Biological: Genetic Factors Flashcards
Explain schizophrenia as a ‘polygenic’ condition
-Genetic explanation sees it as transmitted hereditary.
-It is believed that there is not a single ‘schizophrenic gene’ but that several genes are involved, which increase an individuals overall vulnerability to developing schizophrenia.
-Studies have identified candidate genes - appears it is aetiologically heterogeneous i.e. different combinations of factors can lead to condition.
-Ripke et al (2014) - huge study combining data from investigation into genetic basis of schizophrenia - analysed genetic makeup of 37,000 patients compared to control group 113,000. Estimated 108 separate genetic variations were associated with risk of developing. Genes associated with increased risk included those coding for function of a number of neurotransmitters including dopamine.
What research is there into candidate genes?
-Benzel et al (2007) identified 3 genes: COMT, DRD4, AKT1 - all associated with excess dopamine in specific D2 receptors, leading to acute episodes + positive symptoms.
-Miyakawa et al (2003) studied DNA families affected, fund those with disease more likely to have a defective version of a gene PPP3CC associated with production of calcineurin - regulates immune system.
-Sherrington et al (1998) found a gene located on chromosome 5 linked in a small number of extended families where they have the disorder.
Explain twin studies in relation to genetic factors
-Assumption that MZ twins will show greater concordance rate than DZ twins - both probably share same environment, but vary in genetic similarity.
-MZ genetically identical, 50% genes in DZ are same.
Outline research support from twin studies
-Gottesman + Shields (1966) - investigated concordance rate of disorder in MZ and DZ twins.
-Used patient records from psychiatric hospital spanning 16 years. Found 57 twin pairs able to use in final analysis, participants split roughly half and half between males and females and contained more DZ than MZ.
-Used variety of sources of info (e.g. interviews, personality tests) to split into 4 categories - both twins diagnosed and hospitalised, both hospitalised only one diagnosed, one with schizophrenia and other with a different psychiatric abnormality, one twin had and other clinically ‘normal’.
-Results = if categories 1+2 then concordance rate 54% for MZ, 18% DZ. When category 3 added, rates rose to 79% for MZ, 45% DZ.
-Suggests a genetic link to schizophrenia. Concluded that genetics provide an increased vulnerability to developing it, but other factors should also be considered.
Explain adoption studies in relation to genetic factors
-It is difficult with win studies to separate effects of hereditary from effects of environment as usually raised together.
-These studies allow researchers to look at people who were born with schizophrenic mothers but brought up by adoptive parents with no history of the disorder.
Outline research support from adoption studies
-Kety (1994) - Copenhagen records of 5438 adopted children, found 33 with schizophrenia - interviewed biological and adoptive relatives. Studied a matched control group, found concordance rates higher between schizophrenics and biological relatives (13%) than with adopted relatives (2%). If an adopted parent developed, this did not lead adopted child to be more likely to develop.
-Tienari et al - 1991 identified 155 adopted children whose biological mothers were diagnosed, compared with adopted children with no history of schizophrenia, found about 10% whose mother was diagnosed developed compared to 1% of control group.
2000 - identified 164 adoptees whose biological mothers diagnosed with 11 (6.7%) also diagnose compared to 4 (2%) of 197 control adoptees born to non-schizophrenic mothers.
-Provides strong genetic evidence for a genetic component.
What is a strength of genetic factors?
Genetic explanations use objective techniques:
-It is now easier to assess zygosity accurately, the placenta provides the answer in 2/3 of MZ. If have single outer membrane they must be MZ.
-Most accurate method = DNA probe - to collect tiny amounts of DNA from each twins mouth, lab examine 12 specific markers present in the DNA of each twin.
DZ may share 5 marker patterns by chance MZ will have same patter for all 12. Lab can determine if twins are identical with reliability of 99.99%.
What are 3 limitations of genetic factors?
A reductionist approach:
-Assumes genes are the reason individuals suffer, ignores other key factors e.g. environmental.
-Cannot be argued there seems to be a genetic link, but if it was an entirely genetic condition the it would be presumed there would be 100% concordance rate in MZ twins, where as it is only 48%.
This opens up nature v nurture debate and suggests environment contributes to the onset.
Problems with zygosity rates in twin studies:
-In older reports, different and less objective criteria was used to distinguish between DZ and MZ.
Means we need to view findings of such studies with caution because if same participants were used in a study of modern zygosity assessment techniques then zygosity rates may decrease, altering the findings.
Problems with concordance rates:
-Can also be calculated in different ways and vary widely depending on method used.
-Appears to be related to scientific rigour of studies e.g. studies that have used ‘blind’ interviewers (people who don’t know of diagnosis of patients) have found lower concordance than other studies.
Marshall (1990) suggested the better controlled the study, the lower the concordance rate.
What can we conclude from genetic explanations?
Diathesis-stress model may be a better alternative:
-Considers both biological and social factors.
-e.g. Tienari et al (1987) looked at how childrearing skills affected those at risk from schizophrenia. Found high genetic risk children adopted into homes with healthy rearing patterns were at less risk of developing than those integrated into families with ‘adverse’ parenting. Children classified at lower risk in adverse group did not develop, which shows schizophrenia is developed in response to a number of social factors.
