Advances in biological identification of an individual based on blood markers polymorphism make available the attribution of paternity of a given child to an alleged father. Using Bayes' formula on these data requires previous calculation of conditional probabilities of the child's phenotype under either hypothesis of true paternity and non-paternity of the man. However, exact formulation of those is obtained only by introducing into the both parents-child link. The necessity of the successive use of Bayes' formula in each step for exact calculation of parental genotype weights is proved. The resulting process allows computer identification of possible genotypes and exact computer processing.