Paternity tests in Mexico: Results obtained in 3005 cases

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Highlights

  • Report different statistical parameters from the analysis of 3005 paternity cases.

  • Motherless cases were the most frequent paternity tests (77.3%).

  • The estimated exclusion rate was 29.6%.

  • In motherless cases, ≥ 20 STRs allowed decreasing the number of inconclusive cases and increased IP values.

  • Mutation rates, null alleles and triallelic pattern are described.

Abstract

National and international reports regarding the paternity testing activity scarcely include information from Mexico and other Latin American countries. Therefore, we report different results from the analysis of 3005 paternity cases analyzed during a period of five years in a Mexican paternity testing laboratory. Motherless tests were the most frequent (77.27%), followed by trio cases (20.70%); the remaining 2.04% included different cases of kinship reconstruction. The paternity exclusion rate was 29.58%, higher but into the range reported by the American Association of Blood Banks (average 24.12%). We detected 65 mutations, most of them involving one-step (93.8% and the remaining were two-step mutations (6.2%) thus, we were able to estimate the paternal mutation rate for 17 different STR loci: 0.0018 (95% CI 0.0005–0.0047). Five triallelic patterns and 12 suspected null alleles were detected during this period; however, re-amplification of these samples with a different Human Identification (HID) kit confirmed the homozygous genotypes, which suggests that most of these exclusions actually are one-step mutations. HID kits with ≥20 STRs detected more exclusions, diminishing the rate of inconclusive results with isolated exclusions (<3 loci), and leading to higher paternity indexes (PI). However, the Powerplex 21 kit (20 STRs) and Powerplex Fusion kit (22 STRs) offered similar PI (p = 0.379) and average number of exclusions (PE) (p = 0.339) when a daughter was involved in motherless tests. In brief, besides to report forensic parameters from paternity tests in Mexico, results describe improvements to solve motherless paternity tests using HID kits with ≥20 STRs instead of one including 15 STRs.

Introduction

DNA analysis probably has become the most powerful tool for solving forensic cases related to criminal activity and for biological kinship establishment. Most of the paternity tests involve comparison of genetic profiles between the alleged father and child (including or not the mother), followed by exclusion of paternity or confirmation that require the paternity index (PI) estimation.1 Subsequently, probability of paternity (W) is computed applying the Bayes Theorem. Although the generally accepted minimum standard PI for inclusion of paternity is ≥ 100,2 presently most laboratories require a PI ≥ 10000, which corresponds to W ≥ 99.99% assuming a priori probability of paternity of 0.5.3 Biological kinship relationships can also be established by inclusion of additional relatives (reconstruction cases) and analysis of genetic markers with peculiar inheritance patterns. For instance, paternal kinship can be established between males carrying the same non-recombinant region of the Y-chromosome, and complex cases where at least one female is involved can be solved by means of X-linked STRs (X-STRs).4

Although PCR-based STR typing has been the method of choice for paternity testing,5,6 STRs sporadically are affected by gametic mutations that influence the test interpretation. Therefore, different authors and organizations have estimated mutation rates (μ) for loci commonly used for Human Identification (HID) purposes.7 Problems related to parentage testing gave rise to different national and international organizations, such as the International Society of Forensic Genetics (ISFG), Scientific Working Group on DNA Analysis Methods (SWGDAM), and the American Association of Blood Banks (AABB), among others. These groups have published recommendations and coordinated exercises to compare DNA typing results between participating laboratories.1,3,6,8, 9, 10, 11, 12 However, these reports commonly omit information of some populations, mainly from developing countries. For instance, only two old reports exist regarding the non-paternity frequency in Mexico.13,14 Consequently, the aim of this study is to analyze paternity testing data of a Mexican laboratory during a period of five years, such as type of paternity cases, non-paternity prevalence, number of exclusions and a posteriori information given by some of the HID kits employed in the lab, null alleles, triallelic patterns, and mutation rates.

Section snippets

Information dataset

Information dataset was obtained from paternity cases performed over a period of five years by a Mexican laboratory of genetics (www.dnaprofile.com.mx). During the database creation, personal information of individuals involved in paternity cases was not shared according to legal regulations (Ley federal de protección de datos personales, México). The laboratory participates in two quality control proficiency tests annually organized by the Spanish and Portuguese-Speaking Working Group of the

Type of DNA parentage tests

We collected 3005 cases from different states of Mexico during a period of five years (2010–2015) (Supplementary Table S1). Motherless were the most common tests (77.27%), followed by trios including the mother (20.7%) (Table 1). The remaining 2.04% included different reconstruction cases where the alleged parent is missing and additional relatives were used to evaluate the biological kinship. Most of these cases were solved with lineage markers according to the analyzed biological

Conclusions

This is the first study in Mexico that describes different a posteriori parameters from DNA paternity tests. Interestingly, motherless tests were the most frequently solved (77.3%). The paternity testing exclusion rate was around 30%. The HID kits including ≥20 STRs in motherless tests showed higher PI values to conclude paternity, and lower number of inconclusive cases by isolated exclusions (<3 loci) than one HID kit based on 15 STRs. In general, STR mutation rates estimated herein are in

Funding sources

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of interest

The authors declare no conflicts of interest.

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