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Populations and Statistics in Forensic Genetics
Linköping University, Department of Clinical and Experimental Medicine. Linköping University, Faculty of Health Sciences.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

DNA has become a powerful forensic tool for solving cases such as linking a suspect to a crime scene, resolving biological relationship issues and identifying disaster victims. Traditionally, DNA investigations mainly involve two steps; the establishment of DNA profiles from biological samples and the interpreta-tion of the evidential weight given by theses DNA profiles. This thesis deals with the latter, with focus on models for assessing the weight of evidence and the study of parameters affecting these probability figures.

In order to calculate the correct representative weight of DNA evidence, prior knowledge about the DNA markers for a relevant population sample is required. Important properties that should be studied are, for example, how frequently certain DNA-variants (i.e. alleles) occur in the population, the differences in such frequencies between subpopulations, expected inheritance patterns of the DNA markers within a family and the forensic efficiency of the DNA markers in casework.

In this thesis we aimed to study important population genetic parameters that influence the weight of evidence given by a DNA-analysis, as well as models for proper consideration of such parameters when calculating the weight of evi-dence in relationship testing.

We have established a Swedish frequency database for mitochondrial DNA haplotypes and a haplotype frequency database for markers located on the X-chromosome. Furthermore, mtDNA haplotype frequencies were used to study the genetic variation within Sweden, and between Swedish and other European populations. No genetic substructure was found in Sweden, but strong similari-ties with other western European populations were observed.

Genetic properties such as linkage and linkage disequilibrium could be im-portant when using X-chromosomal markers in relationship testing. This was true for the set of markers that we studied. In order to account for this, we pro-posed a model for how to take linkage and linkage disequilibrium into account when calculating the weight of evidence provided by X-chromosomal analysis.

Finally, we investigated the risk of erroneous decisions when using DNA in-vestigations for family reunification. We showed that the risk is increased due to uncertainties regarding population allele frequencies, consanguinity and compet-ing close relationship between the tested individuals. Additional information and the use of a refined model for the alternative hypotheses reduced the risk of making erroneous decisions.

In summary, as a result of the work on this thesis, we can use mitochondrial DNA and X-chromosome markers in order to resolve complex relationship in-vestigations. Moreover, the reliability of likelihood estimates has been increased by the development of models and the study of relevant parameters affecting probability calculations.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2010. , 55 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1175
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-54742ISBN: 978-91-7393-420-6 (print)OAI: oai:DiVA.org:liu-54742DiVA: diva2:309703
Public defence
2010-05-07, Elsa Brändström-salen, Campus US, Linköpings universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2010-04-08 Created: 2010-04-08 Last updated: 2010-04-08Bibliographically approved
List of papers
1. DNA-testing for immigration cases: The risk of erroneous conclusions
Open this publication in new window or tab >>DNA-testing for immigration cases: The risk of erroneous conclusions
2007 (English)In: Forensic Science International, ISSN 0379-0738, E-ISSN 1872-6283, Vol. 172, no 03-Feb, 144-149 p.Article in journal (Refereed) Published
Abstract [en]

Making the correct decision based on results from DNA analyses and other information in family reunification cases can be complicated for a number of reasons. These include stratified populations, cultural differences in family constellations, families with different population origin, and complicated family relations giving complex pedigrees. The aim of this study was to analyze the risk of erroneous conclusions in immigration cases and to propose alternative procedures to current methods to reduce the risk of making such errors. A simulation model was used to study different issues. For simplicity, we focus on cases which can be formulated as questions about paternity. We present an overview of error rates (of falsely included men as the true father and of falsely excluded true fathers) for fairly standard computations, and we show how these are affected by different factors For example, adding more DNA markers to a case will decrease the error rates, as will the inclusion of more children. We found that using inappropriate population frequency databases had just minor effects on the error rates, but the likelihood ratios varied from an underestimation of 100 times up to an overestimation of 100,000 times To reduce the risk of falsely including a man related to the true father we propose a more refined prior including five hypotheses instead of the two normally used. Simulations showed that this method gave reduced error rates compared with standard computations, even when the prior does not exactly correspond to reality.

Keyword
immigration casework; Bayesian; mutation model; simulation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-53628 (URN)10.1016/j.forsciint.2006.12.015 (DOI)
Available from: 2010-01-26 Created: 2010-01-26 Last updated: 2017-12-12
2. Homogeneity in mitochondrial DNA control region sequences in Swedish subpopulations
Open this publication in new window or tab >>Homogeneity in mitochondrial DNA control region sequences in Swedish subpopulations
2010 (English)In: International journal of legal medicine (Print), ISSN 0937-9827, E-ISSN 1437-1596, Vol. 124, no 2, 91-98 p.Article in journal (Refereed) Published
Abstract [en]

In order to promote mitochondrial DNA (mtDNA) testing in Sweden we have typed 296 Swedish males, which will serve as a Swedish mtDNA frequency database. The tested males were taken from seven geographically different regions representing the contemporary Swedish population. The complete mtDNA control region was typed and the Swedish population was shown to have high haplotype diversity with a random match probability of 0.5%. Almost 47% of the tested samples belonged to haplogroup H and further haplogroup comparison with worldwide populations clustered the Swedish mtDNA data together with other European populations. AMOVA analysis of the seven Swedish subregions displayed no significant maternal substructure in Sweden (F (ST) = 0.002). Our conclusion from this study is that the typed Swedish individuals serve as good representatives for a Swedish forensic mtDNA database. Some caution should, however, be taken for individuals from the northernmost part of Sweden (provinces of Norrbotten and Lapland) due to specific demographic conditions. Furthermore, our analysis of a small sample set of a Swedish Saami population confirmed earlier findings that the Swedish Saami population is an outlier among European populations.

Keyword
Mitochondrial DNA, Sweden, Saami, Control region, Demography
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-54153 (URN)10.1007/s00414-009-0354-7 (DOI)000274521800001 ()
Available from: 2010-02-26 Created: 2010-02-26 Last updated: 2017-12-12
3. Analysis of linkage and linkage disequilibrium for eight X-STR markers.
Open this publication in new window or tab >>Analysis of linkage and linkage disequilibrium for eight X-STR markers.
Show others...
2008 (English)In: Forensic science international. Genetics, ISSN 1878-0326, Vol. 3, no 1, 37-41 p.Article in journal (Refereed) Published
Abstract [en]

X-chromosomal short tandem repeats (X-STR) have proven to be informative and useful in complex relationship testing. The main feature of X-STR markers, compared to autosomal forensic markers, is that all loci are located on the same chromosome. Thus, linkage and linkage disequilibrium may occur. The aim of this work was to study population genetic parameters of eight X-STR markers, located in four linkage groups. We present haplotype frequencies, based on 718 Swedish males, for the four linkage groups included in the Argus X-8 kit. Forensic efficiency parameters have been calculated as well as the allelic association between the tested markers for detection of linkage disequilibrium. To study the occurrences of recombination between the loci, both Swedish and Somali families were typed. A mathematical model for the estimation of recombination frequencies is presented and applied on the family samples. Our study showed that the tested markers all have highly informative forensic values and that there is a significant degree of linkage disequilibrium between the STR markers within the four linkage groups. Furthermore, based on the tested families, we also demonstrated that two of the linkage groups are partially linked. A consequence of these findings is that both linkage and linkage disequilibrium should be accounted for when producing likelihood ratios in relationship testing with X-STR markers.

Place, publisher, year, edition, pages
Elsevier, 2008
Keyword
X-chromosomal STR, Linkage, Linkage disequilibrium, Recombination, Likelihood ratios, Sweden
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-54740 (URN)10.1016/j.fsigen.2008.09.006 (DOI)19083865 (PubMedID)
Available from: 2010-04-08 Created: 2010-04-08 Last updated: 2010-04-09
4. Using X-chromosomal markers in relationship testing: How to calculate likelihood ratios taking linkage and linkage disequilibrium into account
Open this publication in new window or tab >>Using X-chromosomal markers in relationship testing: How to calculate likelihood ratios taking linkage and linkage disequilibrium into account
Show others...
2011 (English)In: Forensic Science International: Genetics, ISSN 1872-4973, E-ISSN 1878-0326, Vol. 5, no 5, 506-511 p.Article in journal (Refereed) Published
Abstract [en]

X-chromosomal markers in forensic genetics have become more widely used during the recent years, particularly for relationship testing. Linkage and linkage disequilibrium (LD) must typically be accounted for when using close X-chromosomal markers. Thus, when producing the weight-of-evidence, given by a DNA-analysis with markers that are linked, the normally used product rule is invalid. Here we present an efficient model for calculating likelihood ratio (LR) with markers on the X-chromosome which are linked and in LD. Furthermore, the model was applied on several cases based on data from the eight X-chromosomal loci included in the Mentype® Argus X-8 (Biotype). Using a simulation approach we showed that the use of X-chromosome data can offer valuable information for choosing between the alternatives in each of the cases we studied, and that the LR can be high in several cases. We demonstrated that when linkage and LD were disregarded, as opposed to taken into account, the difference in calculated LR could be considerable. When these differences were large, the estimated haplotype frequencies often had a strong impact and we present a method to estimate haplotype frequencies. Our conclusion is that linkage and LD should be accounted for when using the tested set of markers, and the presented model is an efficient way of doing so.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
X-chromosome, Linkage, Linkage disequilibrium, Simulation, Haplotype frequency
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-54741 (URN)10.1016/j.fsigen.2010.11.004 (DOI)000294297700023 ()
Available from: 2010-04-08 Created: 2010-04-08 Last updated: 2017-12-12

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