[Praxis of the blood group opinion of parentage: systems with low exclusions potency; evidence of serological exclusion; biostatistical problems. Comments to a paper of H. Ritter in FamRZ 1973, 121 (author's transl)].

Blood group systems with a rare second allel are in praxis ineffective and therefore uneconomical because of their low exclusion potency as well as their low serostatistical value. The evidence of exclusion in particular blood group systems is fixed by the German "Richtlinien". With the new German law, however, one has to differentiate between the requirements in respect of the "offenbar unmöglich" ("fatherhood evidently impossible") in cases of extramatriomonial children, and those in respect of "schwerwiegende Zweifel" ("serious doubts in fatherhood") in cases of illegitimate children. The serostatistical results can give an indication on non-fatherhood, but in no case a real exclusion of paternity. Further evidence, as an anthropological or an HL-A expert opinion, is necessary to decide if the negative hint is reliable or not. To evaluate blood group (and HL-A) findings biostatistically, one uses the BAYES' Theorem with ESSEN-MOLLER's frequencies X and Y (in two-hypothesis cases). As prior plausibility one sets ESSEN-MOLLER's normative 0.5 or a realistic value, calculated from a concrete material of forensic cases. Statistical values on the basis of the exclusion chance as A, WA, Z, P and x do not give the whole information, because the phenotype of the man is not (or not fully) regarded. Therefore these values are of no use in serostatistical evaluation. With appropriate frequencies it is possible to calculate the probability of paternity also in cases in which foreign people is involved. If foreign frequencies are not available, the German "Richtlinien" allow to use european frequencies, if the involved people shares the caucasoid (europid) race.