biological explanations for schizophrenia: genetic basis Flashcards
what have family studies confirmed?
risk of schizophrenia increases in line with genetic similarity to a relative with the disorder
summarise the findings from gottesman’s (1991) large-scale family study
- idential twins (100% of genes shared) = 48% risk
- fraternal twins (50% of genes shared) = 17% risk
- grandchildren (25% of genes shared) = 5% risk
- first cousins (12.5% of genes shared) = 2% risk
- general population = 1% risk
describe the genetic influence for schizophrenia
- schizophrenia is polygenic ie. many different genes are involved
- the most likely candidate genes would be those coding for neurotransmitters or receptors, especially dopamine
genome-wide study of schizophrenia (ripke et al. 2014)
- compared genetic make-up of 37,000 people with a diagnosis of schizophrenia to 113,000 controls
- 108 separate genetic variations were associated with slightly increased risk of schizophrenia
how is schizophrenia aetiologically heterogeneous?
- different studies have identified different candidate genes
- different combinations of factors, including genetic variation, can lead to the condition
the role of mutation
- schizophrenia can have a genetic origin in the absence of a family history of the disorder
- mutations in parental DNA may be caused by radiation, poison or viral infection
where does evidence for the role of mutation come from? (brown et al. 2002)
- positive correlations between paternal age (associated with increased risk of sperm mutation) and risk of schizophrenia
- 0.7% in fathers < 25, over 2% in fathers > 50
evaluation: research support (tienari et al. 2004)
- biological children from parents with schizophrenia are at a heightened risk even if they grow up in an adoptive family
- of 164 adoptees whose biological mothers had been diagnosed with schizophrenia, 6.7% also received a diagnosis, compared to 2% of the 197 control adoptees, who were born to non-schizophrenic mothers
evaluation: research support (hilker et al. 2018)
concordance rate of 33% for MZ twins, 7% for DZ twins
evaluation: research support (joseph 2004)
- pooled data for all schizophrenia twin studies carried out before 2001
- concordance rate of 40.4% for MZ twins, 7.4% for DZ twins
evaluation: research support (kendler 1983)
- 30.9% concordance rate for MZ twins, 6.5% for DZ twins
- suggested that genetic factors are as aetiologically important in schizophrenia as in medical conditions eg. diabetes, hypertension
evaluation: biological risk factors
- birth complications (morgan et al. 2017)
- smoking THC-rich cannabis in teenage years (di forti et al. 2015)
evaluation: psychological risk factors (mørkved et al. 2017)
67% of people with schizophrenia and related psychotic disorders reported at least one childhood trauma as opposed to 38% of a matched group with non-psychotic mental health issues
evaluation: genetic counselling
- if one or more potential parents have a relative with schizophrenia, they risk having a child who would go on to develop the condition
- however, risk estimate from genetic counselling is just an average figure that doesn’t reflect the actual probability of a particular child developing schizophrenia as they will experience a particular environment which also has risk factors
evaluation: MZ twins encounter more similar environments (joseph 2004)
- underlying assumption is that the environment of MZ and DZ twins is equivalent
- J (2004) suggests it is widely accepted that MZ twins are treated more similarly than DZ twins so encounter more similar environments
- difference in concordance rates between MZ and DZ twins only reflect the environmental differences that distinguish the two types of twin
evaluation: adoptees may be selectively placed (joseph 2004)
- assumed in adoption studies that adoptees are not ‘selectively placed’
- adoptive parents who adopt children with a schizophrenic biological parent is assumed to the the same as adoptive parents who adopt children who don’t have this background
- J (2004) claims this is unlikely, particularly in early studies
- in countries like denmark and the US, potential adoptive parents are informed of the genetic background of children prior to selection for adoption
