Using X-chromosomal markers in relationship testing: calculation of likelihood ratios taking both linkage and linkage disequilibrium into account

Recombulator-X: A fast and user-friendly tool for estimating X chromosome recombination rates in forensic genetics

Genetic markers (especially short tandem repeats or STRs) located on the X chromosome are a valuable resource to solve complex kinship cases in forensic genetics in addition or alternatively to autosomal STRs. Groups of tightly linked markers are combined into haplotypes, thus increasing the discriminating power of tests. However, this approach requires precise knowledge of the recombination rates between adjacent markers. The International Society of Forensic Genetics recommends that recombination rate estimation on the X chromosome is performed from pedigree genetic data while taking into account the confounding effect of mutations. However, implementations that satisfy these requirements have several drawbacks: they were never publicly released, they are very slow and/or need cluster-level hardware and strong computational expertise to use. In order to address these key concerns we developed Recombulator-X, a new open-source Python tool. The most challenging issue, namely the running time, was addressed with dynamic programming techniques to greatly reduce the computational complexity of the algorithm. Compared to the previous methods, Recombulator-X reduces the estimation times from weeks or months to less than one hour for typical datasets. Moreover, the estimation process, including preprocessing, has been streamlined and packaged into a simple command-line tool that can be run on a normal PC. Where previous approaches were limited to small panels of STR markers (up to 15), our tool can handle greater numbers (up to 100) of mixed STR and non-STR markers. In conclusion, Recombulator-X makes the estimation process much simpler, faster and accessible to researchers without a computational background, hopefully spurring increased adoption of best practices.

Targeted S5 RNA sequencing assay for the identification and direct association of common body fluids with DNA donors in mixtures

The evidentiary value of DNA profiles varies depending upon the context in which the DNA was found. Linking a DNA profile to a particular cellular phenotype in mixtures may aid in assessing its evidentiary relevance and value. We report the development of two dual-function high-resolution messenger RNA (mRNA) sequencing assays that can each identify the presence of 6 body fluids/tissues (blood, semen, saliva, vaginal secretions, menstrual blood, skin) and, via coding region SNPs (cSNPs) present in the body fluid-specific mRNA transcripts, directly associate particular body fluids with their specific DNA donors in mixtures. The original blood, semen, and saliva (BSS) assay contains 23 cSNPs for blood, semen, and saliva, while the expanded 6F (all 6 fluids/tissues) assay encompasses the BSS assay and also contains 23 additional cSNPs for vaginal secretions, menstrual blood, and skin. Software tools were developed to infer the identity of the body fluids present as well as providing the corresponding cSNP genotypes. Concomitant genomic DNA assays (BSS-d and 6F-d), required to genotype the same cSNPs from persons of interest/inferred contributors to the body fluid mixture, were also developed. Body fluid specificity was demonstrated by the ability to identify the body fluid origin of single-source and two-fluid admixtures. The discriminatory power (European Caucasians) for all body fluids is 0.957-0.997, with linkage disequilibrium considered. Reciprocal body fluid admixtures (mixture pairs with the same two donors but reversed body fluid types) were used to demonstrate the ability to identify the body fluid source of origin as well as associate the donor of each of the two fluids.

Resolving the recombination pattern of 38 X-STRs from Chinese Han three-generation pedigrees

X-chromosomal markers have been proved as a useful tool for solving complex kinship cases due to its sex-linked inheriting feature. Among these markers, tightly linked X-STR clusters forming haplotypes are highly informative. The analysis of the haplotypes requires determination of linkage disequilibrium. In this study, genetic linkage, recombination fractions and mutation rates of 38 X-STR loci in 177 three-generation pedigrees were investigated. Genetic linkage analysis and calculation of recombination fractions were performed within each pair of markers and clusters. Then mutation rates were calculated. The results showed that, a) 22 recombination events happened within the tightly linked X-STR clusters, which span<1.0 Mb; b) significantly linked marker pairs were observed with the LOD (logarithm of the odds) scores > 2.0 (2.0104 to 54.8316); c) the average mutation rate of the 38 X-STR loci was 1.32 × 10^-3 per meiosis in the Chinese Han population, with DXS10135 and DXS8377 presenting notably high mutation rate (6.5 × 10^-3). Our results confirmed that meiotic recombination was not a simple function of physical distance, so that whether recombination occurred at the closely clustered X-STRs or not should be assumed cautiously considering the stability of haplotypes in inheritance process for kinship analysis. This study supplemented the existing database and laid an experimental foundation for the future study on genetic characteristics, recombination, and mutation of the X-STRs.

Extended population genetic analysis of 12 X-STRs - Exemplified using a Norwegian population sample

The use of X-chromosomal markers to resolve questions of relatedness has experienced a significant increase during the last years in forensic genetics. Perhaps primarily due to the emergence of commercial kits, but equally important due to an increased awareness of the utility of those markers. The X-chromosomal inheritance pattern entails that some cases, for instance paternal half-sisters, can potentially be resolved using a few X-chromosomal markers alone. For the statistical assessment in kinship cases it is of importance to have relevant population frequency data. In the present study 631 unrelated males from a Norwegian population sample are analyzed. The resulting haplotypes are compared to previously studied population samples and a deeper analysis of the linkage disequilibrium (LD) structure is conducted. We demonstrate that the power to detect LD will be low when few males, say below 300, are analyzed. We use entropy to describe the degree of LD between multiallelic loci and describe how this measure varies between different studied populations. Large population frequency databases have been recommended when using X-chromosomal markers, and we show that by combining reference databases from genetically similar populations, more precise haplotype frequency estimates can be obtained for rare haplotypes which improves the statistical assessment of the weight of evidence. In addition, we promote the use of simulations to assess the utility of STR markers in contrast to standard forensic parameters. Specifically we perform extensive simulations on cases where X-chromosomal markers are important and illustrate how the results can be used to infer the information gained from these markers.

Linkage, recombination and mutation rate analyses of 16 X-chromosomal STR loci in Sri Lankan Sinhalese pedigrees

This study investigated genetic linkage, recombination fractions and mutation rates of 16 X chromosomal short tandem repeat (X-STR) markers using a recently developed multiplex PCR assay for Sinhalese population of Sri Lanka, by analyzing 81 three-generation families including 81 grandfathers with daughters and 162 grandsons. In addition, 31 two generation families involving mother father daughter trios were included for mutation analysis. The analysis of recombination fractions between marker pairs identified two linkage blocks (maximum LOD scores > 3.0) each spanning a physical distance of 44.35 Mb and 6.04 Mb respectively. Though recombination events are usually rare among closely linked markers, crossovers were observed for markers located < 1.0 Mb apart. The recombination fractions observed are not fully concordant with those reported earlier, including the second-generation Rutgers combined linkage-physical map. This suggests that linkage is not uniform among different populations. However, the overall and marker-specific mutation rates of the present study did not differ from previous reports, though it needs confirmation with a much larger sample set. The findings presented here will provide the baseline information required for biostatistical calculations conducted using X-STR markers, in complex kinship analysis of Sinhalese population.

Comparative evaluation of autosomal STRs and X-chromosome STRs as a complement of autosomal STRs in kinship testing in Southern Han Chinese

Nowadays, kinship testing is very common in forensic caseworks, but the power of autosomal short tandem repeats (A-STRs) may be limited in complex cases. X-Chromosome short tandem repeats (X-STRs), having a unique heritage mode, should be of special use in some deficient cases. To evaluate and compare the potential of A-STR and X-STR as supplement genetic markers in deficient kinship testing, we simulated 10,000 duos for each of 18 kinds of relationships involving full sibling, half-sibling, grandparent-grandchild, and uncle/aunt-nephew/niece. Loci from STRTyper10, PowerPlex 16, and Investigator Argus X-12 were studied in Southern Han Chinese and the distribution of likelihood ratio (LR) values was analysed. With the addition of the X-12 system, the distribution of LR values for the full sisters, paternal half-sisters, paternal grandmother-granddaughters, maternal aunt-nieces, and maternal aunt-nephews separated much more obviously from those of unrelated duos, and the effectiveness was 1.0000, 0.99865, 0.9991, 0.8996 and 0.9634, respectively, which was more efficient than A-STRs. For the individual duos with other relationships, the effects of adding X-STRs and A-STRs were similar. Therefore, for the Southern Han Chinese, X-STRs can be very useful in kinship testing involving full sisters, paternal half-sisters, paternal grandmother-granddaughters, and maternal aunt-nieces/nephews.

Twenty Years Later: A Comprehensive Review of the X Chromosome Use in Forensic Genetics

The unique structure of the X chromosome shaped by evolution has led to the present gender-specific genetic differences, which are not shared by its counterpart, the Y chromosome, and neither by the autosomes. In males, recombination between the X and Y chromosomes is limited to the pseudoautosomal regions, PAR1 and PAR2; therefore, in males, the X chromosome is (almost) entirely transmitted to female offspring. On the other hand, the X chromosome is present in females with two copies that recombine along the whole chromosome during female meiosis and that is transmitted to both female and male descendants. These transmission characteristics, besides the obvious clinical impact (sex chromosome aneuploidies are extremely frequent), make the X chromosome an irreplaceable genetic tool for population genetic-based studies as well as for kinship and forensic investigations. In the early 2000s, the number of publications using X-chromosomal polymorphisms in forensic and population genetic applications increased steadily. However, nearly 20 years later, we observe a conspicuous decrease in the rate of these publications. In light of this observation, the main aim of this article is to provide a comprehensive review of the advances and applications of X-chromosomal markers in population and forensic genetics over the last two decades. The foremost relevant topics are addressed as: (i) developments concerning the number and types of markers available, with special emphasis on short tandem repeat (STR) polymorphisms (STR nomenclatures and practical concerns); (ii) overview of worldwide population (frequency) data; (iii) the use of X-chromosomal markers in (complex) kinship testing and the forensic statistical evaluation of evidence; (iv) segregation and mutation studies; and (v) current weaknesses and future prospects.

Assigning forensic body fluids to donors in mixed body fluids by targeted RNA/DNA deep sequencing of coding region SNPs

Biological traces found at crime scenes are analysed not only to genetically identify the donor(s) but also to determine the composition of the stain. For some cases, it is essential to associate a body fluid with a donor. Especially in mixed body fluid stains, but also in body fluid stains that appear to be single-source, this may be of importance. Linking a DNA profile (sub-source level) with evidence from a presumptive test or mRNA analysis (source level) is not straightforward. Our results support that associating donors and body fluids by means of comparing mixture ratios in RNA and DNA is not recommended. We introduce a set of 35 coding region SNPs (cSNPs) in body fluid-specific mRNA transcripts that represent a direct link between the body fluids and their donors. The discrimination power of the cSNPs was estimated based on allele frequencies calculated from a population sample (n = 188), and we investigated the practical application of the cSNPs in different scenarios. The results demonstrate that more cSNPs are needed to improve the discrimination power. However, the findings are promising as we were able to associate donors with body fluids in mixtures of different body fluids as well as in stains where both donors have contributed the same body fluid, e.g. a blood-blood mixture. In addition, the cSNP assay can be used for body fluid identification. The results of this proof-of-concept study support the use of cSNPs to assign body fluids to the respective donors.

Linkage, recombination, and mutation rate analyses of 19 X-chromosomal STR loci in Chinese Southern Han pedigrees

From Southern Han Chinese samples, we analyzed 19 X-STR markers for linkage, linkage disequilibrium (LD), and mutation rate. The data were collected from two- and three-generation Southern Han Chinese families. These data suggested that both linkage and linkage disequilibrium should be considered while calculating likelihood ratios with X-STR markers in relationship tests. The linkage disequilibrium of these 19 X-STR markers was calculated in our previous research study that was conducted on Southern Han Chinese population. In this study, the recombination fractions between pairs of markers and those obtained from the second-generation Rutgers combined linkage-physical map of the human genome were compared. The observed differences indicated that recombination was not homogeneous along the X chromosome. Therefore, we evaluated the effect on likelihood calculations by referring to haplotype frequencies obtained from allele distributions rather than haplotype counts of Southern Han Chinese population.

A simple method for calculating the likelihood ratio in a kinship test using X-chromosomal markers incorporating linkage, linkage disequilibrium, and mutation

X-chromosomal short tandem repeats (X-STRs) are useful for personal identification and kinship tests. However, it has not yet been fully established how to incorporate linkage and linkage disequilibrium (LD) into the calculation of the likelihood ratio (LR). In this paper, we describe a simple calculation method of LR for X-STRs that incorporated linkage, LD, and mutations. Initially, Japanese population data of 27 X-STRs (DXS6807, DXS9902, DXS6795, DXS6810, DXS10076, DXS10077, DXS10078, DXS10162, DXS10163, DXS10164, DXS7132, DXS981, DXS6800, DXS6803, DXS6809, DXS6789, DXS6799, DXS7424, DXS101, DXS7133, GATA172D05, DXS10103, HPRTB, GATA31E08, DXS8377, DXS10147, and DXS7423) were collected from 748 unrelated individuals to estimate the influence of LD. Significant LD was observed on six pairs of loci. Subsequently, using the population data, a simulation study was performed to evaluate the validity of the present calculation method for LR in cases of father-daughter, full-sisters, paternal half-sisters, maternal half-sisters, and unrelated pairs of females (FD, FS, PHS, MHS, and UR, respectively). As a result, the distribution of LR among FD was completely separated from that among UR. In the sibship test, 98.6% of FS and 98.0% of PHS surpassed the maximum value of UR in combined LR. Even in the FS versus MHS setting, 60.5% of FS had a higher LR than any MHS. We conclude that the present model is powerful in discriminating the relationship and is able to obtain a reasonable LR with fewer computations.

Allele and haplotype diversity of 12 X-STRs in Sardinia

The analysis of clusters of tightly linked X-chromosome short tandem repeat (STR) markers can assist the interpretation of complex kinship cases. However, when linkage disequilibrium (LD) is present in the population of origin of tested individuals, haplotype rather than allele frequencies should be used in likelihood calculations. The diversity of twelve X-STRs arranged in four linkage groups (I: DXS10148-DXS10135-DXS8378; II: DXS7132-DXS10079-DXS10074; III: DXS10103-HPRTB-DXS10101; IV: DXS10146-DXS10134-DXS7423) was tested in a Sardinian population sample (n=516) including three open populations from the Northern, Central and Southern part of the island, and three isolates (Benetutti, Desulo, Carloforte). Evidence of LD was detected in Sardinia within each linkage group. Significant differences in haplotype and allele frequency distribution of X-STR markers was seen between isolates and open populations, which on the contrary appeared highly homogeneous. The percentage of Sardinian haplotypes previously unobserved in a similar dataset compiled for the Italian population was: 76.3% (linkage group I), 61.3% (linkage group II), 54.1% (linkage group III), 58.9% (linkage group IV). Significant pairwise genetic differences were seen between mainland Italy, the three Sardinian isolates, and the open population of Southern Sardinia. The study confirms the presence of high levels and complex patterns of LD along the X chromosome in Sardinia, and provides population-specific haplotype data for biostatistical evaluation in kinship testing.

DNA Commission of the International Society for Forensic Genetics (ISFG): Guidelines on the use of X-STRs in kinship analysis

Forensic genetic laboratories perform an increasing amount of genetic analyses of the X chromosome, in particular to solve complex cases of kinship analysis. For some biological relationships X-chromosomal markers can be more informative than autosomal markers, and there are a large number of markers, methods and databases that have been described for forensic use. Due to their particular mode of inheritance, and their physical location on a single chromosome, some specific considerations are required when estimating the weight of evidence for X-chromosomal marker DNA data. The DNA Commission of the International Society for Forensic Genetics (ISFG) hereby presents guidelines and recommendations for the use of X-chromosomal markers in kinship analysis with a special focus on the biostatistical evaluation. Linkage and linkage disequilibrium (association of alleles) are of special importance for such evaluations and these concepts and the implications for likelihood calculations are described in more detail. Furthermore it is important to use appropriate computer software that accounts for linkage and linkage disequilibrium among loci, as well as for mutations. Even though some software exist, there is still a need for further improvement of dedicated software.

X chromosomal recombination--a family study analyzing 26 X-STR Loci in Chinese Han three-generation pedigrees

The aim of this study is to investigate genetic linkage and recombination fractions of 26 X chromosomal (X-STR) loci with two multiplex PCR systems (MX15-STR and MX12-STR). MX15-STR (including DXS7133, DXS6801, DXS981, DXS6809, DXS7424, DXS6789, DXS9898, DXS7132, GATA165B12, DXS101, DXS10075, DXS6800, GATA31E08, DXS10074, and DXS10079) and MX12-STR (including DXS6854, DXS9902, DXS6800, GATA172D05, DXS7423, HPRTB, DXS6807, DXS6803, DXS6804, DXS6799, DXS8378, and DXS8377) were successful analyzed on 206 two-generation families with two or more children and 33 three-generation families with 72 grandsons. Segregation analysis and calculation of recombination fractions between pairs of markers were performed. Linkage analysis of pairs of markers showed that there existed significant linkage (maximum LOD scores >2.0) within the physical distance of 48.5 Mb. Recombination events could be observed within the clusters of closed linked makers spanning <1.0 Mb. These results indicate that close cluster X-STRs used and recombination fractions of the selected loci will be very useful for biostatistical calculations in complex kinship analysis.

Using object oriented bayesian networks to model linkage, linkage disequilibrium and mutations between STR markers

In a number of applications there is a need to determine the most likely pedigree for a group of persons based on genetic markers. Adequate models are needed to reach this goal. The markers used to perform the statistical calculations can be linked and there may also be linkage disequilibrium (LD) in the population. The purpose of this paper is to present a graphical Bayesian Network framework to deal with such data. Potential LD is normally ignored and it is important to verify that the resulting calculations are not biased. Even if linkage does not influence results for regular paternity cases, it may have substantial impact on likelihood ratios involving other, more extended pedigrees. Models for LD influence likelihoods for all pedigrees to some degree and an initial estimate of the impact of ignoring LD and/or linkage is desirable, going beyond mere rules of thumb based on marker distance. Furthermore, we show how one can readily include a mutation model in the Bayesian Network; extending other programs or formulas to include such models may require considerable amounts of work and will in many case not be practical. As an example, we consider the two STR markers vWa and D12S391. We estimate probabilities for population haplotypes to account for LD using a method based on data from trios, while an estimate for the degree of linkage is taken from the literature. The results show that accounting for haplotype frequencies is unnecessary in most cases for this specific pair of markers. When doing calculations on regular paternity cases, the markers can be considered statistically independent. In more complex cases of disputed relatedness, for instance cases involving siblings or so-called deficient cases, or when small differences in the LR matter, independence should not be assumed. (The networks are freely available at http://arken.umb.no/~dakl/BayesianNetworks.).

Development and population study of the 12 X-STR loci multiplexes PCR systems

To develop a multiplex polymerase chain reaction (PCR) system with 12 X-chromosomal short-tandem repeat (X-STR) loci and to investigate their polymorphism and linkage and/or independence, the 12 loci (DXS6807, DXS8378, DXS9902, DXS6800, DXS6803, DXS6799, DXS6804, GATA172D05, DXS6854, HPRTB, DXS8377, and DXS7423) were simultaneously analyzed in 1,005 unrelated individuals (574 males and 431 females) from Guangdong Han individuals and Kazakh populations living in China. The allele frequencies and mutation rates were investigated. Allele frequency distribution among different populations was compared. Haplotypes of linkage disequilibrium markers (DXS6807-DXS8378-DXS9902) and linked markers (DXS6804-GATA172D05 and DXS8377-DXS7423) were also reported. A total of 117 alleles, ranging from five to 20 for each locus, were observed in our selected populations. Eight cases with mutation of the selected loci were detected in 9,480 meioses. Pairwise comparisons of allele frequencies distribution showed statistically significant differences at most loci among different populations. Haplotype diversity of linked markers was 0.9404-0.9694. The results indicated that this multiplex system is very useful for forensic analysis and may be complementarities for X-12 kits or X-8 kits in forensic case.