Forensic Science International: Genetics Supplement Series

The analysis of X-chromosomal markers can be important in di ﬀ erent situations where the application of autosomal and Y-chromosomal STR markers are not su ﬃ cient to solve the cases. Currently, the Argentine population lacks a representative database on X-chromosomal markers, regarding allele and/or haplotype frequencies


Introduction
X-chromosomal genetic markers are particularly useful in some situations of kinship analysis, especially in deficiency paternity cases.X-STR (short tandem repeats) markers may also be employed in cases of mixed DNA samples, in human identification and to weigh the possibility of an incestuous relationship, in case of daughters [1].Unless mutation, males transmit their X-chromosome unchanged to the daughters, making the X-STR markers excellent candidates to solve cases where the alleged father is absent but his mother or daughter is available to be tested.
Currently, a representative database on X-chromosomal markers, regarding allele and/or haplotype frequencies, and mutation rates, is lacking for the Argentine population.This represents an important limitation for their routine use in laboratories, preventing the achievement of a quantitative, statistically supported, evaluation.
In this work, overall and marker-specific mutation rates were determined for the 12 X-STR markers included in the Argus X-12 kit (Qiagen GmbH, Hilden, Germany).

Sample selection
Anonymous DNA extracts obtained from 345 father-daughter duos were selected from all provinces of Argentina and the biological relationship of paternity was confirmed by the analysis of autosomal STR markers for all duos using the Investigator 24plex QS, Investigator IDplex Plus (Qiagen GmbH, Hilden, Germany) or PowerPlex ® 16 System kits (Promega Corporation, Madison, USA.) for which likelihood ratios higher than 10 6 were achieved.
This project was reviewed and approved by two Ethical Committees, COFyBCF (Colegio Oficial de Farmacéuticos y Bioquímicos de la Capital Federal) and CEIH (Comité de Ética Institucional de Halitus), both from Buenos Aires city, Argentina.
PCR products were separated by capillary electrophoresis in an ABI 3500 or ABI 310 Genetic Analyzer (Applied Biosystems, Foster City, CA, USA) and the electrophoretic data were analyzed using GeneMapper ID-X v 1.2 and Gene Scan Analysis software (Applied Biosystems).

Determination of mutation rates
All father-daughter genetic profiles were compared for incompatibilites.The mendelian incompatibilities identified between them were considered as mutations.
To identify the mutations (or, rather, mendelian incompatibilities between father and daughter genotypes) genetic profiles were compared between father and daughter (Table 1).
X-STR mutation rates were calculated as the number of mutations divided by the total of allelic transmissions analyzed.Confidence interval (CI) was estimated using the exact binomial distribution via spreadsheet formulas provided at http://statpages.org/confint.htlm

Overall mutation rates
A total of 4140 allelic transmissions were analyzed in 345 confirmed father-daughter duos for the 12 X-STR markers, and 21 mendelian incompatibilities were observed across 9 of the 12 markers and in all five Argentine populations groups.
Among the 12 markers analyzed, three of them exhibited no mutations in this study, including two with simple repeat structure (DXS8378, HPRTB) and one with complex repeat structure (DXS7423).On the other hand, the highest number of mutations were observed at DXS10135 and DXS10148, both markers with complex repeat structures.
The overall X-STRmutation rate observed was 5.1 × 10 −3 (95% CI, 3.1 × 10 −3 -7.7 × 10 −3 ) and all the genotypic configurations were explainable by the gain or loss of one single repeat.In three out of the 21 incompatibilities it was not possible to establish if the origin of the mutation was either due to gain or loss of one repetition unit.
It should be noted that the overall mutation rate was higher in this work than in some previous reports [2,3], most likely because only father-daughter duos have been considered.This higher mutation rate in males is usually attributed to the higher number of germ-line divisions they experience [4,5].

Conflicts of interest
None for declare.

Table 1
STR mismatches observed in a total of 4140 allelic transmissions in duo cases.Overall and marker mutation rates.