Mutation rate estimates for 13 STR loci in a large population from Rio Grande do Sul, Southern Brazil

Short tandem repeat (STR) polymorphisms have been extensively used in forensic genetics analysis. Knowledge about the locus-specific mutation rates of STRs improves forensic probability calculations and interpretations of diversity data. To incorporate single-locus diversity information into autosomal STR mutation rate estimations, 13 STR loci were studied during 2007–2009 in 10,959 paternity investigation cases from Rio Grande do Sul, the southernmost state of Brazil, covering an overall number of 284,934 allelic transfers. A total of 355 mutations were identified; 348 repeats were gains or losses of one step, three were gains or losses of two steps, and four were gains or losses of not stepwise mutation. The mutation rates ranged from 4.6 × 10−5 to 2.3 × 10−3, and the overall mutation rate estimate was 1.2 × 10−3. The average of the paternal mutation rate (1.8 × 10−3) was five times higher than the maternal rate (0.36 × 10−3). The observed mutational features for STRs have important consequences for forensic applications, including the definition of criteria for exclusion in paternity testing and the interpretation of DNA profiles in identification analysis.