This chapter argues that there is no one-to-one, direct mapping between specific sets of genes and cognitive-level outcomes. Rather, there are very indirect mappings, with the regulation of gene expression more likely to contribute to very broad differences in developmental timing, neuronal type, neuronal density, firing thresholds, neurotransmitter types, etc. It presents the neuroconstructivist framework where gene/gene interaction, gene/environment interaction and, crucially, the process of ontogeny itself (pre- and postnatal development) are all considered to play a vital role in how genes are expressed and how the brain progressively sculpts itself, slowly becoming specialised over developmental time. The infant brain is not simply a miniature version of the adult brain.