What exactly is it that makes someone a parent? Many people hold that parenthood is grounded, in the first instance, in the natural derivation of one person's genetic constitution from the genetic constitution of others. We refer to this view as "Geneticism". In Part I we distinguish three forms of geneticism on the basis of whether they hold that direct genetic derivation is sufficient, necessary, or both sufficient and necessary, for parenthood. (Call these 'Sufficiency', 'Necessity', and 'Strong' Geneticism, respectively.) Part I also explores the relationship between geneticism and the debate over surrogacy. Parts two through four examine three arguments for geneticism: the Property argument, the Causal argument, and the Parity argument. We conclude that none of these arguments succeeds. The failure of positive arguments for a view cannot demonstrate that the view is false; however, in light of our arguments we provisionally conclude that 'Strong' and 'Necessity' Geneticism are unacceptable. Our arguments do not undermine 'Sufficiency' Geneticism, so this thesis is considerably more promising than the others. But sufficiency geneticism is also compatible with a much more pluralistic account of the nature of parenthood.