[Problems of assessing sibship probabilities by means of genetic analysis]

Nowadays, kinship studies based only on STR analysis are a very common practice. Apart from regular paternity cases, there is a rising number of cases in which the type of sibship between two persons has to be determined. Very often the parents or further relatives are unavailable for testing, so that the probability e. g. for a half-sibship has to be calculated as a deficiency situation. 50 unrelated persons (Group 1), 50 full siblings (Group 2), and 50 half-siblings (Group 3) were genotyped using the 15 STRs comprised by the AmpFlSTR Identifiler multiplex PCR-kit to investigate the reliability of common statistics programs and the indicative value of the calculated probabilities. Using a commercially available statistics program, we calculated probabilities for the following hypotheses: 1. The persons investigated are full siblings; 2. the persons investigated are half-siblings; 3. the persons investigated are unrelated. After STR analysis, half-sibling probabilities between 0.03 %-97.56% for group 1, between 0.01%-78.46% for group 2, and between 24.28%-90.18% for group 3 were calculated. In addition, the authors present two complicated deficiency cases which could be resolved only by extensive DNA typing using X- and Y-chromosomal loci, respectively, as well as the investigation of further relatives. The results of this study suggest that typing of autosomal STRs alone is no sufficient tool for resolving cases of possible half-sibship, even if probabilities of more than 90.0% are obtained. Due to the results of our study we suggest the implementation of mandatory standards for the investigation and interpretation of complicated kinship cases.