Phenotype in Psychiatric Genetic Research

Abstract

Mental illnesses differ from medical conditions in their lack of objectively assessable biological markers for the establishment of a diagnosis. In the absence of clear external validators such as laboratory tests or radiological examinations, accurate assessment of the clinical picture and phenomenology becomes crucial. Common diseases with successful genetic mapping studies are generally characterized by diagnostic assessments that are objective, have a clear biological basis, and measure phenotypic features shared relatively uniformly among affected individuals. For example, type 2 diabetes is diagnosed based on elevation in blood glucose above a generally accepted threshold, as assessed by a simple assay. This phenotypic feature is at the core of the diagnosis, even though other disease components may vary between affected individuals. For mental illness, however, no biological assays are currently available for diagnostic purposes; the phenotypic features are generally assessed by subjective ratings, and individuals are assigned a diagnosis based on report of symptoms, no one of which is present in all individuals assigned that diagnosis. There is now considerable interest in identifying quantitative assessments, which may provide a more objective means of rating psychopathology.