Psychiatric genetics and epigenetics — family counselling MEQ

(i) Heritability vs fatalism. Heritability is the proportion of phenotypic variance in a population attributable to genetic differences, not the percentage of this young man's illness that is genetic and not a statement that outcomes are fixed.[1] Twin meta-analysis estimates schizophrenia liability heritability around 80%, and Danish registry twins show similar high heritability with incomplete MZ concordance — proving environment and other non-genetic factors matter.[1][2] Therefore evidence-based antipsychotic, psychosocial, and educational interventions remain essential; high heritability does not equal therapeutic nihilism.

(ii) Sister counselling structure. Take a three-generation pedigree; explain multifactorial (polygenic + environment) risk; give empiric absolute risks for siblings (elevated above ~1% population risk, classically taught first-degree risks on the order of ~10% depending on source and definition — state uncertainty and avoid false precision); emphasise that most siblings remain well; offer support, early-help education (not scare campaigns); document and offer written summary; involve clinical genetics if a high-impact variant is later found.[1][2]

(iii) GWAS/PRS. GWAS have identified many common risk loci (e.g. PGC 108 loci landmark; larger subsequent maps), supporting a polygenic model.[3] Common polygenic scores capture part of shared risk with bipolar disorder historically shown by the ISC.[4] Current clinical discriminative ability of PRS in the general population is limited; they are not routine diagnostic tests, transfer poorly across ancestries when misapplied, and should not drive commercial "yes/no" family testing.[5]

(iv) Syndromic clues → 22q11.2DS. Cardiac conotruncal defect, hypocalcaemia, and palatal/speech features raise pretest probability of 22q11.2 deletion — a CNV with high relative risk for psychosis and multisystem morbidity.[6][7][8] Action: liaise clinical genetics for appropriate CNV testing; if confirmed, multidisciplinary care (psychiatry, cardiology, endocrinology/calcium, immunology, developmental supports) per 22q guidelines; counsel that psychiatric treatment of psychosis still follows standard principles while medical comorbidities are actively managed.[6][7]

(v) Ethics. (1) Autonomy / non-directiveness in reproductive and testing decisions. (2) Privacy of genetic information and careful handling of relative implications. (3) Justice / non-maleficence — avoid ancestry-biased PRS harm, stigma, and coercive testing; ensure informed consent including limits of utility.[5]