This study proposes the dialogical history of the production and the objecting of logical fallacies spontaneously generated during a Genetic Counseling for Predictive Testing consultation. These logical fallacies are argumentation errors familiar in psychology and rhetoric: « validating the consequent » and « disavowing the antecedent ». A formal model of this dialogue is constructed which elucidates why the fallacy occurs and why corrective efforts finally miscarry. It is demonstrated that the causal connective puisque constitutes a substantial operator of the explored dialogue.