Definition of a behavioural phenotype

Describing a behaviour as part of a behavioural phenotype implies there is a specific genetic cause for that behaviour. Although this point is agreed upon, there has been some debate in the literature as to exactly which syndrome- behaviour associations merit the label “behavioural phenotype".

Flint and Yule (1994) propose that “a behavioural phenotype should consist of a distinctive behaviour that occurs in almost every case of genetic or chromosomal disorder, and rarely (if at all) in other conditions” (p. 666). Secondly they propose that one “should be able to say that this behaviour has a direct and specific relationship to the genetic or chromosomal anomaly that gives rise to the physical manifestations of the syndrome” (p. 666). Adopting this strict definition of behavioural phenotypes Flint (1996), in his critical review of the evidence, found just three syndromes in which a behaviour met the criteria of a behaviour phenotype: self-injurious behaviour in Lesch-Nynan syndrome, over-eating in Prader-Willi syndrome and stereotypic hand movements in RS.

However, RS hand stereotypies do not meet either of Flint and Yule's (1994) criteria for a behavioural phenotype. It is true that the hand stereotypies occur in almost every case of the disorder; hand stereotypies are one of the necessary diagnostic criteria for RS. However, it has yet to be proven that the hand stereotypies are rarely (if at all) seen in other conditions. In addition, the genetic basis of RS had not been confirmed at the time of the Flint (1996) review.

A great deal of variation is seen in both physical and behavioural manifestations of genetic syndromes (Dykens, 1995). Not all individuals with the same syndrome will show a syndrome’s characteristic. This variability may be explained in general terms, as a product of an individual’s entire

genetic endowment (not just the genetic anomaly), their environment, and the complex interplay between the two (e.g. Plomin & Rende, 1991; Scarr & McCartney, 1983). In the light of this within-syndrome variability Dykens (1995) adopts a less strict, and probably more realistic, conceptualisation of behavioural phenotypes. She proposes that “a behavioural phenotype may best be described as the heightened probability or likelihood that people with a given syndrome will exhibit certain behavioural and developmental sequelae relative to those without the syndrome” (p. 523). Although any individual may or may not show a syndrome's characteristic, individuals with the same syndrome may be more likely to show the same behaviour than are those individuals without the syndrome.

Certain behaviours may be quite prevalent or striking in individuals with a particular syndrome and these behaviours may characterise a syndrome. However, these behaviours may not be unique and pathognomonic if they are seen across other syndromes. Dykens (1995) proposes that behaviours of this nature would not be described as behavioural phenotypes. However, before the conclusion is made that a behaviour is common across syndromes, she makes the point that it is important to look for any qualitative differences in behaviours that are shared between different syndromes (Dykens, 1995). Thus Dykens suggests that the properties of behavioural phenotypes include within-syndrome variability and between syndrome similarities and qualitative differences.

Thus, it is not currently clear how strong the association between a genetic syndrome and a behaviour needs to be in order for the behaviour to be described as a behavioural phenotype. Different researchers seem to adopt slightly different definitions. Regardless of the term and definition used, the aim of research in this field is to determine the strength and exact nature of the associations between genetic syndromes and behaviours.

Hodapp, (1997) offers a framework for conceptualising the behavioural effects of genetic disorders in individuals with learning disabilities. He

describes three possible relationships between genetic disorders and behaviour. The first is that genetic disorders have no specific effects on behaviours. As already mentioned, historically this no-specificity view predominated among behavioural researchers in learning disabilities. The opposite position to this is one of total specificity in which each genetic disorder is thought to have one or more unique behavioural characteristics. This position is the one taken by Flint and Yule (1994) (described above) and is what is usually referred to as a behavioural phenotype. Hodapp suggests a third position, one of partial-specificity in which, in contrast to the no­ specificity or total specificity views, a specific connection exists between a small number of genetic disorders and a single outcome. The outcome in turn is not as common in learning disabilities generally. Hodapp makes the point that this partial-specificity position could be thought of as a watered- down version of behavioural phenotypes as defined by Flint and Yule, and that many researchers generally refer to the total-specificity view in their conceptualisations and research. The partial specificity position has some merit as not every genetic disorder will differ from all others on every aspect of behaviour. In summary, the aim of behavioural phenotype research is to find causal links between genetic syndromes and behaviour. This involves both between and within-syndrome investigations.