Genetic Basis of Schizophrenia

• Genetic basis of schizophrenia

  Biological characteristics are inherited from parents
• Genetic basis of schizophrenia

  1. Faulty version of a gene initially occurs due to random mutation

  2. Faulty gene can then be inherited by the individual's offspring

  3. Many candidate genes have been identified that contribute to the risk of developing schizophrenia
• COMT gene


  Gene located on chromosome 22 which controls an enzyme that breaks down dopamine
• A person inherits a low activity variant of the COMT gene


  The enzyme is less effective, so dopamine levels are allowed to increase
• Excessive dopamine activity


  Has been linked to hallucinations, which are a symptom experienced by some schizophrenic patients
• Polygenic explanation


  Symptoms of schizophrenia are a result of a complex combination of many candidate genes and that no single gene is responsible
• Twin studies


  • Involve large samples of monozygotic (identical) twins and dizygotic (non-identical) twins when one of the twins has been diagnosed with schizophrenia
  • Concordance rates are then calculated separately for the MZ and DZ samples, which is the number of twins within each sample who both have the disorder, usually represented as a percentage
• If the concordance rate is significantly higher for MZ twins than for DZ twins, this suggests schizophrenia has a genetic basis because MZ twins share 100% of their genetic material, whereas DZ twins share approximately 50%
• Gottesman (1991) reviewed over 40 twin studies and found the average concordance was 48% for MZ twins and 17% for DZ twins, which suggests schizophrenia is at least partly genetic
• Joseph (2004) conducted a meta-analysis of all twin study data prior to 2001 and reported a MZ twin concordance rate of 40% compared to 7% for DZ twins
• Since MZ twins are genetically identical and the concordance rate is significantly higher compared to DZ twins, this strongly supports the role of genes in schizophrenia
• Because the concordance rate for MZ twins was not close to 100%, this suggests schizophrenia is not purely genetic
• Overall the findings indicate that although genetics are important, schizophrenia is not purely genetic