Biological: The Genetic Basis of Schizophrenia (AQA A Level Psychology): Revision Note

The heritability of schizophrenia & candidate genes

• Heritability refers to the extent to which a condition or trait has been passed on generationally through families via genes (genetic inheritance)

• Gottesman (1991) conducted research into the heritability of schizophrenia and found a concordance rate of 48% for schizophrenia between MZ (monozygotic, i.e., identical) twins

• To put it plainly: the closer the genetic link between you and someone with schizophrenia, the higher the chance you have of also developing the illness

• There is no single gene that has (so far) been identified as causing schizophrenia; rather, it is thought that the condition involves the combined effect of several genes, each of which contribute to an increasing risk of developing schizophrenia

• The genes which may increase a person’s risk of developing schizophrenia are known as candidate genes and, as there are several of them, schizophrenia is referred to as polygenic

• 21st-century research into molecular genetics has identified some candidate genes implicated in schizophrenia e.g. neuregulin 1, dysbindin and proline dehydrogenase (Salleh, 2004)

• The candidate genes cited in the above bullet point are all implicated in the production and regulation of key mood-related neurotransmitters dopamine, serotonin and glutamate

• The Human Genome Project (1990–2003) has contributed hugely to understanding the genetic basis of schizophrenia and it was due to the global scale of this research that much of the current insight into genetics and schizophrenia has been made possible

Research which investigates the genetic basis of schizophrenia

• McCarroll et al. (2016) analysed the genomes of 65,000 people and 700 postmortem brains and found that the strongest known genetic risk for schizophrenia comes from the gene known as C4 

• Haraldsson (2011) conducted a meta-analysis which found strong evidence for the heritability of schizophrenia and that some candidate genes in combination with each other result in a person having an increased vulnerability to schizophrenia compared to other combinations of candidate genes 

Evaluation of the genetic basis of schizophrenia

Strengths

• Methods used to measure a genetic explanation for schizophrenia are clinical, objective and not affected by sources of bias such as individual differences, which means that they have good reliability and validity

• There is now a huge and compelling body of research evidence which points to genetics playing a key role in the development of schizophrenia

Weaknesses

• Genetics is an explanation of schizophrenia but it is not the explanation: if schizophrenia was purely biological, then concordance rates for MZ twins would be 100%, which no research has ever found

• The symptoms of schizophrenia are varied and will not present in exactly the same way per person – the absence of a biomarker means that from a genetic perspective it is very difficult to know definitively if a patient actually has schizophrenia or if they have another disorder altogether

Issues & Debates

• The genetic explanation exemplifies the nature side of the nature vs nurture debate, suggesting schizophrenia is largely inherited through candidate genes

  • However, the fact that MZ twins only show around 48% concordance (Gottesman, 1991) suggests that environmental (nurture) factors also play a key role

  • The most valid explanation is likely to be interactionist, combining genetic vulnerability with environmental triggers (diathesis–stress model)

• Focusing on candidate genes as an explanation for schizophrenia is biologically reductionist

  • This is because it simplifies a complex disorder to genetic and neurochemical factors alone

  • This ignores psychological, social, and environmental influences, such as trauma, family dynamics, or life stress, which can also contribute to the disorder