The use of genetic databases by the police and justice system has risen dramatically over the last 20 years, particularly in France, which has the second largest database in Europe. In such a context, this article analyses the legal and scientific effects of the forensic use of DNA on the formation of individuals’ (bio)identities in France. More specifically, we adopt a line of investigation that builds out from forms of resistance to genetic databases. Our methodology draws on a series of interviews and observations of legal proceedings against people who have refused to give DNA samples. In the first part of this text, we focus on the ‘jurisdiction effects’ of the DNA database being expanded to populations, showing that legal classifications (offender, suspect, etc.) constitute a key issue for these opponents. We then go on to analyse the ‘veridiction effects’ at work among social actors, in terms of the medical information/information related to origin that is conveyed (or not) by DNA profiles. In conclusion, we show that genetic analyses applied in a criminal context to populations, rather than simply individuals, shift (bio)identities through these dual effects, which form the basis of opponents’ resistance to genetic databases.