Genetics is a difficult topic as it involves abstract concepts, which are not directly observable. Studies on using simulations to support the learning of genetics have largely been confined to the use of quantitative simulations to replace experimentation. This study describes the use of a new type of simulation built using an iconic modelling tool, WorldMaker (WM), that can (1) support multiple iconic visualisations of the simulated phenomena, (2) support the visualisation of instantaneous local changes as a simulation is executed as well as the time variation in global changes as the simulation unfolds, and (3) allow the user to examine and modify all of the rules governing the behaviour of the simulations. This study reveals that students' exploration of genetic phenomena using simulations built in WM elicits theorising from them that exposes their prior misconceptions as well as stimulates further exploration and co-construction of genetic understanding that is closer to the scientific view. It helps students to understand operationally difficult concepts like genotypes and phenotypes, as well as to understand genetic ratio as a deterministic value derived from inheritance events that are probabilistic and random in nature. (Contains 5 figures.)