Forensic Science International: Genetics Supplement Series 3 (2011) e206–e207

Interest of X chromosome (Argus X-12 kit) in complex kinship analysis

Laura Caine a,b,c, Raquel Carvalho a, Sergio Costa a, Maria F. Pereira a, Maria F. Pinheiro a,b,c,d,*a Genetic Forensic Service, North Branch of National Institute of Legal Medicine, IP, Porto, Portugalb CENCIFOR-Forensic Science Center, Portugalc Medicine Faculty of the University of Porto, Portugald Faculty of Biomedical Sciences ‘‘Abel Salazar’’, Porto, Portugal

A R T I C L E I N F O

Article history:

Received 25 August 2011

Accepted 29 August 2011

Keywords:

X-chromosomal STRs

Population data

Complex kinship testing

Argus X-12

A B S T R A C T

X-chromosomal short tandem repeats (X-STRs) have proven to be informative and useful in complex

relationship testing. Paternity trio cases can most easily be solved with only autosomal STRs markers,

while test of paternity duos involving more complex family relations could gain from X-chromosomal

testing. The main goal of the present study was to investigate twelve X-STR markers (DXS7132,

DXS7423, DXS8378, DXS10074, DXS10079, DXS10101, DXS10103, DXS10134, DXS10135, DXS10146,

DXS10148, and HPRTB) based on a Portuguese population sample, and their ability to solve complex

kinship cases. Evaluations of statistical parameters of interest were also considered.

� 2011 Published by Elsevier Ireland Ltd.

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Forensic Science International: Genetics Supplement Series

jo ur n al ho mep ag e: www .e lsev ier . c om / lo cate /FSIG SS

1. Introduction

Typing of X-chromosomal short tandem repeats (X-STRs) hasbeen established as a useful strategy in solving complex kinshipcases [1]. This is due to the specic inheritance pattern of the X-chromosome. Females inherit one of their two X-chromosomesfrom their mother and the other from their father, while malesinherit their only X-chromosome from their mother. Physicaldependency between loci (linkage) and dependency betweenalleles at different loci (linkage disequilibrium, LD) are twofeatures that need to be considered during the interpretation andcalculation of probabilities in relationship testing [2]. Thepossibility of occurrence of recombination during meiosis shouldbe considered. The use of X-STRs requires a precise knowledge ofthe genetic LD status among markers. The aim of this study was toevaluate the performance of the 12 X-STRs included in theInvestigator Argus X-12 kit (Qiagen) (DXS7132, DXS7423,DXS8378, DXS10074, DXS10079, DXS10101, DXS10103,DXS10134, DXS10135, DXS10146, DXS10148, and HPRTB) incomplex kinship analysis. The markers studied are located in fourdifferent linkage groups (linkage group 1: DXS10148, DXS10135,and DXS8378; linkage group 2: DXS7132, DXS10079, andDXS10074; linkage group 3: DXS10103, HPRTB, and DXS10101;linkage group 4: DXS10146, DXS10134, and DXS7423).

* Corresponding author at: Delegacao do Norte do Instituto Nacional de Medicina

Legal, Jardim Carrilho Videira, 4050-167 Porto, Portugal. Tel.: +351 222073850;

fax: +351 22332593.

E-mail address: pinheiro.mf@gmail.com (M.F. Pinheiro).

1875-1768/$ – see front matter � 2011 Published by Elsevier Ireland Ltd.

doi:10.1016/j.fsigss.2011.08.103

2. Materials and methods

150 unrelated males from a North Portugal population samplewere typed for the frequency estimation as well as for theevaluation of forensic parameters. We analyzed some casesinvolving the mother, her daughter and the mother of the putativefather. These cases were selected because the analysis of X-STRscould provide additional information. PCR was prepared accordingto the Investigator Argus X-12 handbook and carried out in athermocycler 9700 (Applied Biosystems). Control DNA XX28(Qiagen) was genotyped as standard reference. The amplifiedproducts were detected and separated by capillary electrophoresisin an ABI PRISM 3100 Genetic Analyzer (Applied Biosystems).Fragment sizes and genotypes were determined automaticallyusing the GeneMapperID (Applied Biosystems). Allele frequencies,homozygotie (h), Heterozygosity (HET), Power of Exclusion (PE),Paternity Index (PI), Polymorphism Information Content (PIC),Mean Exclusion Chance (MEC) and Power of Discrimination (PD)calculations were performed using http://www.chrx-str.org/. Todetermine the usefulness of X-STRs in kinship analysis, six caseswhere the alleged father was absent were selected from ourroutine. Testing was done using samples from the mother of theputative father and results were reported as Likelihood Ratios (LRs)and Probability of Paternity (W). In forensics, the LRs are the mostcommonly method used to evaluate genetic evidence [3].

3. Results and discussion

Forensic statistical evaluations for the twelve X-STRs arepresented in Table 1. The values found are consistent withprevious studies that used only 8 X-STRs [4]. In this study,

Table 1Forensic parameters for twelve X-STR loci in a North Portugal population sample. PIC, Polymorphism Information Content.

Linkage group 1 Linkage group 2 Linkage group 3 Linkage group 4

DSX10148 DSX10135 DSX8378 DSX7132 DSX10079 DSX10074 DSX10103 HPRTB DSX10101 DSX10146 DSX10134 DSX7423

PIC 0.872648 0.927277 0.631662 0.663347 0.769805 0.810540 0.671543 0.717868 0.894649 0.895975 0.865379 0.655915

Homozygotie (h) 0.116149 0.068575 0.308048 0.292710 0.203361 0.169422 0.293827 0.240746 0.097388 0.096197 0.123111 0.290584

Heterozygotie (HET) 0.883851 0.931425 0.691952 0.707290 0.796639 0.830578 0.706173 0.759254 0.902612 0.903803 0.876889 0.709416

Power of Exclusion (PE) 0.762563 0.859952 0.415919 0.439627 0.592817 0.656967 0.437875 0.525731 0.800759 0.803195 0.748496 0.442977

Paternity Index (PI) 0.058074 0.034288 0.154024 0.146355 0.101680 0.084711 0.146913 0.120373 0.048694 0.048099 0.061555 0.145292

Power of Discrimination

PD female 0.975306 0.991150 0.844816 0.870378 0.931810 0.951258 0.879035 0.900655 0.982553 0.982919 0.973333 0.862060

PD male 0.883851 0.931425 0.691952 0.707290 0.796639 0.830578 0.706173 0.759254 0.902612 0.903803 0.876889 0.709416

Mean paternity exclusion

MEC Kruger 0.765748 0.931425 0.423356 0.471697 0.611308 0.668814 0.489325 0.529212 0.803754 0.805689 0.756635 0.454067

MEC Kishida 0.872648 0.926746 0.631541 0.663228 0.769805 0.810540 0.671425 0.717632 0.894649 0.895758 0.865367 0.655915

MEC Desmarais 0.872648 0.927277 0.631662 0.663347 0.769805 0.810540 0.671543 0.717868 0.894649 0.895975 0.865379 0.655915

MEC Desmarais Duo 0.783615 0.868680 0.485819 0.519696 0.645536 0.697253 0.528389 0.580766 0.816890 0.818872 0.774154 0.511756

L. Caine et al. / Forensic Science International: Genetics Supplement Series 3 (2011) e206–e207 e207

genotyping of the 150 men revealed the presence of 129, 92, 86 and116 haplotypes for linkage groups 1–4, respectively (data notshown). This can be attributed to the use of an extra locus in eachlinkage group, greatly increasing their discriminating power.However, in kinship testing alleles of closely linked X-chromo-somal loci are analyzed as haplotypes instead of single STR.

In all cases, the use of the Argus X-12 greatly increased the LR,when used in conjunction with the autosomal markers. However,when the Probability of Paternity was calculated, the differencewas not so notorious, and there was no change in the verbalpredicate used to transmit our finding in court (verbal predicateaccording to the modified Hummel’s chart). In one case, the LRusing autosomal markers was low (LR = 14) and the X-STRsprovided decisive information. The combined LR (autosomal andX-STRs) became 6,947,364,312 and the W increased from 93.26% tomore than 99.99999%.

4. Conclusions

All together, the X-chromosome markers included in theInvestigator Argus X-12 kit offer the possibility to solve complex

kinship cases where autosomal STR markers do not provide theinformation needed. The use of X-chromosome data offeredadditional valuable information provided by the autosomal STRs,increasing the LR in all selected cases. Other statistical parametersalso benefit from increased robustness when X-chromosomemarkers were included in the analysis.

Conflict of interest

None.

References

[1] R. Szibor, et al., Use of X-linked markers for forensic purposes, Int. J. Legal Med. 117(2003) 67–74.

[2] J. Ott, Analysis of Human Genetic Linkage, 3rd edition, The Johns HopkinsUniversity Press, Baltimore, 1999.

[3] D.W. Gjertson, et al., ISFG: recommendations on biostatistics in paternity testing,Forensic Sci. Int. Genet. 1 (2007) 223–231.

[4] D. Becker, et al., Population genetic evaluation of eight X-chromosomal shorttandem repeat loci using Mentype Argus X-8 PCR amplification kit, Forensic Sci.Int. Genet. 2 (2008) 69–74.