Specific genetic or hormonal abnormalities during human development produce mental retardation syndromes with specific cognitive patterns, allowing one to map genetic expression onto brain development. Correlational and discriminant functions of brain glucose metabolic rates measured with positron emission tomography (PET) show disrupted brain language areas in young adults with Down syndrome (DS). DS subjects activate similar brain areas during language processing as controls, but to a lesser extent. Subjects with full Turner syndrome (TS) (45,X) have reduced volumes of the hippocampus and of left cerebral hemisphere, measured with magnetic resonance imaging (MRI), and impaired memory and visuospatial abilities. Mosaic Turner subjects have brain volume reductions between full Turner syndrome and controls, indicating gene dosage effects on brain development. Individuals with a premutation for fragile X syndrome [fra(X)] have abnormal patterns of brain glucose metabolism that correspond to behavioral and cognitive changes.