 The relationship between the gene of classical (pre-molecular) genetics, and the gene of modern molecular genetics is a subtle and much discussed topic (Beurton, Falk and Rheinberger (eds.) 2000, Griffiths and Stotz 2006, Moss 2003, Meunier 2022). In molecular genetics, ‘gene’ refers, more or less, to a stretch of DNA that codes for a particular protein—so a gene is a unit of function. But in classical population genetics, ‘gene’ refers, more or less, to a portion of hereditary material that is inherited intact across generations—so a gene is a unit of transmission, not a unit of function. In many cases, the two concepts of gene will pick out roughly the same entities—which has led some philosophers to argue that classical genetics can be ‘reduced’ to molecular genetics (Sarkar 1998). But it is clear that the two concepts do not have precisely the same extension; not every molecular gene is a classical gene, nor vice-versa. Some theorists go further than this, arguing that what molecular biology really shows is that there are no such things as classical genes.