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The author, a renowned geneticist, explores an apparent “determined but free” paradox. By considering the genetic situation of “identical” twins, he demonstrates that each twin is not a clone but a singular being. Indeed, after fertilization, a whole series of stages intervene which are as many possible points of divergence between identical twins, including the random distribution of mitochondrial dna during embryonic development, the inactivation of the X chromosome, and mitotic instability. Finally, because of the possible occurrence of mutations, expansions, or contractions of coding and non-coding genomic dna during cell division in developing tissues (somatic mutations), a mother free of any genetic disease may give birth to identical twins, one of whom—and not the other—will have a malformation, bone fragility, cancer, or neurological disease. This research warns against any temptation to predict from prenatal diagnosis what will happen to the somatic future of the subject. The prognostic uncertainty thus demonstrated as a result of the complexity of scientific knowledge in genetics can only encourage a welcoming of the singular in consultations in pairs bringing together a geneticist and a psychoanalyst.