In the same way as the genetic basis of disease and developmental disorders with behavioural and intellectual features are being discovered, the opportunity of studying how genes may affect conduct is also developing in both dysfunctional and normal situations.The most useful models for studying this problem are the genetic disorders in which the genetic defect has been discovered or is being traced. In parallel with advances in genetics, the behaviour patterns found in each disorder are also being defined and are known as behaviour phenotypes.In this article we aim to illustrate information currently available in the field of behaviour phenotypes. The behaviour and intellectual characteristics of the following syndromes are therefore described: fragile X, premutation of the fragile X syndrome, velocardiofacial, Prader Willi, Rett, Angelman and Williams.