What is the range of the clinical phenotype transmitted

The fact that an individual can have the same genes as their schizophrenic co-twin but have a better than evens chance of remaining non-psychotic indicates that it is not schizophrenia per se which is inherited but rather a susceptibility to it. Further evidence in support of this comes from a study which showed that the offspring of the identical but well co-twins of schizophrenic individuals have a risk of the disorder similar to that of the offspring of the affected. ^ Thus the predisposition is transmitted without being expressed as schizophrenia.

As noted earlier, sometimes the predisposition may be expressed as non-psychotic spectrum disorders. In addition, family studies show that relatives of schizophrenic patients also have an excess of other psychotic conditions such as schizoaffective disorder, atypical and schizophreniform psychoses, and affective psychosis with mood-incongruent delusions. Thus, the clinical phenotype transmitted encompasses a range of psychotic conditions, as well as schizotypal personality disorder.

Within schizophrenia, researchers have asked whether the classical Kraepelinian subtypes are differentially inherited. The results have in general been negative which is not surprising since clinicians know that an individual patient can appear predominantly hebephrenic on one admission and schizoaffective on another. However, there has been a consensus that paranoid schizophrenia is less familial than other types and is associated with a lower monozygotic twin concordance.

Recently, it has been shown that schizophrenic symptoms can be summarized as three main factors: delusions and hallucinations (reality distortion), negative symptoms (psychomotor poverty), and disorganization or positive thought disorder.(89) Is schizotypal personality particularly closely related to one of these three core syndromes? Mata et al. (19 showed that schizotypal personality scores in non-psychotic relatives were significantly correlated with the presence of delusions and hallucinations in the probands; indeed, they were also correlated with premorbid schizoptypal traits in the childhood of the probands. Thus, it seems that certain families transmit schizotypal traits which manifest themselves in childhood; some family members remain schizotypal throughout life but in others this deviant personality type is then compounded by other (genetic or environmental) factors so that the individual passes a threshold for the expression of delusions and hallucinations.

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