The new genomics has begun to play an increasing role in the arbitration of social identities. By facilitating the transcription of older notions that heritable substances determine identity and relatedness into a novel biotic idiom supposedly beyond social maneuver, this molecular–biological knowledge stakes out claims in the domain of the historical. Arguing from the highly publicized case of the genomic “resolution” of the question of Thomas Jefferson's paternity of his slave Sally Hemings's children and from the emergence of commercial personal genomic history services targeting African American consumers, I seek to expose the epistemological and methodological problems inherent in biotechnologically driven “ancestry projects” (however oppositional and empowering they may be in certain cases). I also aim to show how the divinatory logic of applications of genomic technologies of knowledge production to the validation of modes of social identification replicates racial essentialisms such as U.S. ideologies of hypodescent in a manner oddly reminiscent of the “invisible essences” that, according to classic ethnographic descriptions, underlie systems of witchcraft detection.