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
Gottesman’s (1991) findings show the percentage likelihood of a person developing schizophrenia based on their relationship to each of the groups shown.
- 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
Understanding the human genome has helped researchers to identify key candidate genes which may code for schizophrenia.
Exam Tip
You do not need to identify any specific genes (candidate or otherwise) in an exam question but you do need to demonstrate that you are confident with the key terminology associated with this topic e.g. heritability, concordance rates, polygenic in order to elevate your answer.
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
Worked example
Virgina, age 33, has been reading about schizophrenia. She herself has no apparent symptoms of the condition. Her doctor has explained to her that her susceptibility to develop schizophrenia is quite high as both her parents are schizophrenic. Virginia is now concerned that any children she has will also have the disorder.
Briefly describe the genetic explanation for schizophrenia and explain why Virginia may be right to be concerned about having children. [8]
AO1 = 4 marks, AO2 = 4 marks
For 6-8 marks the answer should clearly and concisely explain the genetic explanation for schizophrenia using a good level of detail. Effective use of examples should be given to support the points made. There should be confident use of terminology. Reference to the stem should be effective.
For 4-6 marks the answer should give a coherent account of the genetic explanation for schizophrenia. There may be some lack of expansion or development of ideas. There should be some use of terminology. Reference to the stem will be present but may lack depth or detail.
For 1-3 marks the answer will be only partially successful in describing the genetic explanation for schizophrenia. Terminology will be sparse and there may be some vagueness or ambiguity to the response. Reference to the stem is likely to be sparse or absent altogether.
Possible answer content could include:
- AO1:
- Close (first-degree) relatives are more likely to share genes which means that they are more likely to share a vulnerability to schizophrenia with MZ twins having the highest risk
- There is a 6% chance of developing schizophrenia if your parents have it compared to only 2% if your cousin has the disorder
- Schizophrenia is polygenic i.e. no one, single gene codes for it, rather it is a product of candidate genes, meaning that different combinations of genes can produce schizophrenia
- AO2:
- It is not surprising that Virginia is concerned that any children she has may also have the disorder as her susceptibility is high given her own parents’ schizophrenia
- Virginia’s doctor could tell her she could have children who do not develop schizophrenia but that she would have no way of knowing this before getting pregnant
- Virginia has reached the age of 33 without, apparently, showing any symptoms of schizophrenia so it is possible that any children she has may not inherit the specific combination of candidate genes which increase vulnerability to schizophrenia
