schizophrenia: bio explanation - genetics

Question 1

what are monozygotic twins?

Answer 1

• result from the fertilisation of a single egg by a single sperm
• identical
• share same genome and nearly always the same sex

Question 2

what are dizygotic twins?

Answer 2

• fraternal
• two separate eggs fertilise by two separate sperms
• will be no more alike than siblings born at separate times
• can be either same or different sex

Question 3

what is the concordance rate?

Answer 3

• rate of agreement
• likelihood that when one twin demonstrates something, the other twin also does

Question 4

what are genetics?

Answer 4

• child inherits two sets of genes - one from each parent
• gene can be passed onto next generation
• genes may come in different variations known that alleles, expressing different traits

Question 5

what is nature?

Answer 5

• genetics

Question 6

what is nurture?

Answer 6

• all of the external/environmental factors that affect human development

Question 7

what is the link between genetics and schizophrenia?

Answer 7

• there is evidence from twin and family studies for a genetic explanation of schizophrenia
• if a family has more members than usual with schizophrenia, that suggests a genetic link
• if mz twins share schizophrenia more than dz twins, that suggests a genetic link
• recent estimate for the heritability of schizophrenia is 79% (Hilker et al. 2018)
• this suggests a large role for genetic factors in determining disorder

Question 8

what does the risk of developing schizophrenia depend on?

Answer 8

• what degree relatives they are and how much gene is shared
• 1st degree = 50% (e.g. identical twins)
• 3rd degree = 12.5% (e.g. first cousins)

Question 9

how much does the human genome comprise? and what does it consist of?

Answer 9

• around 23,000 genes, each gene carrying instructions about how to build a unique person
• genes consist of dna strands and dna holds ‘instructions’ for general physical features of an organism such as height or eye colour and specific physical features that relate to psychological functioning such as neurotransmitters or the size of brain structures

Question 10

what is one approach to understand the genetic underpinnings of schizophrenia?

Answer 10

• to identify responsible genes that are present in family members who have been diagnosed with the disorder
• the search for such genes has identified a very large number of possibilities
• Wright 2014 indicated that **as many as 700 genes have been linked to schizophrenia

Question 11

what two specific genes have been identified that are linked to the presence of schizophrenia?

Answer 11

• COMT gene
• DISC1 gene

Question 12

what is the COMT gene?

Answer 12

• provides instructions for the creation of catechol-O-methyltransferase, an enzyme which breaks down neurotransmitters such as dopamine in the prefrontal cortex
• deletion of COMT gene would mean that dopamine levels are poorly regulated resulting in schizophrenic symptoms

Question 13

what is the DISC1 gene?

Answer 13

• people with an abnormality to the gene Disrupted-in-Schizophrenia 1 (DISC 1) are 1.4 times more likely to develop schizophrenia than people without this abnormality (Kim et al. 2012)
• codes for the creation of GABA, a neurotransmitter which regulates other neurotransmitters such as glutamate and dopamine in the limbic system

Question 14

evidence for genetics as an explanation: Gottesman + Shields 1966

Answer 14

• wanted to establish probability of both twins developing schizophrenia
• carried out research over 16 years
• 48% concordance rate for identical twins
• 17% concordance rate for non-identical twins
• if the most severe cases of schizophrenia were looked at, the concordance rate for mz twins was between 75% and 91%
• study therefore strongly suggests that there is at least some genetic basis for schizophrenia

Question 15

evidence for genetics as an explanation: Heston 1966

Answer 15

• aim was to see whether schizophrenia has a genetic predisposition
• 50 adoptees whose biological mothers had been diagnosed with schizophrenia compared to control of 47 adoptees whose mothers had not been diagnosed with schizophrenia
• all had been separated from their mothers during first few days of life
• incidence of schizophrenia in the general population is about 1% and this was similar for those who were adopted with no family history of schizophrenia but five, which is approximately 10% of the adopted children with a family history of schizophrenia (e.g. mother diagnosed with schizophrenia) were later diagnosed with schizophrenia.
• therefore, this research shows that schizophrenia may have a strong genetic component

Question 16

evidence for genetics as an explanation: Dahoun 2017

Answer 16

• reviewed 14 studies and concluded that DISC1 is associated with presynaptic dopamine disregulation, key factor in schizophrenia
• this shows how genetic variation can impact neurochemical differences which can predispose a person towards schizophrenia

Question 17

strengths: supporting evidence

Answer 17

• twin studies provide strong genetic evidence. as the mz twins are genetically identical whilst dz twins are no more alike than ordinary people siblings, this supports the involvement of genetic factors. if these were not important, and instead nurture was the key determinate, then there should be no difference in the concordance rates between mz and dz twins
• Gottesman + Shields supporting evidence, + concordance rate, CA: difficult to replicate (sample size)

Question 18

weakness: other things

Answer 18

• sample size. sample size of twin, family and adoption studies casts doubt on the generalisability of findings and so population validity may be low. gene-mapping study was based on a restricted number of families and has failed to be replicated, which illustrates problems of sample bias. also twins share more than their dnas, dzs more likely to be treated same as they look same and all twins often share same environment
• reductionist. focus on only biological factors ignores role of psychological factors. research on twins with identical genes shows far from 100% concordance and so schizophrenia cannot be just explained by genetics. explanation as to why one identical twin develops schizophrenia and other doesn’t is likely to be psychological rather than biological. thus, we must consider more than one level of explanation, biological explanation alone is too reductionist (simplistic)