Identification of half-sisters from different mothers by autosomal and X chromosomal short tandem repeats: A case study

Jinpei Zhang; Shicheng Hao; Yan Liu; Li Yuan

doi:10.4103/jfsm.jfsm_30_21

CASE REPORT

Year : 2021 | Volume : 7 | Issue : 2 | Page : 66-69

Identification of half-sisters from different mothers by autosomal and X chromosomal short tandem repeats: A case study

Jinpei Zhang¹, Shicheng Hao¹, Yan Liu², Li Yuan¹
¹ Key Laboratory of Evidence Science (China University of Political Science and Law), Ministry of Education; Collaborative Innovation Center of Judicial Civilization, Beijing, China
² Key Laboratory of Evidence Science (China University of Political Science and Law), Ministry of Education, Beijing, China

Date of Submission	22-May-2021
Date of Decision	16-Jun-2021
Date of Acceptance	23-Jun-2021
Date of Web Publication	26-Jul-2021

Correspondence Address:
Li Yuan
Key Laboratory of Evidence Science (China University of Political Science and Law), Ministry of Education, Beijing 100088; Collaborative Innovation Center of Judicial Civilization, Beijing
China

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/jfsm.jfsm_30_21

Abstract

Complex kinship identification such as half-sibling identification is a difficult task in forensic biology Here we represented an approach in dealing with half-sisters from different mothers, with the combination of autosomal and X chromosomal short-tandem repeats (STRs) data. X chromosomal STRs can offer additional information, especially in some cases where autosomal STRs alone may not provide enough information for an accurate opinion. In this case, half-sister or unrelated relationship between two women (S₁ and S₂) with different mothers were distinguished. 23 autosomal and 31 X chromosomal STRs of S₁, S₂, S₁'s mother (M₁), S₂'s mother (M₂) and S₁'s grandmother (G₁) were profiled with three different commercial kits. As to X-chromosome STRs, likelihood ratios (LRs) were calculated by FamLinkX with consideration of linkage, linkage disequilibrium, and mutations. When only the profiles of the two individuals (S₁ and S₂) were available, LRs between S₁ and S₂ were 1.1110 × 10² based on 23 autosomal STRs and 3.2257 om10⁷ based on 31 X chromosomal STRs. When the maternal genotypes were taken into consideration, LRs increased to 2.5297 × 10³ and 3.0563 × 10¹⁸. Therefore, both the DNA profiles of each mothers and X chromosomal STRs are important in dealing with the identification of half-sisters from different mothers.

Keywords: Forensic case, half-sisters, likelihood ratio, short tandem repeat, X chromosomal short tandem repeat

How to cite this article:
Zhang J, Hao S, Liu Y, Yuan L. Identification of half-sisters from different mothers by autosomal and X chromosomal short tandem repeats: A case study. J Forensic Sci Med 2021;7:66-9

How to cite this URL:
Zhang J, Hao S, Liu Y, Yuan L. Identification of half-sisters from different mothers by autosomal and X chromosomal short tandem repeats: A case study. J Forensic Sci Med [serial online] 2021 [cited 2022 May 16];7:66-9. Available from: https://www.jfsmonline.com/text.asp?2021/7/2/66/322340

Introduction

Over the past decades, there has been an increase in the cases for half-sibling identification due to the deaths of the fathers. Although likelihood ratio (LR) is widely used to compare the possibilities of two different hypothesis in terms of the putative relationships under the observed DNA profiles, the identification of half-siblings has always been a difficult task in forensic science. In cases involving half-sisters identification, an approach for determining the probability of sibship in two-person without the other relatives has been described previously.^[1],[2],[3] Without parents' genotypes, it is hard to draw a reliable conclusion merely based on autosomal short tandem repeats (STRs) results, since this approach requires for a large number of autosomal loci for reliable discrimination. Mayor and Balding^[4] proposed that 47–54 autosomal STR loci were required to ensure the misclassification rates were under 2% when only the profiles of the two individuals were available in half-sibling identification. If the profiles of mothers were available, the paternal alleles of the query individuals could be inferred from the maternal genotypes, so that only 22–24 STR loci were necessary.

Female has two X-chromosomes that are inherited from their mother and father, respectively. While male has only one X chromosome that originates from his mother and will inherit this one to his daughter. Due to the specific inheritance pattern of the X-chromosome, it could be useful in the identification of half-sisters having different mothers. If two daughters do not share the same alleles at a certain amount of the examined loci which cannot be interpreted by mutation, the half-sister relationship could be excluded simply; if not, the exclusion could not be supported, where the genotypes of the mother could be used to infer the putative alleles of the father; moreover, additional genotype of the certain grandmother can also assist in this process, which is crucial for the accurate inferring. Hence, X-chromosomal STRs (X-STRs) may provide more information than autosomal STRs in the identification of half-sisters.^[5] This article reports a case that half-sisters relationship was identified with the combination of autosomal and X chromosomal STRs data.

Materials and Methods

Samples

One case of inheritance dispute, which aimed to identify whether daughter 1 (S₁) and daughter 2 (S₂) were half-sisters, was accepted. In the case, S₁ and S₂ have different mothers. S₁'s father (F₁) was dead. Blood samples were collected from S₁, S₂, S₁'s mother (M₁), S₂'s mother (M₂) and S₁'s grandmother (G₁).

Short tandem repeat typing

DNA was directly typed by the STR commercial kits, including GSTAR^™ 25 kits (Beijing Jinmashenghe Technology Co., Ltd, Beijing, China), Microreader^™ 19X Direct ID System (Microreader, Suzhou, Jiangsu, China) and AGCU X19 STR kits (AGCU ScienTech Incorporation, Wuxi, Jiangsu, China). PCR was performed according to the manufacturers' recommendations. PCR products were typed by ABI PRISM 3130 XL Genetic Analyzer (ThermoFisher, USA). Genotypes were analyzed using GeneMapper ID-X 1.4 analysis software (ThermoFisher, USA). The laboratory has been accredited by the China National Accreditation Service for Conformity Assessment (CNAS).

Likelihood ratio calculation

The chances of S1 and S2 share the same father or not have been calculated firstly. The LRs for their half-siblings were represented by the ratio of the chances. LR values of autosomal STRs and X chromosomal STRs were calculated, separately.^[1],[4] FamLinkX software was used to calculate the LR involving X chromosomal data.^[6] All calculations were based on the published^[7],[8],[9] and in-house population genetic data in Chinese.

Results and Discussion

Results of 23 autosomal short tandem repeats

Genotypes of 23 autosomal STRs in GSTAR^™ 25 kits were selected and are listed in [Table 1]. [Table 1] shows that M₁, S₁ and M₂, S₂ in accordance with Mendelian law. Thus, LRs was calculated by the ratio of the chances that S₁ and S₂ were half-sisters from the same father with the hypothesis that they were unrelated. Unlike paternity analysis, no paternal allele can be used to directly confirm or exclude a relationship. When only hypothetical siblings (S₁ and S₂) are available for the case, the LR for each locus is derived based on the state of the shared alleles of the two individuals and Bayes' theorem.^[1] According to the results of the 23 autosomal STRs, LR was 1.1110 × 10² when using only the profiles of S₁ and S₂. When the maternal profiles were taken into consideration, the high utility of maternal genotypes allowed the paternal alleles to be identified,^[4] LR increased to 2.5297 × 10³. Thus, provided higher discriminatory power than only profiling two individuals.

Table 1: 23 autosomal short tandem repeats of five participants

Click here to view

Results of 31 X chromosomal short tandem repeats

According to the X-chromosomal STRs markers (X-STRs) of G₁, S₁ and M₁, the profile of F₁ could be generated, as listed in [Table 2].

Table 2: 31 X chromosomal short tandem repeats of 5 participants and putative father

Click here to view

If only M₁ and S₁ were considered, the genotypes of F₁ at GATA172D05 and GATA165B12 cannot be certainly determined. However, by taking G₁ into consideration, both of them could be determined as “10.” Except DXS10148 and DXS7424, the other 29 X-STRs genotypes of F₁ were determined. Alleles against inheritance law were not found when comparing F₁'s X-STRs with S₂'s. Although DXS10148 and DXS7424 were not determined, they cannot exclude the paternity of F₁ and S₂. As for DXS10148, S₁ was “20, 26.1” and M₁ was “20, 23.1,” suggesting F₁ was “26.1.” S₂ was “24.1, 25.1” and M₂ was “24.1, 27.1,” suggesting father of S₂ was “25.1.” In parentage testing, the situation in which only a single mismatch was observed, was often assumed to result from mutation, rather than genuine exclusion. While G₁ was “24.1, 25.1” at DXS10148. This indicates a mutation event may occur during the inheritance from G₁ to F₁ or from F₁ to S₁. Given the putative father of S₂ was “25.1” at DXS10148, the mutation was more likely to occurring in the process of inheritance from F1 to S1, instead of the process of G1 to F1 or F1 to S2.

When X-chromosomal markers were used for LR calculation, linkage should be paid attention.^[10],[11] According to the website (http://www.chrx-str.org/), there were four linkage groups in the X chromosome. 31 X-STRs analyzed in the case, including 12 markers (DXS10148, DXS10135, DXS8378, DXS7132, DXS10079, DXS10074, DXS10075, DXS10103, HPRTB, DXS10101, DXS7423, and DXS10134) in the four linkage groups. In such situation, haplotype frequencies can be used. If one simply assumes that all X chromosome STRs are independently inherited, this may naturally overestimate the evidence. Moreover, if only one genetic marker in the linkage group is used, this would lose the information of the discard data. According to the 31 X chromosomal STRs, LR between S₁ and S₂ was determined as 3.2257 × 10⁷ and the value increased to 3.0563 × 10¹⁸ when the maternal genotypes were taken into consideration.

According to the results of 23 autosomal and 31 X chromosomal STRs, the hypothesis that S₁ and S₂ were half-sisters was supported. X-STRs have strong exclusion ability due to their specific inheritance pattern and play an important role in the identification of half-sisters from different mothers. As the number of commercial X-STRs kits increases, X-STRs can be the first choice in the identification of half-sisters from different mothers, which are especially suitable for the case where other relatives were absent. As the number of participants increases, and with the increase of autosomal and sex chromosomal markers used, the more information will be recovered.

Financial support and sponsorship

This work was financially supported by the project of Humanities and Social Sciences Research of the Ministry of Education (No. 19YJA820050).

Conflicts of interest

There are no conflicts of interest.

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