In biological genetics the genotype is the actual gene structure , in contrast to the pheonotype, which is the observable characteristics of the organism. In computational genetic algorithms there is a similar difference between the way in which the state is encoded, for example, the locations of appliances in a kitchen, and the way that gives rise to, say, plumbing and wiring issues within the kitchen. Often the mapping between genotype and pheonotype is effectively a one-way function, where it is hard to work out the geotype given the phenotype. However, for a genetic algorithm to work it is essential that there is at least some pattern to the relationship otherwise it will be no better than a Monte Carlo techniques.
Used on Chap. 4: page 77