Resolving Paternity Relationships Using X‐Chromosome STRs and Bayesian Networks

Abstract

X-chromosomal short tandem repeats (X-STRs) are very useful in complex paternity cases because they are inherited by male and female offspring in different ways. They complement autosomal STRs (as-STRs) allowing higher paternity probabilities to be attained. These probabilities are expressed in a likelihood ratio (LR). The formulae needed to calculate LR depend on the genotype combinations of suspected pedigrees. LR can also be obtained by the use of Bayesian networks (BNs). These are graphical representations of real situations that can be used to easily calculate complex probabilities. In the present work, two BNs are presented, which are designed to derive LRs for half-sisters/half-sisters and mother/daughter/paternal grandmother relationships. These networks were validated against known formulae and show themselves to be useful in other suspect pedigree situations than those for which they were developed. The BNs were applied in two paternity cases. The application of the mother/daughter/paternal grandmother BN highlighted the complementary value of X-STRs to as-STRs. The same case evaluated without the mother underlined that missing information tends to be conservative if the alleged father is the biological father and otherwise nonconservative. The half-sisters case shows a limitation of statistical interpretations in regard to high allelic frequencies.

Forensic Applications of Markers Present on the X Chromosome

“…In the joint typing of a father and his daughter, the ChrX haplotypes of both daughters can be fully determined because they always inherit the full paternal X chromosome (ChrX) [29]. Because males possess only one X chromosome and fathers pass their entire X chromosome to their daughters, X-STR markers are valuable supplements to Ychromosomal STR markers (Y-STRs) and autosomal short tandem repeat markers (STRs) in cases where genetic relationships need to be established [30]. Additionally, some X-STR loci exhibit strong linkage disequilibrium (LD) and group as haplotypes [31].…”

Development and validation of a new 18 X‐STR typing assay for forensic applications

“…Inference of relationships involving father–daughter, mother–son, or mother–daughter maybe solved more easily when X‐STRs were included [1]. In particular, questioned forensic cases such as criminal incest investigations or paternity cases involving blood relatives could be greatly improved by the recruitment of X‐STRs [2–4]. X‐STRs can also differentiate pedigrees that could not be resolved via unlinked autosomal STR markers alone [5].…”

Genetic polymorphism of DNATyper X19 kit in Northern Chinese Han people

“…Association with sex-linked genetic characteristics has made X-STR markers good candidates for efficient complementing of autosomal and Y-chromosomal STR markers in solving kinship-deficient cases involving father-daughter, mother-son, and sister-half-sister relationships, as well as those involving segregation and incest [1][2][3]. In 2018, Yu et al [4] used X-STR to solve two cases, confirming that inclusion of more X-STRs may help solve complex kinship cases, which could not be resolved via autosomal STR markers.…”