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The study of behavioural phenotypes could be said to go back to the days of the syndrome first described by Down.1 He believed that affected children (whose genetic anomaly was of course not known to him) had distinctive personality characteristics, including “a lively sense of the ridiculous”, among others. Subsequent studies have failed to confirm these stereotypes.2 Nevertheless, there have since been numerous attempts to discover, among individuals with known chromosomal or genetic anomalies, mental features that are causally related to the underlying condition. Opinions are divided, between those who regard the study of behavioural phenotypes as being worthwhile3and those who have their doubts.4 Before discussing the arguments, a definition is required.
What is a behavioural phenotype?
There is no consensus. Flint2 suggested that it is: “a behaviour, including cognitive processes and social interaction style, that is consistently associated with, and specific to, a syndrome which has a chromosomal or a genetic aetiology”. He added “where there is little doubt that the phenotype is a consequence of the underlying anomaly”, but others feel that definition is too restrictive.5 6 In practice, most behavioural phenotypes are associated with chromosomal aneuploides and microdeletions rather than point mutations in a single gene.
The study of behavioural phenotypes aims to identify bona fide links between genotype and phenotype. A few moments' reflection only is necessary to realise that there are many problems with the concept. First, is the term to be restricted only to disorders in which a genetic anomaly has been identified? There are many conditions (such as Rett syndrome7) that appear to have a genetic origin and characteristic behavioural features, but the nature of the genetic deficit is not known. Second, is it likely that any cognitive or behavioural features will be found consistently, unless they are part of …