandsforone-minuteand
then the poor shedder gripped an object for 10 seconds.40 One pairing in particular
consistentlyfoundthatthegoodsheddermorethanoftenhadtheirfullprofiletransferred
andthepoorshedder,thevector,couldnotbedetected.40Byputtinginplacea30second
19
timedelaybetweentransferevents,mixedprofileswerediscoveredwhere70%werefrom
theoriginalsource.40AlthoughthisstudydidconfirmeventsofsecondaryDNAtransfer
occurring, it is performed under controlled conditions where pairings were created to
optimizethedesiredoutcomeandtypicallytheremaybehoursordaysbetweentransfer
eventsoccurring.
Farmenetal.additionallyconductedastudywheretwoindividualsundertookahandshake
for30secondsfollowedbythegrippingofbeakersfor30seconds.14Atotalof60swabs
weretakenandallresultedinaDNAprofileofsomeform,eitherpartial,fullormixed.14
Althoughnotspecified,theauthorssuggestthatallsamplesweremixturescontainingboth
profilesoftheknowncontributors,indicatingthatsecondaryDNAtransferhadsuccessfully
occurred.14ThisisinagreeancewiththepreviousstudyconductedbyLoweetal.where
theyalsosawsecondarytransferoccur,withbothstudiesseeingthatmixturescanhave
highercontributionofindividualswhowerenotthevector.14,40Typically,inthescenarioof
secondaryDNAtransfer,theindividualwhodepositstheDNAatthelasttouchwillleavea
higheramountoftheirDNAasthattransferisprimary.ThoseDNAprofilesthathavegone
throughmultiple transfer before the final depositwill typically decrease significantly in
concentration.
AstudyconductedbyMeakinetalin2015alsoshowedinstancesofthesecondarytransfer
ofDNAoccurringthroughahandshakeasthevectorandaknifeasthefinaltransferevent.
A“regularly-used”wasartificiallycreatedoveraperiodoftwodaysandafterthattime
period two volunteers shook hands then immediately stabbed their knife into foam
occurringforthenextthreedayswiththreedifferentknives.41Thestudyfoundthatfor
20
threeofthefoursetsofknivesmixtureswerefoundcontainingthreedifferentprofiles.41
Thefirsttwocouldbeattributedtotheregularuseroftheknifeandthehandshaker,but
the other was an unknown profile, possibly the cause of secondary transfer from the
regularuserthroughothermeansorpossiblytertiarytransferfromthehandshaker.41As
with the study conducted by Lowe et al., this study has tested the persistence of the
secondarily-transferredDNAandfoundthatthebestchanceofrecoveringthatDNAwould
be to sample as soon as possible after the final transfer event had occurred which is
consistentwithLoweetal.’sfindings.40,41
Withanew-foundgap inthe literature,Caleetal.performedastandardstudy intothe
secondarytransferofDNAwithakitofincreasedsensitivity.Theauthorsaimedtostudy
howthepresenceofsecondaryDNAtransferwouldaffecttheinterpretationofDNAtyping
results.11Inaddition,theeffectsbetweensurfacetexturewasalsoexplored.11Participants
washed hands before wearing glove for 1.5 hours before shaking hands with another
participantfor2minutesandthenhandlingapre-cleanedknifeforafurther2minutes.11
Significantresultsofthisstudyconcludedthattexturedidnotappeartohaveasignificant
effect in addition to the data obtained from five of the 24 samples that suggest that
individualscanhavetheirDNAdepositedonobjectsinsufficientquantitiesthattheycan
be identifiedas theonlyormajor contributor, suggesting that theyhad come indirect
contactwith the object.11 These results seem to be a trend through numerous studies
whereifgoodsheddersarethedonorDNAprofile,theywilltypicallyoverpowertheDNA
of that of the vector.9,42-44 All these papers discussed within this section are highly
unrealisticandlackscientificrigour.Inaneverydayscenario,individualswouldbarelyever
21
shakehands for30 secondsand furtherhandleanotherobject immediatelyafterwhen
committingacrime.
2.3INDIRECTFINDINGSOFSECONDARYTRANSFER
TherearealsoinstancesinwhereprimarytransferorpersistenceofDNAmayhavebeen
experimentedonduringastudyalthoughunknownallelesmaybedetectedontheobject
or individual.Hereall the studies thathavedetectedsecondary transferofDNAduring
anotherexperimenthavebeencollated.However,inthesestudies,unlesstheauthorshave
specificallystatedwedonotknowwherethesecondarytransferhasoccurredandwhothe
unknownallelesarefrom.Additionally,theunknownallelesmaybepartofthebackground
DNApresentonanobjectunlesstheobjectofindividualhasbeenpre-cleanedbeforehand.
ThefirstinstanceofaccidentaloccurrenceofsecondaryDNAtransferwasduringastudy
conductedbyLoweetal.in2002wheretheauthorstestedshedderstatusesofindividuals.
Secondary transferwasdetectedwhenparticipantshelda tube for10 secondsbut the
extent was dependent on the subject pairings.42 In an additional two experiments,
secondary transfer was not conclusively shown as DNA recovered varied considerably
betweenreplicates.42Overall,theauthorsconcludethatthesecondarytransferofDNAwas
showntooccurunderidealconditions.42Afewyearslater,Phippsetal.conductedastudy
ingreatsimilaritywheretheyfoundthattheoccurrenceofsecondarytransferwaslowand
notlikelybutpossible.43
AstudyconductedbyRuttyfoundaninstanceofpossiblesecondaryDNAtransferoccurring
wheremale-female pairs were used in order to simulate a strangulation event.45Men
22
placedtwofingerpadsonthewomen’sneck,anactionthatwasrepeatedatotalof29
timescreating116samples.45Twoswabsofthemale’sfingers(controls)yieldedaprofile
ofafemaleinthestudyalthoughthesetwoindividualshadneverbeenpairedorcomeinto
contactwithoneanother.45Partialprofileswerealsofoundfromoneormoreindividuals
in pairs of controls up to 10 days after the experiment.45 Although in a different
circumstancetothatofLoweetal.,bothstudiesshowthepresenceofadditionalalleles
thatmaybeattributedtosecondarytransfer.
Usinganotherdifferentexample,Petricevicetal.usedfivevolunteerswhosleptintheir
ownbedbutwithanewlowerbedsheet.46Anotherscenariosawthatindividualssleptin
a bedwhere they never had previously.46When individuals slept in their own beds, a
second individual’s DNA profile was found in at least one sample from 3 out of the 5
volunteers.46 The authors were able to attribute these from the volunteer’s partners,
possiblytransferrinfromotherbeddingusedorfromthevolunteerthemselves.46When
individuals slept in an unknown bed, all unknown samples could be attributed to the
regularuser’sprofilewhichdidnotindicatesecondaryDNAtransfer.
AnothersimilarexperimenttothatconductedbyLoweetal.andPhippsetal.,Djuricetal.
hadvolunteersholdplastictubesandindividual’sanklesfor10seconds.47Whenvolunteers
heldtubes,outofthesevenprofilesobtains,oneresultedinapartialprofilenotmatching
thevolunteer’s,indicatingtheoccurrenceofsecondarytransfer.47Whenankleswereused
asdepositionsites,nooccurrenceofthesecondarytransferofDNAwasnoted.47Gorayet
al.conductedaverysimilarexperimentagainbuttestedthedepositionontoglassplates
ratherthanplastictubes.Atotalof40samplesweretakenandnon-selfDNA(unexpected
23
alleles) was seen in 79% of the samples.9 Surprisingly, 7 samples had the primary
contributorprofileexcludedfromthemixture.9Gorayetal.hypothesizedthatthemost
commonexplanationforthepresenceofnon-selfDNAindepositswereduetothetransfer
ofnon-selfDNApickedupthroughevery-dayactivities.9Theauthorsalsoranthemixtures
through staff eliminationdatabasesandotherworkerswhowereknown tobe in close
proximitytoparticipantsmatchedtothenon-selfDNAdeposited.9
ApaperpublishedbyOldonietal.appliedtheconstantunknownfromcontactstainsat
break-instoconductedanexperiment.Here,volunteerswouldhandleanobjectregularly
over 8-10 days then a second userwould handle the same object for a pre-determine
period of time.48 A total of 231 samples were created where approximately 2-10%
containedunknownalleles,possiblesuggestingsecondarytransfer.48Additionally,intwo
ofthesimulations,therewerefullDNAprofilespresentthatdidnotcorrelatetoindividuals
directlyinthosesimulationsbutratherothervolunteersfromthestudy.48
Manyof thesestudiesconducted in thesecondary transferofDNAarecarriedoutata
forensic laboratory and involve volunteers who work in an office/laboratory based
environment. As these individualsmay be at high risk for transferring DNA to forensic
exhibits if working with them, an exploration into the ways in which volunteers may
transfer DNA between themselves was conducted by Fonneløp in 2015. Here four
volunteershadtheirkeyboardsandmouseswappedwithanother’swithswabstakenfrom
bothobjects andvolunteers at thebeginningand throughout the study.49 Swabs taken
before the study had begun showed DNA present from unknown individuals, but
unfortunatelyitcannotbeconcludedifthisDNAwassecondarilytransferredorthrough
24
directcontact.Theauthor’sattributedthisoccurrencetobackgroundDNAbeingpresent
oneverydayobjects.49Inaddition,thestudyalsoshowedthatsecondarytransferofDNA
occurredthroughthepresenceoftheoriginaluser’sDNAonthehandsoftheseconduser
upto8daysafterreceivingthecomputerequipment.49
Oneof the simplest ofways todetermine thepossibility of secondaryDNA transfer or
backgroundDNApresentwouldbetodirectlyswabanindividual’shandwhichwascarried
out by Lacerenza et al. 120 sampleswere directly collected from60 individual’s hands
where56sampleshadoneormoreunknownallelesand36ofthosecouldbeclassifiedas
mixtures(64.3%).50AstheseDNAsamplespresentinthemixtureswereforeigntothatof
thevolunteer,itcouldbeconcludedthateithersecondaryDNAtransferoccurredorthat
theseprofileswereapartofthebackgroundDNApresentonthatindividual’shand.50
AnewsubstratewasthenusedbyvandenBergeetal.where individualsdraggedeach
othermimickinganactivity-relatedscenario.Samplesweretakenfromtheknee-areaof
thetrouserswherethedraggerdidnottouchresultingin26samples,3ofwhichdidnot
resultinover7alleles.51Donoralleleswerepresentin100%ofthesamplesandnon-donor
in71%.51Theauthorsconcludedthatmorecaremustbetakenwheninterpretingmixtures
asallelesmatchingaperpetratormaynotbedistinguishablefrombackgroundsignals.51
Inaveryrecentstudy,Mageeetal.usedcollarsandcuffsofuppergarmentstodetermine
the presence of DNA.52 Out of 55 samples taken, non-wearer DNA was recovered, an
averageof1.3ngfromthecollarand2.7ngfromthecuffs.52Fromoneparticularsamples,
anon-wear contributedapproximately57%ofDNA toamixture, followedby thewear
25
contributing37%andasecondnon-wear,7%.52Threeothernon-wearersampleswereable
tobeattributedtospousesofthevolunteers.52
2.4SHEDDERSTATUS
Shedderstatusisoneofthemorerecentconceptsundertheumbrellaofthetransferof
DNAandonethatwillcontinuetogrowasmoreresearchisconductedintothetopic.The
first study to directly carry out an experiment on individual’s shedder status’ was
conductedbyLoweetal.in2002andisconsideredthebackbonestudytothistopic.The
authorsgotparticipantstogripasteriletube,15minutes,2hoursand6hourperiodsafter
hand-washing.42 They determined that there were differences between individuals in
regardtohowmuchDNAthattheydepositedonasurface.42Goodshedderswerethen
classediftheindividualleftanentireprofileafter15minutesofhand-washing.42Allother
individualswereclassedaspoorshedders.
Afair fewyears later,Phippsetal.conductedasecondstudythat investigatedshedder
statusandusedalmostidenticalexperimentstothatofLoweetal.Plastictubeswereused
for deposits and hand washing occurred 15 minutes prior to the experiment.43
Unexpectedly,thestudyhadshownthatindividualsdonotproduceconsistentquantities
ofDNAovertime,varyingasmuchwithinthemselvesthancomparedtootherpeople.43
Finally,usingthecriterialaidoutinLoweetal.,thisstudydiscoveredno“good”shedders
andallwereclassifiedas“poor”shedderssuggestingthislevelofclassificationmaynotbe
allassimpleasoncethought.42,43
26
ThefollowingyearFarmenetal.conductedastudyonshedderstatusinconjunctionwith
anexperimentonsecondaryDNAtransfer.Authorsmeasuredindividuals’shedderstatus
justbasedonswabsfromtheirhands.44Thisunfortunatelydoesn’ttestthequantityofDNA
thattheindividual’sactuallydepositonasurface,notclassingasatruerepresentationof
shedderstatus.Outofthenineindividualswhoparticipatedinthestudy,twoindividuals
had a low DNA yield and seven individuals produced profiles that were able to be
analysed.44
Morerecentlyin2016,Gorayetal.carriedoutanexperimentthatexploredtheeffectof
thevariableshanddominance,handsizeandgenderonindividual’sshedderstatuses.240
sampleswerecollectedfromhandprintsonglassplateswhereonlyfourofthesampleshad
nodetectablequantitiesofDNA.9 Twoofthetenparticipantscouldbeclassedasgood
sheddersbutthiswasdoneunderdifferentcriteriaandexperimentalcriteriaastothebase
studyconductedbyLoweetal.9Theonlycorrelationorsignificancethatthestudyfound
with the variables tested was that of gender, where a significant difference was seen
betweengendersintheamountofDNAthattheydeposited.9
2.5OTHERFORMSOFDNAINVOLVEDINSECONDARYTRANSFER
2.5.1TRANSFEROFSPERMATOZA
Oneof the first studiescarriedout in thebroader realmof the transferofDNAwasby
Kafarowskietal.in1996.Thiswasconductedbeforethefirstoccurrenceofthesecondary
transfer of touch DNA andwas one of the first of its kind. It aimed to determine the
likelihoodofthetransferofspermatozoabetweenarticlesofclothingduringamachine-
cyclewash.53Althoughtheretentionofspermatozoaduringmachinewashhadpreviously
27
beenexplored,thetransferofittootherarticlesofclothinghadn’t.54,55Theauthorsused
underpants as a means for the primary deposit of spermatozoa, reflecting real-life
situationsthatmaydepictasexualassault.Thispairofunderpantswaswashedwiththree
additional clean pairs and then machine dried.53 In all three trials, trace quantities of
spermatozoawerefoundwhereaminimumofonespermheadandmaximumofeightper
sample.53AsthiswasoneofthefirststudiesperformedinthesecondarytransferofDNA,
there are many downfalls to it that have improved over time with knowledge and
technology.
2.5.2TRANSFEROFSALIVA
SalivaisanadditionalbiologicalsubstancethatcontainshumanDNAthathasthepossibility
tobetransferredthroughobjectsandindividualstocauseasecondarytransfer.Thefirst
thatexploredintothissecondarytransferofsalivawasconductedbyvanOorschotetal.
wheretheyexploredthetransferofsalivathroughasquirrel-hairfingerprintbrush.Inone
experimentasinglebrushwassweptover19sheetsofpaperwithdried-salivapresentthen
brushedoveranother20clean sheetsofpaper.38Outof the80 samplesgenerated,13
resultedinpartialprofiles,1innoprofileandtheremaining66resultedinfullprofiles.38
The second experiment saw both the powder type and DNA extraction tested in the
transferofsalivafrombrushtosixsheetsofplastic.38Forthecomparisononextraction
techniques,outof6samples,3werefullprofiles,2partialprofilesand1noresult.38Any
presenceofaprofileinthisinstanceconfirmsthesecondarytransferofDNAhasoccurred
fromthepaper,tofingerprintbrushandthenbacktothepaper.
28
Nextresearchesdecidedtocarryoutexperimentverysimilarinnaturetothoseconducted
withtouchDNAbutwithsalivainstead.Wiegandetal.transferredsalivaontoeitherpaper,
cottonorplasticsurfaces,leftthemtoairdrythenrubbedathumbontothestrain.34The
thumbwasthenplacedontoanotherpieceofpaperandthentheareawasswabbed.34
Whenpaperwastheprimarysurface,onlyabout50%ofthetransferredsalivaproduced
DNAprofilesalthoughtheywereinverylowconcentrations(max.1pg/µL).34Intotalonly
3outof96gavecompleteprofiles,allfromplasticastheoriginalsourcebutonlyonebeing
fromsaliva.34
Thesecondarytransferofsalivawasadditionallytestedthroughavectorofpensandplastic
tubes for theoverall aimof determining if salivawouldbe amoreprevalent biological
materialduringtransferevents.16Theirstudyconcludedthatsecondarytransferofsaliva
did occur, in greater levels of retention than touch DNA and that moist surfacesmay
facilitateDNAtransfertoagreaterdegreethanadrysurfaceaspreviouslydiscoveredin
Gorayetal.2010.16,56TheauthorsadditionallystatedafindingofbackgroundDNApresent
onanindividual’shandwhendealingwiththetransferofDNA.16
In2015,thetransferofsalivathroughlaunderingwasstudied,somethinginwhichtouch
DNAhadbeenexploredmultipletimes.Individualsdepositedsalivaontotextileclothsand
thentheseweresubsequentlywashedwithcleantextilecloth.35Betweentheexperiment
additionallyconductedwithbloodandtouchDNA,atotalof46independentwasheswere
completed both by hand and machine with/without detergent.35 Saliva was seen to
undergotransfereventswhichsupportsotherstudiesperformed.16,34,35,38,56
29
The most recent study conducted in relation to the transfer of saliva was through
individual’s hands as vectors for the transfer. 14 areas of the individuals palm were
swabbed inaddition to theglassplatewhich the salivawas transferred to.57 The study
showeda significantdecrease in thequantityofDNA through the final transferevents,
wherepreviouslyanaverageof9.16ngwasseenandposttransfertherewasanaverageof
0.225ng.57Thestudyalsoyieldedanumberofunknownallelesonthefinalplatewherethe
authorscontributedthistopriorinteractionwithindividuals,supportingthefindingthat
secondarytransferofDNAhadoccurred.57
2.5.3TRANSFEROFBLOOD
Followingonfromthesecondarytransferofsaliva,Wiegandetal.additionallyconducted
astudyintotheriskofthetransferofbloodstainsfromsurfacessuchasplastic,cottonand
paper.Thebloodstainswerelefttoairdrythenanindividualplacedtheirthumbonthe
stain thenontoanotherpieceofpaper.34Fromthecottonsurface,DNAconcentrations
aftersecondarytransferoccurredwereremarkablysimilartothosefollowingtheprimary
transferevent.34Whengloveswereused insteadofbarehands, theDNAconcentration
followingDNAtransferincreased.34Boththeseresultssupportedthefindingsofsecondary
transfer of DNA through blood occurring. Lehmann et al. conducted a very similar
experimentbutall transfereventswereconductedwith substratesonlyandnohuman
interaction. The experiment supported the existence of the secondary transfer of DNA
throughbloodandadditionallyfoundthatifthebloodwerewet,itwouldtransferfurther
andwouldendinahigherconcentrationasmoresubstancewastransferred.28
30
AstudyinvestigatedsecondarytransferofDNAthroughlaunderinginawashingmachine
orbyhandbyKamphausenetal.in2015.Artificialbloodstainswerealsocreatedontextile
cloth, then dried and these were then washed with clean textile cloths.35 In total 46
independent washes were completed both by hand and machine in addition to
with/withoutdetergent.35 Transferofblood cellswere seen in this study in addition to
numerouspreviousones.28,34,35,36
Agapwasthenidentifiedintheliterature,wherenoneofthesepreviousstudieshadreally
thoroughlytestedtheeffectthearidityofthebiologicalsubstances,herespecificallyblood.
vanOorschotetal.researchedintothisusingcottonasthesecondarysubstrate.36Drying
time, temperature and humidity all acted as dependent variables in this study.36 The
secondarytransferofbloodwasseentohaveoccurredwithsignificantdifferencesinthe
transferpercentageofbloodbetween5and60minutes.36
2.6TERTIARYTRANSFEROFDNAANDBEYOND
Although the secondary transfer of DNA is still in the process of gaining a greater
understandingofitsmechanisms,throughthesestudiesthediscoveryoftransferevents
occurring after a second transfer have occurred. These studies are still in their very
preliminarystagesandlikesecondaryDNAtransfer,stillrequireagreaterunderstanding
intothemechanismssurroundingtheprocess.Thefirststudythatexploredtheoccurrence
ofthetertiarytransferofDNAwasconductedbyWarshaueretal.whereanobjectwasthe
original vector andwas further transferred from an individual to another individual or
objectasthetertiarytransferevent.16TheywereabletoprovethattertiarytransferofDNA
couldoccurbutwereunabletorecoverenoughDNAtoestimatetherateofDNAlostduring
31
the additional transfer step.16 Out of the additional transfer steps conducted, 87.5%
displayed less than half of the expected alleles.16 These results correlated with those
producedbyFonneløpetal.whereafterthefinaltransfersteponly17%ofsampleswere
highqualitythatcouldbereportedandgothroughdatabasesearching(5outof30transfer
events).17
In the study that was conducted by Fonneløp et al. they also tested the instance of
quaternarytransferoccurring,whichwasafirst.Theyfoundthatonceagain,itoccurred
butaswithmostof the issueswiththestudiesexplored inthis review,all surfacesand
objectswerecleanedthoroughlytobeginwithtoeliminatethebackgroundDNApresent.
Atotalof17outofthe108analysedsamplescontainunknownallelesandtheseunknown
allelescouldnotbeattributedtothepresenceofbackgroundDNAasitwaseradicated.17
Therewasadditionallyonetransfersequencethattheauthorswereabletodeterminethe
donoroftheDNApresentinthetertiarytransferevent.17ThesourceoftheDNAwasfrom
thegirlfriendofthevolunteer,asamplefoundinalltransferevents.17Previously,thesetwo
individuals had not been in contact with each other for more than 10 hours and the
volunteerhadadditionallywashedhishandspriortotheexperimentbeginning.17
In2016,Helmusetal.performedthefirststudysinglytestingthetertiarytransferofDNA
duetothegapidentifiedwithintheliterature.Threepairsofindividualswereusedwitha
cotton cloth as the vector and final depository where a total of 180 samples was
generated.19 Out of those 180 samples generated, only 72 had detectable DNA that
identifiedthattertiarytransferhadoccurred(40%).19Inastudycompletedtheyearearlier
theyhadadditionallyshowntheoccurrenceofthetertiarytransferofDNAbuttheauthors
32
notedthattheywereunabletodistinguishbetweentertiarytransferorinterferencefrom
alow-levelofnon-donorDNA,somethingthatmayhavetodowithtypeofsystemused,
thePromegaESI-17FastPCR.18
Inamorerecentstudy,tertiarytransferofDNAevolvedtoamorecomplexmechanism
thanthatofindividual-objectinvolvedtransfer.Sixnewunwornsocksandat-shirtwere
launderedtogetherwithnoadditionalitemsinamachinethathadbeenpreviouslyused.20
ResultsshowedthattherewerenoDNAresultsrecoveredfromquantity(abovethreshold)
throughtoprofilingthereforenotsuggestingthat tertiary transfercouldoccur.20 In this
case,either thekitswith improvedsensitivityneedtobecreatedor this is thepointat
whichtheextentoftertiarytransferwilloccur.Thisishoweverunknownwithoutadditional
studiesconductedinthisarea.
2.7CASESTUDIES
In2002,apaperwaspublishedbyAnsellexploringacasestudyinvolvingthesecondary
transfer of seminal constituents. A rape occurred in Sweden whereby a woman was
allegedlyrapedbyamanwheremixedDNAprofileswerefoundinboththevaginalswabs,
underwearswabsandpenilesamples.58Bothvaginalandunderwearswabsexcludedthe
manas a suspect ashisDNAwasnot found (hada vasectomy), although thewoman’s
boyfriendDNAwasadditionallyfoundduetoconsensualintercoursethepreviousday.58
Thepenileswabsalsoshowedamixedprofilebutthatofthewomen’sDNAinadditionto
thewomen’sboyfriendsDNA.58Theallegedrapeofthiswomenwasabletobeprovedin
courtbythesecondarytransferoftheboyfriend’sDNAtothesuspect.58
33
Morerecently,in2017,anarticlewascreatedfollowingtheincidenceofcontaminationby
PoliceOfficer’sinAustriawhichwithouteliminationdatabases,manyinstancesofindirect
orsecondarytransferofDNAmaygoundetected.59Duringtheperiodof2000to2016,a
totalof46,000tracesamplesweresubmittedthroughDNAandoutofthoseatotalof347
contamination incidents were detected through the use of the elimination database
(0.75%)withamajorityfromthecauseofindirecttransfer.59Theauthorsfurtherwentinto
detailregarding3separateincidentsthattherewereabletopinpointwherethetransfer
eventshadoccurred.Thefirstoccurrencehappenedthroughsharedcameraequipment
that hadbeenusedby a PoliceOfficer completely unrelated to the case.59 The second
showstheindirecttransferbyaPoliceOfficerthroughasharedvehicletoavolumecrime
caseandthethirdthroughpackagingofevidencematerialonanunrelatedPoliceOfficer’s
desk.59
3. PERFORMANCEOFMETA-ANALYSIS
3.1METHOD
Athoroughreviewoftheliteraturewasconductedthroughtheuseofonlinedatabasesto
gatherarticles inthesecondarytransferofDNA.Databaseswerechosenbasedontheir
relevancetothetopicofForensicScience.DatabaseschosenwereScopus,WebofScience
CoreCollection,ProQuest,Medline,ScienceDirectandResearchGate.Somedatabases
weremorespecifictoForensicSciencebutgeneralsciencedatabaseswereincluded,such
asScienceDirect,inordertobroadenthesearchresultstoensuregreatercoverageofthe
entireliteratureintothesecondarytransferofDNA.
34
Thekeytermsusedforsearchingthedatabasesincludedthefollowing:
• “Secondarytransfer”AND“DNA”AND“Forensics”
• “DNAtransferevents”AND“Forensics”
• “Tertiarytransfer”AND“DNA”AND“Forensics”
• “DirectDNAtransfer”AND“Forensics”
• “Shedderstatus”AND“Forensics”
• “Indirecttransfer”AND“DNA”AND“Forensics”
Noadditionalsearchtermswereexcludedduetothespecificityofthesedatabasesand
theirsmallerrangeofjournalarticlesavailable.
Theonlyrestrictionsthatweremadeonthedatabasesearcheswerethatofeliminating
types of articles, e.g. book chapters, encyclopedias etc.No additional restrictionswere
placedonthedatabasesearches.Alldatabasesalreadyhadrestrictionssuchaslanguage
anddatesappliedtosearches,whereEnglishwasselectedasthelanguageandbeginning
datesofsearcheswenttothebeginningofthedatabase.Thisdatewasmostcommonlyin
the1990’sandsearchresultswenttoApril2018.
3.1.1SCREENINGOFARTICLES
After thecompletionof thedatabasesearches,allarticleswerethenscreenedfor their
appropriatenesstothetopicofthesecondarytransferofDNA.Atotalof307articleswere
identifiedthroughthedatabasesearch.Thesearticleswerecompiledofallyieldedsearch
resultsfromallsixsearchtermsthroughallsixdatabases.These307articlesthenhadtheir
reference lists screened yielding a further 103 eligible articles, creating a total of 410
articlesthroughtheliteraturesearch.
35
Thenextstepoftheprocesswastoscreenall410articlesfoundasaresultoftheliterature
searchbasedontheirtitlesandabstracts.Forthis, inclusionandexclusioncriteriawere
usedtonarrowdownthepossiblearticles.Themainexclusioncriteriathatwasusedwasif
thearticledidnotinvolvetransfereventofDNAinanyform(biologicalsubstancesuchas
touch,spermatozoa,bloodandsaliva)itwouldbeexcludedfromthemeta-analysis.Other
exclusioncriteriaincludedstudiesthatinvolvesacasestudy,thosethatarearesponseto
ajournalarticleandaformofwritingthatwasnotajournalarticleortheses(e.g.bookor
abookchapter).Fromthisscreeningprocess,atotalof141articleswereselectedbased
ontheirtitleandabstracts.
These 141 articles then went through a further screening process that involved the
inclusionoftheentirebodyoftext.Articleswereagainexcludedbasedoninclusionand
exclusion criteria, where if an article had a recorded or measured occurrence of the
secondarytransferofDNAinanyform,itwasabletobeencompassedinthemeta-analysis.
ArticlesthatalsofoundanoccurrenceofthesecondarytransferofDNAbutindirectly(was
notthemainvariabletestingfor)werealsoexcluded.Thesecondscreeningresultedina
total of 38 articles that were able to be continued through to themeta-analysis. This
resultedin103studiesexcludedfromthisscreeningprocess.
3.1.2DATAEXTRACTION
Data was extracted from all 38 articles discovered through the screening process and
collatedintoatableusingMicrosoftExcel.Thisdataincludedthefollowing:
• Author;
• Year;
36
• Conditionsoftheexperiment(Controlled,semi-controlled,uncontrolled);
• NumberofParticipants;
• Numberofsamplestaken;
• Typeofvector;
• FinalobjectoftheDNAdeposition;
• Timebetweentheprimaryandsecondarytransferevent;
• Timehandlingthevector;
• Timehandlingthefinalobject;
• Typeandpressureofcontactofvector;
• AveragetotalDNArecovered(ng);
• TotalDNAconcentration(ng/µL);
• Totalnumberofunknownallelesdetected(indicativeofsecondarytransfer);
• Numberofmixturesseen;
• Ifareferencesampleofvolunteershadbeentaken;and
• IfsecondarytransferofDNAinanyformwasdetected.
3.2RESULTSANDDISCUSSION
Itwasoriginallyhypothesizedthatthroughtheexplorationofliteratureandcompletionof
ameta-analysisthatanappropriateandconciseguidewillbeproducedtodeterminethe
chancesofasecondarytransfereventoccurringandwhatconditionsarerequired,toaid
Biologists inexperttestimony.Thishypothesiswasrejectedonthebasisthatthemeta-
analysiswasfoundnotbeanadequatemodelforthedatathatwasavailable.Thescreening
processofarticlesexploringthesecondarytransferofDNAwassufficientforthisfieldof
37
science,asarticleswereabletobeeasilyincludedandexcludedbasedonasetofcriteria.
Dataextractionwasadditionally able tobe carriedout to someextent asmost journal
articlesgiveaccuraterepresentationsofthestudyandhowitwasperformed.However,
through the creation of a table, incorporating all of the included studies for themeta-
analysisandtheconductionofthemeta-analysis,itwasdeterminedthatthemeta-analysis
wouldnotbeadequateforthedatathatwasavailable.
Forthismeta-analysis,itwashopedtouseaverageamountoftotalDNArecovered(ng)as
oneofthequantitativevariables.Articleshoweverdidnotallgiveresultsthatwouldbe
sufficienttocalculateanaveragefrom.SomegaveallvaluesoftotalDNArecoveredfrom
thefinalobjectsaftersecondaryDNAtransfer,whereanaveragewasabletobecalculated.
Conversely,somearticleswouldstatearangeofthetotalamountofDNArecoveredwhich
anaveragecouldnotbecalculatedfromastherawresultswerenotpresent.Therefore,
therewasnotaconsistentsetofvaluesthatwouldbeusedtorepresentthetotalamount
ofDNArecoveredthroughallincludedstudies.
Asasecondquantitativevariablewasrequired,outofthedataavailabletoextract,time
handlingthevector,finalobjectortimebetweenhandlingthevectorandfinalobject,were
theonlytwooptionsavailable.Althoughthisvaluedifferedbetweenstudies,withinastudy
itwashighlylikelythatonlyasingletimevaluewasutilized.Forexample,Gorayetal.used
oneminuteforthetimehandlingthevectorandthefinalobjectforallvolunteers.27Two
different vector types were used within the study with two different times between
handlingthevectorandfinalobject(immediatelyand24hoursafter).27BothtouchDNA
and bloodwas used as the biological substances, creating four possible scenarios.27 As
38
thesequantitativevariablesdidnotvary substantiallywithin thestudy, itwouldnotbe
possibletocomparethevariableswithinthestudy.Otherstudiesadditionallyonlyhadone
vector typeandbiological substance,andthereforeonlyonequantitativevalueof time
handlingvectorsorobjects.28,29,35,41,45
Reflecting back onto the hypothesis, it was stated that it was anticipated that “an
appropriateandconciseguidewillbeproducedtodeterminethechancesofasecondary
transfereventoccurringandwhatconditionsarerequired”.Asmentionedpreviously,the
frameworkthatameta-analysisworksonisthecomparisonoftwoquantitativevariables
inthehopetoperformstatisticaltestssuchasCohen’sDorPearson’scoefficient.Inorder
to determine the conditions required for the secondary transfer of DNA to occur,
quantitativevalueswouldneedtobegathered.TheconditionthatsecondaryDNAtransfer
are conducted under would include things such as the vector type, final object type,
conditionsoftheexperiment,backgroundDNApresentandtypeofpressureandcontact
ofthevectororfinalobject.Thesevariablesunfortunatelyarequalitativeanddonothave
an option to measure them quantitatively. In that case, performance of comparisons
betweenthesevariablesandquantitativevariablessuchastotalDNA,DNAconcentration
ortimehandlingobjectscannotbecarriedout.
4. LIMITATIONSOFTHESECONDARYTRANFEROFDNA
4.1CONTROLLEDNATUREOFTHESTUDIES
Withavastnumberofpapersexploredinthisliteraturereview,noneofthemtestedthe
occurrenceofsecondaryDNAtransferhappeningwithcompletelyuncontrolledscenarios.
39
In my opinion, the closest study that was to an uncontrolled scenario was a study
conductedbyGoray&vanOorschotin2013.Theyusedasmallgroupofindividualswitha
smallnumberofobjects(glasses,jugs,tables,chairs)participatinginsocialinteractionfor
20minutes.31Priortotheexperimentoccurringtheauthorsdidpre-cleanallsurfacesand
itemsandthemajorityofcontrolsproducedanegativeresultasexpected.Ascloseasthis
studywastobeinguncontrolled,theoccurrenceofpre-cleaningdiminishesallbackground
DNApresentonthoseitems,notreflectingwhatweretooccurina“real-life”situation.
4.2TIMEBETWEENTAKINGSAMPLES
Thereseemstobeacommonthemebetweenstudiespublishedthatmostsamplesfrom
transfereventsaretakenalmostimmediatelyaftertheeventoccurs.Asstatedinoneof
thestudies,“thebestchanceofrecoveringtheDNAwouldbetosampleassoonaspossible
afterthefinaltransfereventhasoccurred”.41Thisunfortunatelyisnotpossibleinreal-life
situations as often a crime scenemay not be discovered for hours or days after it has
occurred. This same principle can be applied for the time between transfer events
occurring.Loweetal.hadfoundwhenplacinga30seconddelaybetweentransferevents
the final sample went from containing a full profile with all the first individuals’ DNA
presenttoonlyhave70%ofthatsameindividualspresent.42Evenwiththat30secondtime
delay,thesituationisstillnotindicativeofreal-lifeasindividualswouldnotshakehands
immediatelypriortocommittingacrime.Judgingofftheseresults, itcouldbesaidthat
unlessthesesituationsweretohaveoccurredandtheadditionofsomeothervariablesit
wouldbehighlyunlikelythatDNAfoundatacrimescenewouldbeanyoneelse’sother
thanthatindividualcarryingoutaprimarytransfer.
40
4.3REFERENCESAMPLES
Innearlyallofthestudiesexploredthroughoutthisreview,referencesamplesweretaken
from thevolunteers.This couldbeconsiderednormal for studies in this field,although
whentryingtobestrepresentwhatoccursinthereal-world,thisactiondoesnotcorrectly
reflect that.When interpretingmixturesor just complex single sourceprofiles, forensic
scientistswillmorelikelythannotneverhaveareferencesampleforcomparisonpurposes.
Hereitmaythenbemoredifficulttopulloutmultiplecontributorsofamixturewithout
knowledgeofthemajorcontributor,oranycontributor.Byusingareferencesampleofthe
volunteerduringthesestudies,essentiallymostoftheworkforensicscientistsconductin
regardtoprofileinterpretationisvoid.
5. LIMITATIONSOFMETA-ANALYSIS
5.1NEWAPPLICATIONTOTHISFIELD
Meta-analysesareaverycommonmethodappliedtothatofthemedicalfieldcurrently
andaspreviouslymentioned,ithasnotyetpreviouslybeenappliedtothefieldofForensic
Science.Becauseofthisitwasnotknownifthemethodcouldbeappliedinthesameway
asithasdonebeforeinthemedicalfield.Asdiscoveredthroughattemptingameta-analysis
on thesecondary transferofDNA, itwasnotanappropriatemethodon these typesof
studiespublished.
5.2TYPEOFRESULTSPUBLISHED
Throughconductingameta-analysisofthesecondarytransferofDNA,itwasdetermined
thatthestudiesconductedandpublishedwerenot inaformthatdataextractioncould
41
occurefficiently.Notallstudiespublishedtheirresultsinthesamewayanditwasdifficult
to obtain two quantitative variables for comparison. Close to none of the 38 studies
publishedinthesecondarytransferofDNAandincludedinthemeta-analysis,published
theirrawresultsofvariablessuchastotalamountofDNArecovered.Duetothis,averages,
meansorstandarddeviationswerenotabletobecomputedforresultsincludedinthese
studies,andthereforeameta-analysiswasnotabletobecarriedout.
6. THEFUTUREFORTHESECONDARYTRANSFEROFDNA
Themainpointthatshouldbetakenoutofthisreviewisthatinthisfieldweneedmore
studies thatexamine theoccurrenceof the secondary transferofDNA inmore life-like
scenarios.Untilthen,onlysomuchcanbespeculatedaboutifthistransfereventoccurs
enough to be a valid argument in court for why an individuals’ DNAmay be present.
Although performing these uncontrolled studies may not produce high instances of
secondaryDNAtransferoccurring,throughanapplicationofameta-analysisresultsfrom
eachstudycanbecombinedtoproduceahigherstatisticalpower.
7. CONCLUSION
Atthisstage,itcanbeconcludedthatthehypothesis,throughtheexplorationofliterature
andcompletionofameta-analysisthatanappropriateandconciseguidewillbeproduced
todeterminethechancesofasecondarytransfereventoccurringandwhatconditionsare
required,toaidBiologistsinexperttestimony,canberejected.Datapublishedinthe38
studiesincludedforthemeta-analysiswasnotadequateenoughtoperformquantitative
comparisonsbetweenvariables inorder tocombineresults tostrengthenthepowerof
42
eachstudy.Infuture,itshouldbeconsideredthatarticlespublishedinthefieldofForensic
Sciencedisplaytheirresultsindifferentwaysorgiveaccesstoreadersoftheirrawresults
forfurtherinterpretationpurposes.
43
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49
PARTTWO
MANUSCRIPT
META-ANALYSISOFTHESECONDARYTRANSFEROFDNA
50
ABSTRACT
Deoxyribonucleicacid(DNA)iscomplexmoleculepresentinnearlyeverycellofthehuman
bodythatcontainsallofanindividual’sgeneticmaterial.TraceDNAhasallowedDNAtobe
detectedoneverydayobjectssuchasdoorhandles,tablesandchairs.Currently,theidea
ofthesecondarytransferofDNAhasarisen,involvingthetransferofgeneticprofilesfrom
anindividualthroughavectorandthentofinalindividualorobject.Thishasbecomeare-
occurringterminthecourtsystem,asindividualsstandingtrialusethisargumentforthe
presence of their DNA. It is therefore hypothesized that through the exploration of
literatureandcompletionofameta-analysisthatanappropriateandconciseguidewillbe
produced to determine the chances of a secondary transfer event occurring andwhat
conditions are required, to aid Biologists in expert testimony. Database searcheswere
conductedtogainresourcesinthetopic,followedbyanumberofscreenstodetermine
the suitability of articles for the meta-analysis. With the resultant 38 articles, data
extractionwasadditionallycarried.However,duetothelackofquantitativevariablesand
results,ameta-analysiswasnotabletobeconducted.It issuggestedthatinthefuture,
studiespublishtheirresultsinamorescientificallyrigorousmannerorallowtheaccessof
rawresultsforexternalinterpretationpurposes.
Keywords:forensicscience,secondarytransfer,DNAtransfer,indirecttransfer,touchDNA,
DNA
51
TABLEOFCONTENTS
ABSTRACT………………………………………………………………………………………………....................50LISTOFABBREVIATIONS………………………………………………………………………………...............52INTRODUCTION………………………………………………………………………………............................53MATERIALSANDMETHODS………………………………………………………………..........................55
DATABASESEARCHES……………………………………………………………...............................55SCREENINGOFARTICLES………………………………………………………...............................56DATAEXTRACTION………………………………………………………………................................57
RESULTSANDDISCUSSION……………………………………….....…............................................58LITERATUREREVIEW……………………………………………......…….....................................58
OBJECTASVECTOR………………………………………………....................................58 INDIVIDUALASVECTOR…………………………………………...................................61 INDIRECTFINDINGSOFSECONDARYTRANSFER……………...........................62 SHEDDERSTATUS…………………………………………………….................................64 OTHERFORMSOFDNAINVOLVEDINSECONDARYTRANSFER…………….....65 TERTIARYTRANSFEROFDNAANDBEYOND…………………………....................67 CASESTUDIES………………………………….........……........………….........................69 LIMITATIONS……………………………………………………………................................69META-ANALYSIS…………………………………………………......……...........……………..............71
CONCLUSION……………………………………………………………………………………………..................74REFERENCES…………………………………………………………………………………………......................75
52
LISTOFABBREVIATIONS
DNA=DeoxyribonucleicAcid
ng=Nanograms
ng/µL=Nanogramspermicroliter(concentration)
pg=Pictograms
STR=Shorttandemrepeat
µL=Microliter
UV=Ultraviolet
53
INTRODUCTION
Deoxyribonucleicacid(DNA)ismoleculepresent innearlyeverycellofthehumanbody
thatpossessallofanindividual’sgeneticmaterial.1Eachcellhasapproximatelythreebillion
basepairswhichisessentialforthecharacterizationofeachindividual.1Thesebasepairs
cancreatepolymorphicsequences,forexampleashorttandemrepeat(STR),arepeated
sequenceofDNAthatisusuallyaroundonetosixbasepairsinlength.2Theseareasareof
greatinterestinthefieldofforensicscience,astheircombinationsarevariablebetween
individuals.InForensicScience,STRtypingisusedtoinferthenumberofrepeatsofDNA
where today kits such as PowerPlex 21® and GlobalFiler® are used within forensic
laboratoriesinAustralia.
TraceDNAhasbecomemoreestablishedoverthepastcoupleofdecadesinthefieldof
forensic science. It is defined as cells off the skin surface that have been carried onto
anothersurfacethroughphysicalcontact,allowingdetectionofDNAoneverydayobjects.3
TraceDNAwasfirstexploredingreatdepthbyvanOorschotandJonesin1997,wherethey
found that they could obtain DNA profiles from swabbing objects that were regularly
handledbyaparticular individual.4Previouslyapproximately1ngofDNAwasneededto
gainafullprofilebutnowapproximatelyonly100pgisneeded.5,6Duetothisincreasein
sensitivityofthetechnology,conceptssuchassecondaryandtertiarytransferofDNAhave
nowdeveloped.
ThesecondarytransferofDNAisdefinedasthetransferofDNAfromonepersonorobject
toanotherviaan intermediatepersonorobject,not involvingadirect linkbetweenan
individualandthetargetsurface.6-9Withcontinuedresearchcanresult inanincreaseof
54
knowledge,wherenowconceptssuchastertiaryandquaternarytransferhasarisenthough
secondary transfer is still beingexplored. Similar to secondary transferofDNA, tertiary
transferinvolvesthetransferofDNAfromonepersonorobjecttoanotherthroughtwo
intermediary vectors.10 Quaternary transfer requires an additional transfer events,
resultinginatotaloffourtransfersofDNA.Alltransfereventshavebeenshowntooccur
innumerouspublishedpapers,butnotconsistently.11-15
Since itsfirstuse inthecourtsystemsin1986,DNAhasbeenahighlyregardeditemof
evidence, something that has been very hard to argue against. AsDNAbecomesmore
understood,scientistsareabletogainagreater insight intowhohasbeenpresentata
crimesceneandincreasedsensitivityofkitsallowsforensicscientiststogainDNAprofiles
fromthesmallestofbiologicalsubstances.ClaimsofthesecondarytransferofDNAcan
nowoccurduetothis increasedknowledgeofDNAandsensitivityofkits,meaningthat
nowanindividualmaynotdirectlydeposittheirDNAontoanobject,butrathertransferred
throughavector.This isbecoming increasingly commonwithin thecourt systemanda
need for a concise guide to determine the possibility of secondary transfer has now
emerged.
Aproposedsolutionisthroughameta-analysis,ananalysisthatpoolsresultsfrommultiple
studiestogethertoincreaseitsstrengthandsamplesize,thusthepowerofthestudies.3,16
Tocarryoutacorrectmeta-analysis,itshouldbedoneasapartofasystematicreviewand
includefeaturessuchasacomprehensive,reproduciblesearchandtransparentselection
criteria.16Therearetwomaintypesofreviews,systematicandnarrative.Narrativereviews
exploremorebroader topicsandqualitatively summariseevidence,whereassystematic
55
reviewsfocusmoredeeplyonprimarystudieswhoexploreclinicalquestions.16Thereare
twotypesofmodels thatareusedto forameta-analysis, randomeffector fixedeffect
models.16 The fixed effect model assumes that the ‘true’ effects are fixed whereas a
randomeffectmodelassumesthatonlyarandomsampleisincluded.16Therandomeffect
modelismorepreferredforameta-analysisasitbetterreflectsrealityasstudiesbarely
evermimiceachother.16Thisisthereforewhyarandomeffectmodelwillbeusedforthis
meta-analysis.
Theaimofthis literaturereviewwastoproduceacombinedreviewintothesecondary
transferofDNAinadditiontodrawingmultiplestudiestogethertoattempttoprovidean
analysis of the literature. It is therefore hypothesized that through the exploration of
literatureandcompletionofameta-analysisthatanappropriateandconciseguidewillbe
produced to determine the chances of a secondary transfer event occurring andwhat
conditionsarerequired,toaidBiologistsinexperttestimony.
MATERIALSANDMETHODS
DATABASESEARCHES
Aliteraturereviewwasconductedinconjunctionwiththemeta-analysiswhereresources
weregatheredusingonlinedatabasesintothesecondarytransferofDNA.Databaseswere
chosenbasedontheirrelevancetothefieldofForensicScience.Chosendatabasesincluded
Scopus,WebofScienceCoreCollection,ProQuest,Medline,ScienceDirectandResearch
Gate.Theseincludedbothspecificandgeneraldatabasestoensurehighcoverageofall
56
published literature inthesecondarytransferofDNA.Keytermsusedforsearchingthe
databasesincludedthefollowing:
• “Secondarytransfer”AND“DNA”AND“Forensics”;
• “DNAtransferevents”AND“Forensics”;
• “Tertiarytransfer”AND“DNA”AND“Forensics”;
• “DirectDNAtransfer”AND“Forensics”;
• “Shedderstatus”AND“Forensics”and
• “Indirecttransfer”AND“DNA”AND“Forensics”.
Noadditionalsearchtermrestrictionswereappliedduetothespecificityofthedatabases
andthe lackof functionprovidedtodoso.Theonly restrictionthatwasappliedto the
database searches were those that eliminated specific types of resources, e.g. book
chapters&encyclopedias.Alldatabasesalreadyhadrestrictionsappliedtosearchessuch
aslanguageanddates,whereEnglishwasthelanguageanddatesrangedfromthe1990’s
toApril2018.
SCREENINGOFARTICLES
Atotalof307articleswereidentifiedthroughthedatabasesearch,compiledofallyielded
searchresults.Allarticlesthenhadtheirreferencelistsscreenedresultinginafurther103
eligiblearticles,creatingatotalof410articlesidentifiedthroughtheliteraturesearch.The
nextstepwastoscreenall410articlesusingtheirtitlesandabstracts,basedoninclusion
andexclusioncriteria.Themainexclusioncriteriathatwasappliedwasifthearticledidnot
involvethetransfereventofDNAinanyform(e.g.touchDNA,spermatozoa,bloodand
saliva) it would be excluded from the meta-analysis. Other exclusion criteria included
papersthatinvolvesacasestudy,thosethatarearesponsetoajournalarticleandaform
57
ofwritingthatwasnotajournalarticleortheses(e.g.bookorabookchapter).Fromthis
screeningprocess,atotalof141articlesweredeemedsuitable.These141articlesfurther
had theentirebodyof text screened, againusing inclusionandexclusion criteria. If an
articlehadarecordedormeasuredoccurrenceofthesecondarytransferofDNAinany
form, it was able to be encompassed in themeta-analysis. Articles that discovered an
occurrence of the secondary transfer ofDNAbut indirectly (was not themain variable
testingfor)werealsoexcluded.Thisscreenresultedinatotalof38articlesthatwereable
tobecontinuedthroughtothemeta-analysis,and103studiesexcluded.
DATAEXTRACTION
Data was extracted from all 38 articles discovered through the screening process and
collatedintoatableusingMicrosoftExcel.Thisdataincludedthefollowing:
• Author;
• Year;
• Conditionsoftheexperiment(Controlled,semi-controlled,uncontrolled);
• NumberofParticipants;
• Numberofsamplestaken;
• Typeofvector;
• FinalobjectoftheDNAdeposition;
• Timebetweentheprimaryandsecondarytransferevent;
• Timehandlingthevector;
• Timehandlingthefinalobject;
• Typeandpressureofcontactofvector;
58
• AveragetotalDNArecovered(ng);
• TotalDNAconcentration(ng/µL);
• Totalnumberofunknownallelesdetected(indicativeofsecondarytransfer);
• Numberofmixturesseen;
• Ifareferencesampleofvolunteershadbeentaken;and
• IfsecondarytransferofDNAinanyformwasdetected.
RESULTSANDDISUCSSION
LITERATUREREVIEW
RegardinganytransfereventofDNA,thewayinwhichitoccursisrelativelysimilaracross
theboard.TheremustalwaysbeaprimarytransfereventinvolvingthemovementofDNA
from an individual to either an object or another individual. There then must be a
subsequenttransferevent,wherethatsamegeneticprofilewillbetransferredeithertoan
individual or an object. In this case, it could be hypothesized that the more common
scenariowouldbethatofanindividualasthevector.Belowstudiesusingbothinstances
asvectorswillbeexploredandreviewedtogainagreaterunderstandingastohowthis
mechanismoccursanditslikelihood.
OBJECTASVECTOR
When the secondary transfer of DNA is put forth as a possible explanation, it ismore
sensibletohaveoccurredthroughanobjectratherthananindividual.Oneofthefirststudy
thatperformedsecondaryDNAtransferthroughanobjectwasperformedin2012.Thefirst
59
experimentsawhandscomeincontactwithatoyorasingletandthenthatobjectcoming
incontactwithalaboratorycoat.17Resultsshowedthatin19outof20repeats,DNAfrom
thedonorwasobserved.17AuniqueapproachwasusedbyFrenchetal.whereUltraviolet
(UV)powderwasusedthroughthetransferexperiment.Threeparticipantssatarounda
tabledrinkingfromabottlewiththeirownthreeglassesfor30minuteswhereonlyone
individualstartedwiththeUVpowder.18Therewasnodirectcontactbetweenindividuals
butthestudyshowedtheindirecttransferoftheUVpowdertoallindividuals.18
GorayandvanOorschotperformedaverysimilarexperimenttoFrenchetal.thefollowing
yearwherethesame“uncontrolled”conditionswereused.Herehowevertheydidnotuse
UVpowderbuttestedfortheDNAitself.In27%ofthetablesamplesandthen33%ofthe
chairsampleshadaDNAprofilesfromparticipantsthatdidnotcomeintocontactwiththe
surfaceindicatingthattheseprofilesmighthavebeentransferredfromthejug.19Afurther
58% of table samples and 42% of chair samples had DNA profiles from unknown
individuals.19
Margiottaetal.usedglovesasthevectorfortransferwhereinhalfthecasestestedfor,
they found a mixture of DNA containing alleles belonging to an unknown individual
different to that of the volunteer.20 In an additional 15%of the cases just alleles from
another unknown individual were recovered not attributed to the volunteer.20 Here
secondarytransferhasoccurred,althoughauthorsdidpointoutsomeof theirnegative
controlsfromsupposedly“clean”glovesdidyieldalleles.20Szkutaetal.furtheredworkof
otherstudiesthatfocusedonthequantityofDNAthatisleftonvectorsafteratransfer
eventhasoccurred.Secondary transfereventswereconductedbetweentwo“exhibits”
60
usingtoolssuchasforceps,scissorsandglovesasthevector.21Itwasshownthatsecondary
transfercouldoccurfromexhibittoexhibitbyDNA-freevectorsbutthemainfindingwas
thepresenceofasufficientquantityofDNAonthevectorforfurthertransferevents.21
Fonneløp et al. investigated the transfer of DNA based on a case study, using three
volunteerswhowereknowntobegoodshedders.12Thefirstindividualhandledanobject
for 30 seconds followed by a second individual.12 Results showed for the first transfer
event,83%oftheDNAwashighenoughqualityforprofiledetectionandforthesecond
transfer event, that figure dropped to 53%.12 In 2017, Buckingham et al. furthered a
previousstudywhereporouswereusedratherthannon-poroussurfacestoinvestigatethe
resulting yield and profile of DNA. Some results produced evidence of secondary and
tertiary transfer ofDNA although the proportion to the depositor’sDNAwas less than
10%.22
Theuseofalaundrymachineforthetransferofspermatozoaisarelativelywellcovered
topic.AstudyconductedbyKamphausenetal.in2015wasoneofthefirsttestingforthe
transferoftouchDNAhoweverthesecondarytransferoftouchDNAwasdeemedalmost
impossibleforreliableSTRanalysis.23Asecondstudyconductedin2018sawunwornsocks
werewashed inanormal loadofhousehold laundry.1522%of samplesyieldeda result
higher than theminimal threshold forcasework (0.06ng/µL).15Foursamplesmatcheda
profile of a female from thehousehold indicative of the transfer ofDNAoccurringbut
unknownastoeitherdirectorindirecttransfer.15
61
VanOorschotetal.wasthefirsttotesttheextenttowhichDNAtransfermayoccurusing
fingerprintbrushes.2473currently-usedfingerprintbrushedwerebrushedover5sheetsof
plastic, includingonecontaininga freshdepositedhandprint.24Theresultsshowedthat
verylimitedpickupandtransferofDNAoccurredbythebrusheshoweverwhenchanges
weremadetotheDNAanalysis,significantpickupandtransferofDNAwasseen.24Asecond
studywasconductedbyProffetal.wheretheydeterminedthatDNAcontaminationwith
fingerprint brushes was quite common (85%) and for the secondary transfer, 25% of
surfaceshadDNAdetectedwithonefullprofile.25Aswiththepreviousstudy,theyalso
foundalimitedriskofthesecondarytransferofDNAthroughfingerprintbrushes.24,25
INDIVIDUALASVECTOR
This scenario appears to be themore common scenario to occur although is the least
researched.ThefirststudythatusedanindividualasavectorforthetransferofDNAwas
conductedby Loweet al. in2003. The study found thatwhenpoor shedderswere the
vector,more often than not, the original individual had their full profile transferred.26
Additionally,witha30secondtimedelaybetweentransferevents,mixedprofileswere
discovered where 70% were from the original source.26 Farmen et al. additionally
conductedastudywheretwoindividualsundertookahandshakefollowedbythegripping
ofbeakers.9Althoughnotspecified, theauthorssuggest thatall samplesweremixtures
containingbothprofilesoftheknowncontributors,indicatingthatsecondaryDNAtransfer
hadsuccessfullyoccurred.9
AstudyconductedbyMeakinetal in2015alsoshowedthesecondary transferofDNA
throughahandshakeandthenhandlingaknife.Thestudyfoundthatthreeofthefoursets
62
ofknives,mixtureswerefoundcontainingthreedifferentprofiles.27Twoprofilescouldbe
attributedtothetwovolunteers,buttheotherwasanunknownprofile,possiblythecause
of secondary or tertiary transfer.27 Cale et al. performed a standard study into the
secondarytransferofDNAwithanewkitwithincreasedsensitivityduetoagapfoundin
theliterature.TheauthorsaimedtostudyhowthepresenceofsecondaryDNAtransfer
wouldaffecttheinterpretationofDNAtypingresults.6Dataobtainedfromfiveofthe24
samples suggest that individuals can have their DNA deposited on objects in sufficient
quantitiesthattheycanbeidentifiedastheonlyormajorcontributor,suggestingthatthey
hadcomeindirectcontactwiththeobject.6
INDIRECTFINDINGSOFSECONDARYTRANSFER
TherearealsoinstancesthatoccurinwhereprimarytransferorthepersistenceofDNA
thathavebeenexploredduringastudyalthoughunknownallelesaredetected.Here,these
studies thathavedetected secondary transferofDNAduring another experimenthave
beencollated.However,inthesestudies,unlesstheauthorshavespecificallystatedwhere
thesecondarytransferhasoccurredandwhotheunknownallelesbelongto,wedonot
knowtheoutcome.Additionally,unknownallelesattributedtobackgroundDNApresent
onaregularly-usedobject.
ThefirstinstanceofaccidentaloccurrenceofsecondaryDNAtransferwasduringastudy
conductedbyLoweetal.in2002wheretheauthorstestedshedderstatusofindividuals.
Secondary transferwasdetectedwhenparticipantshelda tube for10 secondsbut the
extentwasdependentonthesubjectpairings.28Afewyearslater,Phippsetal.conducted
astudyingreatsimilaritywheretheyfoundthattheoccurrenceofsecondarytransferwas
63
lowandnotlikelybutpossible.29Only14allelesoutof581couldnotbeattributedtothe
volunteersequatingto0.02%.29AstudyconductedbyRuttyfoundaninstanceofpossible
secondaryDNAtransferoccurringwheremale-femalepairswereusedinordertosimulate
astrangulationevent.30Twoswabsofthemale’sfingers(controls)yieldedaprofileofa
female inthestudyalthoughthesetwoindividualshadneverbeenpairedorcomeinto
contactwithoneanother.30
Petricevicetal.usedfivevolunteerswhosleptintheirownbedbutwithanewlowerbed
sheetor individualsslept inabedwheretheyneverhadpreviously.31When individuals
sleptintheirownbeds,asecondindividual’sDNAprofilewasfoundinatleastonesample
from3outofthe5volunteers.31Whenindividualssleptinanunknownbed,allunknown
samplescouldbeattributedtotheregularuser’sprofilewhichdidnotindicatesecondary
DNAtransfer.Djuricetal.hadvolunteersholdplastictubesandindividual’sanklesfor10
seconds.32Whenvolunteersheldtubes,outofthesevenprofilesobtains,oneresultedin
a partial profile not matching the volunteer’s, indicating the occurrence of secondary
transfer.32Whenankleswereusedasdeposition sites, nooccurrenceof the secondary
transferofDNAwasnoted.32Gorayetal.conductedaverysimilarexperimentagainbut
testedthedepositionontoglassplatesratherthanplastictubes.Atotalof40sampleswere
takenandnon-selfDNA(unexpectedalleles)wasseenin79%ofthesamples.33
ApaperpublishedbyOldonietal.appliedtheconstantunknownfromcontactstainsat
break-ins to conducted an experiment. A total of 231 samples were created where
approximately2-10%containedunknownalleles,possiblesuggestingsecondarytransfer.34
In2015,FonneløpshowedthatsecondarytransferofDNAoccurredthroughthepresence
64
oftheoriginaluser’sDNAonthehandsoftheseconduserupto8daysafterreceivingthe
computerequipment.35Lacerenzaetal.directlyswabbedindividual’shand’swhereoutof
120samples,56hadoneormoreunknownallelesand36ofthosecouldbeclassifiedas
mixtures(64.3%).37InastudyconductedbyvandenBergeetal.,individualsdraggedeach
othermimickinganactivity-relatedscenario.Outof26samples,3ofwhichdidnotresult
inover7alleles.37Donoralleleswerepresent in100%ofthesamplesandnon-donorin
71%.36Morerecently,Mageeetal.usedcollarsandcuffsofuppergarmentstodetermine
the presence of DNA.38 Out of 55 samples taken, non-wearer DNA was recovered, an
averageof1.3ngfromthecollarand2.7ngfromthecuffs.38
SHEDDERSTATUS
Shedderstatusisoneofthemorerecentconceptsundertheumbrellaofthetransferof
DNAandonethatwillcontinuetogrowasmoreresearchisconductedintothetopic.The
first study to directly carry out an experiment on individual’s shedder status’ was
conductedbyLoweetal.in2002andisconsideredthebackbonestudytothistopic.The
authorsgotparticipantstogripasteriletube,15minutes,2hoursand6hourperiodsafter
hand-washing.29 They determined that there were differences between individuals in
regardtohowmanyallelesthattheydepositedonasurface.29Goodshedderswerethen
classediftheindividualleftanentireprofileafter15minutesofhand-washing.29Allother
individualswereclassedaspoorshedders.
Phippsetal.conductedasecondstudythatinvestigatedshedderstatusandusedalmost
identical experiments to that of Lowe et al. Unexpectedly, the study had shown that
individualsdonotproduceconsistentquantitiesofDNAovertime,varyingasmuchwithin
65
themselvesthancomparedtootherpeople.30UsingthecriterialaidoutinLoweetal.,this
studydiscoveredno“good”sheddersandallwereclassifiedas“poor”shedderssuggesting
thislevelofclassificationmaynotbeallassimpleasthought.29,30Farmenetal.measured
individuals’ shedder status just based on swabs from their hands.39 It did not test the
quantityofDNAthattheindividual’sactuallydepositonasurface,soitmaynotclassasa
truerepresentationofshedderstatus.Outofthenineindividualswhoparticipatedinthe
study,twoindividualshada lowDNAyieldandsevenindividualsproducedprofilesthat
were able to be analysed.39 More recently, Goray et al. collected 240 samples from
handprintsonglassplateswhereonlyfourofthesampleshadnodetectablequantitiesof
DNA.33Twoofthetenparticipantscouldbeclassedasgoodsheddersbutthiswasdone
underdifferentcriteriaandexperimentalcriteriaastothebasestudyconductedbyLowe
etal.33
OTHERFORMSOFDNAINVOLVEDINSECONDARYTRANSFER
Oneof the first studiescarriedout in thebroader realmof the transferofDNAwasby
Kafarowskietal.in1996.Conductedbeforethefirstoccurrenceofthesecondarytransfer
oftouchDNA,itaimedtodeterminethelikelihoodofthetransferofspermatozoabetween
articlesofclothingduringamachine-cyclewash.40Althoughtheretentionofspermatozoa
duringmachinewashhadpreviouslybeenexplored,thetransferofittootherarticlesof
clothinghadn’t.41,42Theauthorsusedunderpantsasameansfortheprimarydepositof
spermatozoa,washingthemwiththreeadditionalcleanpairsandthenmachinedried.40In
allthreetrials,tracequantitiesofspermatozoawerefoundwhereaminimumofonesperm
headandmaximumofeightpersample.40
66
SalivaisanadditionalbiologicalsubstancethatcontainshumanDNAthathasthepossibility
tobetransferredthroughobjectsandindividualstocauseasecondarytransfer.Thefirst
thatexploredintothissecondarytransferofsalivawasconductedbyWiegandetal.where
salivawastransferredontoeitherpaper,cottonorplasticsurfaces,leftthemtoairdrythen
athumbrubbedontothestain.43Thethumbwasthenplacedontoanotherpieceofpaper
andthentheareawasswabbed.43Intotalonly1outof96gavecompleteprofiles,allfrom
plasticastheoriginalsource.43Vectorsofpensandplastictubeswereusedinanadditional
study.Theirstudyconcludedthatsecondarytransferofsalivadidoccur,ingreaterlevels
of retention than touch DNA and thatmoist surfacesmay facilitate DNA transfer to a
greaterdegreethanadrysurfaceaspreviouslydiscoveredinGorayetal.2010.11,43
In2015,thetransferofsalivathroughlaunderingwasstudiedwhereindividualsdeposited
saliva onto textile cloths and then these were subsequently washedwith clean textile
cloth.23 A total of 46 independentwasheswere completed both by hand andmachine
with/without detergent for saliva, blood and touchDNA.23 Salivawas seen to undergo
transfereventswhichsupportsotherstudiesperformed.11,23,24,43,44Themostrecentstudy
conducted was using individual’s hands as vectors for the transfer. 14 areas of the
individuals palm were swabbed in addition to the glass plate which the saliva was
transferredto.45Thestudyyieldedanumberofunknownallelesonthefinalplatewhere
theauthorscontributedthis toprior interactionwith individuals,supportingthefinding
thatsecondarytransferofDNAhadoccurred.45
Wiegandetal.conductedastudyintotheriskofthetransferofbloodstainsfromsurfaces
suchasplastic,cottonandpaper.Thebloodstainswereairdriedthenanindividualplaced
67
theirthumbonthestainthenontoanotherpieceofpaper.43SecondarytransferofDNA
wasfoundtooccuronthecottonsurface,bothwithandwithoutgloves.43Lehmannetal.
conducted a very similar experiment but all transfer events were conducted with
substratesonlyandnohumaninteraction.Theexperimentsupportedtheexistenceofthe
secondarytransferofDNAthroughbloodandadditionallyfoundthatifthebloodwerewet,
itwouldtransferfurtherandwouldendinahigherconcentrationasmoresubstancewas
transferred.46
Kamphausen et al. investigated the secondary transfer ofDNA through laundering in a
washingmachineorbyhand.Artificialbloodstainswerecreatedontextilecloth,driedand
thenwashedwithcleantextilecloths.23Through46independentwashes,transferofblood
cellswasseensupportingpreviousstudies.23,43,46,47Agaphadthenbeenidentifiedinthe
literature,wherenoneofthepreviousstudieshadthoroughlytestedtheeffectofaridity
ofblood.vanOorschotetal.useddryingtime,temperatureandhumidityasdependent
variablesandcottonas the secondary substrate.47The secondary transferofbloodwas
seen to have occurred with significant differences in the transfer percentage of blood
between5and60minutes.47
TERTIARYTRANSFEROFDNAANDBEYOND
Although the secondary transfer of DNA is still in the process of gaining a greater
understandingofitsmechanisms,throughthesestudiesthediscoveryoftransferevents
occurring after a second transfer have occurred. These studies are still in their very
preliminarystagesandlikesecondaryDNAtransfer,stillrequireagreaterunderstanding
intothemechanismssurroundingtheprocess.Thefirststudythatexploredtheoccurrence
68
ofthetertiarytransferofDNAwasconductedbyWarshaueretal.whereanobjectwasthe
original vector andwas further transferred from an individual to another individual or
objectasthetertiarytransferevent.11TheywereabletoprovethattertiarytransferofDNA
couldoccur,where87.5%displayedlessthanhalfoftheexpectedalleles.11Theseresults
correlatedwiththoseproducedbyFonneløpetal.whereafterthefinaltransfersteponly
17% of samples were high quality that could be reported and go through database
searching(5outof30transferevents).12
In the study that was conducted by Fonneløp et al. they also tested the instance of
quaternarytransferoccurring.Theyfoundthatonceagain,itoccurredwhereatotalof17
out of the 108 analysed samples contain unknown alleles.12 In 2016, Helmus et al.
performedthefirststudysolelytestingthetertiarytransferofDNA,agapidentifiedwithin
theliterature.Outof180samplesgenerated,only72haddetectableDNAthatidentified
thattertiarytransferhadoccurred(40%).12Inastudycompletedtheyearearliertheyhad
additionallyshowntheoccurrenceofthetertiarytransferofDNAbuttheauthorsnoted
thattheywereunabletodistinguishbetweentertiarytransferorinterferencefromalow-
levelofnon-donorDNA.13Inamorerecentstudy, tertiary transferofDNAevolvedtoa
morecomplexmechanismthanthatofindividual-objectinvolvedtransfer.Sixnewunworn
socksandat-shirtwerelaunderedtogetherwithnoadditionalitemsinamachinethathad
beenpreviouslyused.15ResultsshowedthattherewerenoDNAresultsrecoveredfrom
quantity (above threshold) through to profiling, suggesting that tertiary transfer didn’t
occur.15
69
CASESTUDIES
In 2017, an article was published following the incidence of contamination by Police
Officer’s in Austriawhichwithout elimination databases,many instances of indirect or
secondarytransferofDNAmaygoundetected.48Duringtheperiodof2000to2016,atotal
of 46,000 trace sampleswere submitted throughDNA and out of those a total of 347
contamination incidents were detected through the use of the elimination database
(0.75%) with a majority from indirect transfer.48 The authors further went into detail
regarding3separateincidentsthattheywereabletopinpointwherethetransferevents
hadoccurred.Thefirstincidentoccurredthroughsharedcameraequipmentthathadbeen
usedbyaPoliceOfficercompletelyunrelatedtothecase.48Thesecondshowstheindirect
transferbyaPoliceOfficerthroughasharedvehicletoavolumecrimecaseandthethird
throughpackagingofevidencematerialonanunrelatedPoliceOfficer’sdesk.48
LIMITATIONS
Limitations of the conduction of this literature review included the level of control of
studies,timebetweentakingsamplesandtheacquisitionofreferencesamples.Nostudies
presented in this literature reviewhad tested for the secondary transferofDNAunder
completelyuncontrolledscenarios.Thecloseststudytohavingacompletelyuncontrolled
study was conducted by Goray & van Oorschot in 2013. They used a small group of
individualswith a small number of objects (glasses, jugs, tables, chairs) participating in
socialinteractionfor20minutes.19Priortotheexperimentoccurringtheauthorsdidpre-
clean all surfaces and items. As close as this study was to being uncontrolled, the
occurrenceof pre-cleaningdiminishes all backgroundDNApresent on those items, not
reflectingwhatweretooccurina“real-life”situation.
70
Acommonthemebetweenstudiessawsamplesfromobjectsor individualsbeingtaken
almostimmediatelyafterthetransfereventoccurring.Asstatedinoneofthestudies,“the
bestchanceofrecoveringtheDNAwouldbetosampleassoonaspossibleafterthefinal
transfereventhasoccurred”.27Thisunfortunatelyisnotpossibleinreal-lifesituationsas
oftenacrimescenemaynotbediscoveredforhoursordaysafter ithasoccurred.This
sameprinciplecanbeappliedforthetimebetweentransfereventsoccurring.Loweetal.
hadfoundwhenplacinga30seconddelaybetweentransfereventsthefinalsamplewent
fromcontainingafullprofilewithallthefirstindividuals’DNApresenttoonlyhave70%of
thatsameindividualspresent.28Evenwiththat30secondtimedelay,thesituationisstill
not indicative of “real-life” as individuals would not shake hands immediately prior to
committingacrime.Basedofftheseresults,itcouldbesaidthatunlessthesesituations
weretohaveoccurredandtheadditionofsomeothervariablesitwouldbehighlyunlikely
thatDNAfoundatacrimescenewouldbeanyoneelse’sotherthanthatindividualcarrying
outaprimarytransfer.
Anothervariablewhichdoesnotmimic“real-life”situationsistheacquisitionofreference
samplesofvolunteersduringstudies.ForinterpretinganyDNAprofile,forensicscientists
willmostlikelynothaveaccesstoareferencesampleforcomparisonpurposesunlessa
known suspect had been previously profiled. Here itmay bemore difficult to pull out
multiple contributors of amixturewithout knowledgeof themajor contributor, or any
other contributor. By using a reference sample of the volunteer during these studies,
essentiallymostoftheworkforensicscientistsconductinregardtoprofileinterpretation
isvoid.
71
META-ANALYSIS
Itwasoriginallyhypothesizedthatthroughtheexplorationofliteratureandcompletionof
ameta-analysisthatanappropriateandconciseguidewillbeproducedtodeterminethe
chancesofasecondarytransfereventoccurringandwhatconditionsarerequired,toaid
Biologists inexperttestimony.Thishypothesiswasrejectedonthebasisthatthemeta-
analysiswasfoundnotbeanadequatemodelforthedatathatwasavailable.Thescreening
processofarticlesexploringthesecondarytransferofDNAwassufficientforthisfieldof
science,asarticleswereabletobeeasilyincludedandexcludedbasedonasetofcriteria.
Dataextractionwasadditionally able tobe carriedout to someextent asmost journal
articlesgiveaccuraterepresentationsofthestudyandhowitwasperformed.However,
through the creation of a table, incorporating all of the included studies for themeta-
analysisandtheconductionofthemeta-analysis,itwasdeterminedthatthemeta-analysis
wouldnotbeadequateforthedatathatwasavailable.
Forthismeta-analysis,itwashopedtouseaverageamountoftotalDNArecovered(ng)as
oneofthequantitativevariables.Articleshoweverdidnotallgiveresultsthatwouldbe
sufficienttocalculateanaveragefrom.SomegaveallvaluesoftotalDNArecoveredfrom
thefinalobjectsaftersecondaryDNAtransfer,whereanaveragewasabletobecalculated.
Conversely,somearticleswouldstatearangeofthetotalamountofDNArecoveredwhich
anaveragecouldnotbecalculatedfromastherawresultswerenotpresent.Therefore,
therewasnotaconsistentsetofvaluesthatwouldbeusedtorepresentthetotalamount
ofDNArecoveredthroughallincludedstudies.
72
Asasecondquantitativevariablewasrequired,outofthedataavailabletoextract,time
handlingthevector,finalobjectortimebetweenhandlingthevectorandfinalobject,were
theonlytwooptionsavailable.Althoughthisvaluedifferedbetweenstudies,withinastudy
itwashighlylikelythatonlyasingletimevaluewasutilized.Forexample,Gorayetal.used
oneminuteforthetimehandlingthevectorandthefinalobjectforallvolunteers.17Two
different vector types were used within the study with two different times between
handlingthevectorandfinalobject(immediatelyand24hoursafter).17BothtouchDNA
and bloodwas used as the biological substances, creating four possible scenarios.17 As
thesequantitativevariablesdidnotvary substantiallywithin thestudy, itwouldnotbe
possibletocomparethevariableswithinthestudy.Otherstudiesadditionallyonlyhadone
vector typeandbiological substance,andthereforeonlyonequantitativevalueof time
handlingvectorsorobjects.23,28,29,27,30
Reflecting back onto the hypothesis, it was stated that it was anticipated that “an
appropriateandconciseguidewillbeproducedtodeterminethechancesofasecondary
transfereventoccurringandwhatconditionsarerequired”.Asmentionedpreviously,the
frameworkthatameta-analysisworksonisthecomparisonoftwoquantitativevariables
inthehopetoperformstatisticaltestssuchasCohen’sDorPearson’scoefficient.Inorder
to determine the conditions required for the secondary transfer of DNA to occur,
quantitativevalueswouldneedtobegathered.TheconditionthatsecondaryDNAtransfer
are conducted under would include things such as the vector type, final object type,
conditionsoftheexperiment,backgroundDNApresentandtypeofpressureandcontact
ofthevectororfinalobject.Thesevariablesunfortunatelyarequalitativeanddonothave
an option to measure them quantitatively. In that case, performance of comparisons
73
betweenthesevariablesandquantitativevariablessuchastotalDNA,DNAconcentration
ortimehandlingobjectscannotbecarriedout.
Otherlimitationsofthemeta-analysisincludedthenewapplicationtothefieldofforensic
science,typesofresultspublishedandthesourceoftheDNA.Asmentionedpreviously,
meta-analysesaremostcommonlyusedwithinthemedicalfieldandhasnotyetbeenused
withinthefieldofForensicScience.Therefore,itwasnotknowniftheapplicationofthis
methodcouldbeappliedthesamewaythatishasdonepreviouslyinthemedicalfield.As
discovered through thispaper, itwasdeemedan inappropriateanalysis for the typeof
studiesthatarepublishedonthesecondarytransferofDNA.Thisismainlyduetothetype
of results thatarepublished.Resultswerenotpublished ina formthatdataextraction
couldoccurefficiently,additionallymakingitdifficulttoobtaintwoquantitativevariables
forcomparison.Closetononeofthe38studiespublishedinthesecondarytransferofDNA
and included in themeta-analysis,published their rawresultsofvariables suchas total
amountofDNArecovered.Duetothis,averages,meansorstandarddeviationswerenot
abletobecomputedforresultsincludedinthesestudies,andthereforeameta-analysis
wasnotabletobecarriedout.
ItwasalsodifficulttopinpointastowheretheDNAhadcomefrom,asthepresenceof
DNA on an object or individual could either be a result of secondary DNA transfer or
backgroundDNA.Inthiscase,onlystudieswhocanspecificallytestedforthesecondary
transferofDNAcouldbeincludedinthemeta-analysis,asitwascertainthatthenon-donor
DNAhadcomefromthatspecificevent.However,thisthandecreasedtheexperiment’s
74
applicability to the real world, where now background DNAwas not present, possibly
causinganunrealisticresult.
CONCLUSION
Atthisstage,itcanbeconcludedthatthehypothesis,throughtheexplorationofliterature
andcompletionofameta-analysisthatanappropriateandconciseguidewillbeproduced
todeterminethechancesofasecondarytransfereventoccurringandwhatconditionsare
required,toaidBiologistsinexperttestimony,canberejected.Datapublishedinthe38
studiesincludedforthemeta-analysiswasnotadequateenoughtoperformquantitative
comparisonsbetweenvariables inorder tocombineresults tostrengthenthepowerof
eachstudy.Infuture,itshouldbeconsideredthatarticlespublishedinthefieldofForensic
Sciencedisplaytheirresultsindifferentwaysorgiveaccesstoreadersoftheirrawresults
forfurtherinterpretationpurposes.
75
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