It is increasingly recognized that comorbidity is common in all fields of psychiatry, and furthermore, it is acknowledged that a large number of individuals with genetically determined conditions have associated behavioural phenotypes, and are more susceptible to particular psychiatric and psychological comorbidities than others. It is also recognized that the identification of such phenotypes enables clinicians to be more aware of the potential difficulties an individual may experience, and hence, facilitate early diagnosis, effective management and prevention, appropriate allocation of resources and psychoeducation for the individual and their family. We describe the case report of a girl with cleidocranial dysostosis (CCD), and comorbid intellectual disability and attention deficit hyperactivity disorder (ADHD), and suggest the possible existence of a behavioural phenotype. We also highlight the lack of an evidence base for the management of ADHD within the learning-disability population, and describe successful management utilizing the current evidence base, which exists for those of average intellectual ability.