Behavioural phenotypes

Behavioural phenotypes are specific psychological characteristics with a known genetic aetiology. Like their somatic counterparts, the identification of behavioural phenotypes is potentially of clinical value. Various genetic mechanisms are associated with characteristic cognitive and behavioural profiles. These include: normal functional variations (polymorphisms); genetic mutations (with associated loss of function); structural anomalies and chromosomal deletions. Most descriptions of behavioural phenotypes concern the outward manifestations of seriously comprised gene expression. Even in such cases, there is rarely a close relationship between the genetic anomaly and an affected ability or behaviour; it is usually non-specific. For example, autistic features of behaviour have been associated with many conditions of genetic origin including Tuberous sclerosis, Turner syndrome and Fragile-X syndrome. The aetiological mechanisms can be direct, in the sense that the genetic condition causes abnormal neural functioning (e.g. transcriptional silencing of the FMR1 gene). Alternatively, it may be indirect, causing secondary structural anomalies (e.g. mesial temporal lobe tubers) that compromise neural integrity. By studying the relationship between genetic anomalies and their associated behavioural or cognitive phenotypes research has strived to understand how genes build brains, and influence performance. Many recent developments attempt to map atypical genotypes to specific cognitive processes, by way of exploring the impact on neural function. Examples considered here include Huntington's disease (autosomal dominant single gene disorder) and Velocardiofacial syndrome (a microdeletion). Future developments in this area are likely to derive success from an increasing understanding of the mechanisms linking genetic influences on brain development and their resulting cognitive or behavioural expression.