biological explanation of schizophrenia: genetics

Question 1

what is the risk of developing schizophrenia in the general population?

Answer 1

• the risk is less than 1% over a lifetime

Question 2

how does having a relative with schizophrenia affect your risk of developing it?

Answer 2

• second degree relative (e.g. aunt/uncle, niece/nephew) → risk is 2-6%
• first degree relative (e.g. parent, sibling, dizygotic twin) → risk is 6-17%
• identical twin with schizophrenia → risk is 48% (Gottesman, 1991)
• greater genetic relatedness = higher risk, suggesting a strong genetic element to schizophrenia

Question 3

what do twin and family studies suggest about the genetic explanation of schizophrenia?

Answer 3

• families with more members having schizophrenia suggest a genetic link
• identical (monozygotic) twins share schizophrenia more than non-identical (dizygotic) twins, supporting a genetic link to schizophrenia

Question 4

what does the heritability estimate of schizophrenia suggest?

Answer 4

• recent estimate for heritability is 79% (Hilker et al. 2018)
• suggests a large role for genetic factors in determining schizophrenia

Question 5

what does the human genome consist of and what does it determine?

Answer 5

• the human genome has around 23,000 genes
• genes contain DNA strands with instructions for both physical features (e.g. height, eye color) and psychological functioning (e.g. neurotransmitters, brain structure size)

Question 6

what is one approach to understanding the genetic basis of schizophrenia?

Answer 6

• identifying responsible genes in family members with schizophrenia
• 700 genes have been linked to schizophrenia (Wright, 2014)

Question 7

specific genes that have been identified that are linked to the presence of schizophrenia:

Answer 7

• the COMT gene
• the DISC1 gene

Question 8

COMT gene

Answer 8

• COMT gene provides instructions for creating catechol-O-methyltransferase
• this enzyme breaks down dopamine in the prefrontal cortex
• deletion of the COMT gene leads to poor dopamine regulation, causing schizophrenic symptoms

Question 9

DISC1 gene

Answer 9

• disc1 = disrupted-in-schizophrenia 1
• disc 1 gene abnormality = 1.4x more likely to develop schizophrenia (Kim et al 2012)
• disc 1 gene makes GABA
• gaba regulates glutamate and dopamine in the limbic system

Question 10

what do family studies suggest about the genetic and environmental factors in schizophrenia? (ao3)

Answer 10

• family studies show an inherited component in schizophrenia risk increases with genetic similarity
• however, no single recessive or dominant gene has been identified as the cause
• schizophrenia likely results from multiple genes rather than one (Harrison & Owen, 2003)
• up to six genes may be involved in susceptibility (Harrison & Owen, 2003)
• genes may affect synapse functions and brain circuitry (Harrison & Weinberger, 2004)
• however, family studies don’t fully consider environmental influences on schizophrenia risk
• dysfunctional family communication and negative emotions may increase schizophrenia relapse rates

Question 11

supporting: gottesman and shields

Answer 11

• 48% concordance rate for identical (monozygotic) twins
• 17% concordance rate for non-identical (dizygotic) twins
• for severe cases of schizophrenia, MZ twins had a concordance rate between 75% and 91%
• study suggests a genetic basis for schizophrenia

Question 12

supporting: Heston

Answer 12

• aim: whether schizophrenia has a genetic predisposition
• 50 adoptees with biological mothers diagnosed with schizophrenia
• 47 control adoptees with mothers who had no schizophrenia diagnosis
• 1% incidence of schizophrenia in the general population
• 10% of adoptees with a family history of schizophrenia (mother diagnosed) developed schizophrenia
• suggests schizophrenia has a strong genetic component

Question 13

supporting: Dahoun

Answer 13

• Dahoun (2017) reviewed 14 studies
• concluded DISC1 is linked to presynaptic dopamine dysregulation
• dopamine dysregulation is a key factor in schizophrenia
• shows how genetic variation can affect neurochemical differences, increasing schizophrenia risk

Question 14

supporting: twin studies

Answer 14

• twin studies provide strong genetic evidence
• MZ twins are genetically identical, while DZ twins are like ordinary siblings
• if nurture were the key factor, there would be no difference in concordance rates between MZ and DZ twins
• the difference in concordance rates supports the involvement of genetic factors in schizophrenia

Question 15

other: sample size (weakness)

Answer 15

• sample size in twin, family and adoption studies may limit generalisability and population validity
• gene-mapping study had a small sample and hasn’t been replicated, showing sample bias
• DZ twins may be treated the same due to their similar appearance, affecting results
• twins often share the same environment, which complicates the interpretation of genetic influence

Question 16

other things: reductionist

Answer 16

• focuses only on biological factors, ignoring psychological factors
• twins with identical genes show less than 100% concordance, so schizophrenia can’t be explained by genetics alone
• the difference between identical twins with and without schizophrenia is likely psychological
• the biological explanation is too simplistic and fails to consider multiple levels of explanation

Question 17

other things: Heston (strength and CA)

Answer 17

+ concordance rate
- difficult to replicate (sample size)