What makes an ideal model system to study embryonic development? The answer to this question has varied in the course of scientific quest, depending on the available experimental approaches. Yet, at the beginning of the new millennium, most of the mainstream model organisms have one feature in common, they are amenable to genetic analysis. Indeed, the genetic approach has proven remarkably effective in the dissection of complex biological phenomena, starting with the regulation of the lactose operon in Escherichia coli, the decision between the lysogenic and lytic cycles of the lambda phage...