This paper proposes an algorithm through which the development of computationally generated forms can be externally directed towards both functional objectives and intuitive design targets. More precisely, it presents a special version of Interactive Genetic Algorithm, which introduces Weighted Mutation as a method to support the long life of genes corresponding to favored phenotypic characteristics. At the same time, optimization processes towards operational goals are also enabled. A set of experiments is conducted on the case study of a building facade, which appears to provide a suitable means for the investigation of functional, as well as aesthetic issues. The results are positively assessed, since they prove that this new methodology broadens the capacities of standard Interactive Genetic Algorithms, shedding light on how a constructive human-machine relationship can benefit the design process.