Behavioural phenotypes and genetic syndromes (MEQ)

(i) Definition. A behavioural phenotype is the heightened probability that people with a given genetic syndrome will show particular cognitive, social, communicative and psychiatric patterns. Features are neither universal nor exclusive and do not replace a DSM/ICD psychiatric diagnosis.^[1]

(ii) Likely diagnosis and risk. 22q11.2 deletion syndrome (VCFS/DiGeorge spectrum): conotruncal heart disease, palatal speech issues, hypocalcaemia history, ID and new psychosis. Adult schizophrenia-spectrum rates are very high in landmark VCFS work (Murphy) and consortium data support substantial psychosis risk into adulthood — longitudinal psychiatric care is mandatory.^[2][3][4]

(iii) Comparative phenotypes. Down syndrome (trisomy 21): depression and early Alzheimer dementia risk with age; medical confounders common. Fragile X (FMR1 full mutation, typically greater than 200 CGG): ID, ASD features, ADHD, anxiety; X-linked; premutation relatives may have FXPOI/FXTAS clues. Prader-Willi (loss of paternal 15q11-q13 gene expression): hyperphagia, OCD-like behaviours, temper outbursts; psychosis especially with maternal UPD. Angelman (loss of maternal UBE3A): severe ID, minimal speech, seizures, happy demeanour, ataxia.^{[5][6][7][8][9]}

(iv) Now. Medical: calcium/PTH, ECG, infection screen, substances, metabolic baseline before antipsychotic. Confirm/arrange 22q11.2 genetic documentation (often chromosomal microarray) and clinical genetics. Multisystem review (cardiac, immune, speech). Collateral MSE; risk assessment; capacity and least-restrictive legal status under local law. Early psychosis pathway with ID-adapted communication.^[3][4]

(v) Psychopharmacology. Treat clear psychosis with an antipsychotic — start low, go slow in ID (e.g. risperidone oral 0.25–0.5 mg daily initially with slow titration, or alternative with attention to QTc/metabolic risk); monitor weight, glucose/lipids, EPS, prolactin as relevant; check calcium and cardiac status. Do not use antipsychotics as first-line for pure communication-driven behaviour without psychosis. Combine with psychosocial supports and carer education.^[3][4]