The Biological explanations for Schizophrenia (genetic basis)

Family studies (1)

Family studies have confirmed that risk of schizophrenia increase in line with genetic similarity to a relative with the condition.

 

This relationship is shown in this graph: and parents the findings from Irving Gottesman’s (1991) large-scale family study.

For example, someone with an aunt with schizophrenia has a 2% chance of development it, increasing to 9% if the individual is a sibling and 48% if they are an identical twin.

Family studies (2)

Of course family members tend to share aspects of their environment as well as many of their genes, so the correlation represents both - but the family studies still give good support for the importance of genes in schizophrenia.

• The closer a relative, the more susceptible you are to developing schizophrenia

Candidate genes (1)

Candidate Genes = specific individual genes researchers believe are associated with an increase risk of inheriting a psychological condition or behaviour.

 

The next logical step is to identify candidate genes. Early research in this area looked for a single genetic variation in the belief that on faulty gene could explain schizophrenia.

 

However it appears that a number of different genes are involved (schizophrenia is polygenic). The most likely genes would be those coding for neurotransmitters including dopamine.

Candidate genes (2)

In a large study Ripke (2014) combined all previous data from genome-wide studies (looking at the whole human genome) of schizophrenia.

 

The genetic make-up of 37,000 people with a diagnosis of schizophrenia was compared to that of 113,000 controls.

 

Findings = they found that 107 separate genetic variations were associated with slightly increased risk of schizophrenia.

 

Because different studies have identified different candidate genes, it also appears that schizophrenia is aetiologically heterogenous. (Different combination of factors, including genetic variation, can lead to the condition)

Role of mutation (1)

Schizophrenia can also have a genetic origin in the absence of a family history of the condition. One explanation for this is mutation in parental DNA which can be caused by radiation, poison or viral infection.

 

Evidence for mutation comes from positive correlations between paternal age (associated with risk of sperm mutation) and risk of schizophrenia, increasing from around 0.7% with fathers under 25 to over 2% in fathers over 50

 

• Combined effect of several genes, each of which contribute to an increasing risk of developing schizophrenia.

A03 - strength (evidence based)

One strength of the genetic explanation is the strong evidence base. Family studies show that risk increases with genetic similarity to a family member with schizophrenia.

 

Teinari 2000

Investigated how genetic vulnerability interacts with rearing environments to influence the development of the disorder.

 

164 adoptees whose biological mothers has been diagnosed.

 

6.7% of the 164 also received a diagnosis of SZ, compared to just 2% of the 197 control adoptees (those born to non-SZ mothers)

 

Shows that biological children of parents with SZ are at heightened risk even if they grow up in an adoptive family. Some people are more vulnerable to SZ as a result of their genetic makeup

A03 - Limitation (risk in increase with genetic similarity to a family member with SZ)

Irving gottesman (1991) conducted a large -scale family study.

 

Found that the likleihood of SZ increased depending on how closely someone was genetically related to a person with SZ.

 

Found that someone with an aunt with schizophrenia has a 2% chance of development it, increasing to 9% if the individual is a sibling and 48% if they are an identical twin.

 

This supports the genetic explanation because those who share more genes with an affected relative have a higher risk of developing SZ.

 

However the concordance rate fro identical twins was only 48% suggesting that environmental factors must also play a role.