Biologists, physicists, and other researchers explore the bases of Mendelian dominance as a pre-requisite for understanding the more complex non-Mendelian inheritance. Their topics include the phenomenology and mechanisms of dominance, phenotype and stochastic gene expression, mathematical models of haplo-insufficiency, biological consequences of dosage-dependent gene regulation in multicellular eukaryotes, and lessons from a genetic network about the causes of dominance.