Calculating the exclusion probability and paternity index for X-chromosomal loci in the presence of substructure

Impact of a chromosome X STR Decaplex in deficiency paternity cases

Deficiency paternity cases, characterized by the absence of the alleged father, are a challenge for forensic genetics. Here we present four cases with a female child and a deceased alleged father in which the analysis of a set of 21 or 22 autosomal STRs (AS STRs) produced results within a range of doubt when genotyping relatives of the alleged father. Aiming to increase the Paternity Index (PI) and obtain more reliable results, a set of 10 X-linked STR markers, developed by the Spanish and Portuguese Group of the International Society for Forensic Genetics (ISFG), was then added. Statistical analysis substantially shifted the results towards the alleged fatherhood in all four cases, with more dramatic changes when the supposed half-sister and respective mother were the relatives tested.

Effects of genotyping errors on parentage exclusion analysis

Genetic markers are widely used to determine the parentage of individuals in studies of mating systems, reproductive success, dispersals, quantitative genetic parameters and in the management of conservation populations. These markers are, however, imperfect for parentage analyses because of the presence of genotyping errors and undetectable alleles, which may cause incompatible genotypes (mismatches) between parents and offspring and thus result in false exclusions of true parentage. Highly polymorphic markers widely used in parentage analyses, such as microsatellites, are especially prone to genotyping errors. In this investigation, I derived the probabilities of excluding a random (related) individual from parentage and the probabilities of Mendelian-inconsistent errors (mismatches) and Mendelian-consistent errors (which do not cause mismatches) in parent-offspring dyads, when a marker having null alleles, allelic dropouts and false alleles is used in a parentage analysis. These probabilities are useful in evaluating the impact of various types of genotyping errors on the information content of a set of markers in and thus the power of a parentage analysis, in determining the threshold number of genetic mismatches that is appropriate for a parentage exclusion analysis and in estimating the rates of genotyping errors and frequencies of null alleles from observed mismatches between known parent-offspring dyads. These applications are demonstrated by numerical examples using both hypothetical and empirical data sets and discussed in the context of practical parentage exclusion analyses.

Genetic polymorphisms of eight X-chromosomal STR loci in the population of Japanese

The genetic polymorphisms of eight X-chromosomal short tandem repeats (STR) DXS10135, DXS8378, DXS7132, DXS10074, HPRTB, DXS10101, DXS10134 and DXS7423 were analyzed in a sample of 492 unrelated males (283) and females (209) from the Japanese population. Multiplex PCR amplification was performed using the Mentype Argus X-8 PCR amplification kit. The haplotype frequencies within the four linkage groups were studied for the 283 examined Japanese males. Allele frequencies of eight X-STR loci were calculated separately for males and females, and exact tests demonstrated no significant deviations from Hardy-Weinberg equilibrium. Several microvariant and rare alleles were observed, and forensic efficiency parameters were calculated. The combined powers of discrimination of the loci in men and women were 0.999995 and 0.9999999999988, respectively.

Resolving Paternity Relationships Using X-Chromosome STRs and Bayesian Networks

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.

The expected performance of single nucleotide polymorphism loci in paternity testing

We discuss the utility of single nucleotide polymorphism loci for full trio and mother-unavailable paternity testing cases, in the presence of population substructure and relatedness of putative and actual fathers. We focus primarily on the expected number of loci required to gain specified probabilities of mismatches, and report the expected proportion of paternity indices greater than three threshold values for these loci.