This paper discusses design families and design individuals. Phase space is introduced to complement state space. Phase transition as a generic method of generating design families is proposed and then demonstrated through an example of simple designs with one component. Based on phase transition in the biological world, a development model for complex designs (with multi-components) is established, by which different phase transitions for changing complex design families are analysed and simulated. Two concepts are drawn from the biological analogy: through gene mutation and regulation the individuals of complex design systems can be varied and searched within their state space; and by changing the interpretation process of genes, the families of such systems can be explored within their phase space.