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.

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Although the birth of twins has always attracted attention, there are no known genetic or environmental factors that can determine the birth of monozygotic (MZ) twins. And even for dizygotic (DZ) twins, genetic influences are not completely understood. A previous study from our group has shown that the C allele of polymorphism rs1042522 in the TP53 gene was more frequent in the mothers of twins than in the mothers of singletons in a small village in South Brazil. In order to clarify whether this was an isolated factor, we performed a population-based, observational case-control study. Samples were selected from a state-funded program of paternity investigation. Samples were considered cases when two of the children had the same date of birth, whereas controls were those samples in which at least two children were born in different dates. The first subsequent sample fulfilling control criteria was included after each case. From 2007 to 2013, 32,661 records were searched and 283 (0.9%) twins were found (119 MZ and 164 DZ). Genotypic and allele frequencies were not different between mothers of twins or mothers of singletons. However, mothers of MZ twins showed a higher frequency of GG genotype and lower frequency of the C allele when compared to mothers of DZ twins. Also, the proportion of MZ twins (42%) was higher than usually reported (30%). Finally, the proportion of twins found in this study seems to be more realistic, as this sample was allegedly not from users of assisted reproduction techniques.

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Analysis of recently transfused patients is usually postponed to avoid spurious results because of contamination with donor's cells. However, little is known about the extent of this influence in routine molecular diagnostic tests. To elucidate this question, we tested a mix of blood samples from 2 α-1-antitrypsin-deficient patients diagnosed as Pi*Z homozygous with 1 normal donor at 1:1, 1:10, 1:20, and 1:30 proportions. Human identification panel and Pi*Z allele detection were used to establish the detection limit of a blood mixture. Mixtures of 1:1 and 1:10 were easily detected with both techniques, whereas for 1:30, it was necessary to change the equipment settings to identify the mixture. Moreover, the heterozygous pattern observed for the mixtures on Pi*Z genotyping was weaker at this level of mixture. We further evaluated the degree of mixture detectable in 20 transfused patients who received 1 blood unit (concentrate of irradiated or nonirradiated red blood cells) using the human identification panel. Two days after the transfusion, the presence of the donor's markers was not detected, suggesting that after this time point the levels of admixture are below 1:30. The methods applied in the present study showed adequate sensitivity to identify alleles of the so-called "smaller population" of cells up to 3%, approximately. The same result was obtained in a "diagnostic situation," in which the blood mixture was submitted to a PCR-RFLP protocol to detect a mutation.

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