X-chromosomal STRs (X-STRs) are a valuable complement for resolving complex kinship cases, particularly relevant in cases such as paternal half-sister or grandmother-granddaughter hypotheses, where autosomal markers may prove uninformative. However, their widespread forensic application is often hindered by the complexities of their haplodiploid inheritance pattern and consequent non-independent segregation of markers (linkage).

This study provides a dual-purpose solution to enhance X-STR use in forensic routine casework. First, we established a theoretical mathematical framework based on identity-by-descent (IBD) sharing to identify the complete set of kinship hypotheses independent of linkage. Our results demonstrate that for pedigrees where only a single IBD partition is possible (such as unrelated pairs, parent-child transmissions, or paternal half-sisters), the likelihood ratios remain unaffected by recombination, regardless of the genotypic configuration. Second, for cases where linkage must be accounted for, we present the largest phased recombination dataset to date. Through a GHEP-ISFG collaborative exercise involving 350 maternal grandfather-mother-son trios, we estimated recombination rates for the 10 X-STRs in the Decaplex system using the Recombulator-X tool and Kosambi’s mapping function. Crucially, we demonstrate the retrospective inter-usability of these estimates across major commercial multiplexes, such as SureID® X37 and Microreader™ 19X, either by directly applying them to the same marker pairs or to markers within the same linkage groups.

By defining the kinship problems that are not affected by linkage and providing robust and transferable recombination data for the cases where linkage needs to be considered, this work simplifies X-STR implementation. Our findings offer a definitive guide for forensic experts navigating the haplodiploid complexities of the X chromosome.

Authors

• Nádia Pinto (Instituto de Investigação e Inovação em Saúde da Universidade do Porto, Portugal)
• Catarina Xavier (Instituto de Investigação e Inovação em Saúde da Universidade do Porto, Portugal)
• Marisa Faustino (Faculdade de Ciências da Universidade do Porto, Portugal)
• María Zarrabeitia (Unit of Legal Medicine, Hospital Universitário Marqués de Valdecilla-IFIMAV, RETICEF, University of Cantabria, Spain)
• Maria Castañeda-Fernandez (Unit of Legal Medicine, Hospital Universitário Marqués de Valdecilla-IFIMAV, RETICEF, University of Cantabria, Spain)
• Maria Fátima Pinheiro (Instituto Nacional de Medicina Legal, Portugal)
• Maria João Porto (Instituto Nacional de Medicina Legal, Portugal)
• Pedro Brito (Instituto Nacional de Medicina Legal, Portugal)
• Pedro Brito (Instituto Nacional de Medicina Legal, Portugal)
• Pedro Brito (Instituto Nacional de Medicina Legal, Portugal)
• Pedro Brito (Instituto Nacional de Medicina Legal, Portugal)
• Mariel Abovich (Banco Nacional de Datos Genéticos, Argentina)
• Gabriela Berardi (PRICAI - Fundación Favaloro, Argentina)
• Ulisses Toscanini (PRICAI - Fundación Favaloro, Argentina)
• Regina Cicarelli (Laboratório de Investigação de Paternidade-NAC, Faculdade de Ciências Farmacêuticas, Universidade Estadual Paulista, Brazil)
• Bárbara Kawamura (Department of Immunology, Institute of Biomedical Sciences, University of Sao Paulo, Brazil)
• Michael Nothnagel (Cologne Center for Genomics (CCG), University of Cologne, Germany)
• Leonor Gusmão (DNA Diagnostic Laboratory (LDD), State University of Rio de Janeiro, Brazil)