2 Neuroscience2.11 Integrated neurobiology of syndromes and states
2.11.15 Genetic syndromes and intellectual disability
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
Linking development and mechanism
Intellectual disability means impaired intellectual and adaptive functioning (everyday living skills) that starts during development. A genetic cause does not decide every aspect of a person's ability or behaviour.
Genetic syndromes disturb development through altered gene dosage, synaptic function, transcriptional regulation and genomic imprinting.
Syndrome | Genetic mechanism | Recognisable associations |
|---|---|---|
Fragile X syndrome | FMR1 full mutation becomes methylated, so less FMR1 protein is made | Intellectual disability, autistic features and anxiety |
Prader-Willi syndrome | Paternal genes at 15q11-q13 are not expressed | Infantile hypotonia, later hyperphagia (overeating) and characteristic behavioural difficulties |
Angelman syndrome | Maternal UBE3A is not expressed in neurones | Severe speech impairment, ataxia and seizures |
Rett syndrome | Usually a pathogenic MECP2 variant | Developmental regression, loss of purposeful hand use and stereotyped hand movements |
Down syndrome | Extra chromosome 21 material | Developmental disability and more Alzheimer pathology in later life |
22q11.2 deletion syndrome | Copy-number loss affecting several genes | Variable developmental difficulties, congenital abnormalities and a higher risk of psychosis |
Interpreting a syndrome
Imprinting explains why losing expression in the same chromosomal region can cause different syndromes, depending on which parent the affected copy came from. Prader-Willi and Angelman syndromes, both at 15q11-q13, are the standard example.
Behavioural phenotypes tell you what is more likely, not what must be present, so do not use them as diagnostic rules. Epilepsy, sensory impairment, pain and environmental stress can all change the presentation.
Regression (loss of skills already gained) needs investigation for neurological, metabolic and other acquired causes, even when a genetic diagnosis is already known.
New psychiatric symptoms should be judged against the person's own developmental baseline and way of communicating. Do not dismiss them as part of the disability.
Molecular confirmation and genetic counselling clarify the mechanism. This matters because recurrence risk can differ between a deletion, uniparental disomy (both copies of a chromosome inherited from one parent), an imprinting defect and an inherited variant.