Schizophrenia Q&A

Questions – Schizophrenia

These questions address the Schizophrenia Bulletin article entitled “Schizophrenia, Psychiatric Genetics, and Darwinian Psychiatry: An Evolutionary Framework” by Peralson and Folley, 2008.

1. Is schizophrenia a qualitative or quantitative trait?  What is the estimated heritability of this phenotype?

This is most likely a multi-gene trait with common alleles that are risks to causing this disease.  By themselves, those genes would not cause the disease, but when they all come together the disease is manifested (polygenic disease).  All genes are needed for the disorder to be present.  This makes it a quantitative trait.  No specific genes are linked to Schizophrenia, but a number of genes contribute to causing the disorder.  They increase the probability of the disease manifesting in a person. 

            In concordance studies with twins, in identical twins, they are forty to sixty percent likely to get the disease, as opposed to fraternal twins that are ten to twenty percent likely to get the disease.  This indicates that genetics has a lot to do with the cause of the disease, but environment still plays a point (“The Heritability of Schizophrenia”).  Disequilibrium studies have also been performed.  In these studies, large families affected by the disease are studied and their chromosomes are compared to families who are not affected.  From these studies, it has been shown that no specific chromosomal locations have been found that specifically cause the disease.  A variety of loci contribute to causing the disease.  When assessing families with Schizophrenia, the family genes are combined for assessment because of the smaller family size.  This can cause issues to identifying specific genes that cause the disorder because some families may have different genes specifically causing the disease than another family (“Schizophrenia and Genetics”). 

2.  
1. What kind of inheritance explains the observed patterns seen in schizophrenia?

It is the unfavorable combination of specific genes that contribute to neuron and brain development.  These changes in the genes are due to copy number variance and single nucleotide polymorphisms within the normal range (refer back to question 1 for more specifics).  

2. Why do schizophrenia risk genes persist in evolution?

The genes themselves are not selected against when inherited singly.  They are “below the radar” of selection.  Some inherited individually have a compensatory advantage or are neutral, keeping them in the population.  These genes are only selected against when the right combination is inherited and the disease occurs. 

3. What are the possible heterozygote advantages associated with schizophrenia?  Do you agree?

These alleles alone can be associated with normal or increased fertility, making them advantageous.  Another advantage associated with Schizophrenia for the genes would be in an ancestral environment.  An example of how these genes came to be would be through “broken genes”.  For example, some genes provided protective advantage with salt retention when salt was not readily available in the environment.  Overtime, keeping this trait when sodium is now plentiful contributes to the elevated levels of hypertension seen in society today.  There is a similar idea with diabetes.  In the ancestral environment, these genes expressed hormones that caused insulin resistance that helped with more effective fat storage, helping with limited availability of food.  Now this trait is disadvantageous and causes elevated levels of obesity in people with this disease because of unnecessary fat storage.  These genes are associated with causing diabetes.  There might have been adaptations that Schizophrenia susceptibility genes caused, but they have yet to be identified.  Schizophrenia might have been advantageous in causing paranoia in ancestral environments, helping protect the person from predators or other possible dangers.  We agree with these ideas the paper presents.  As the environment changed over time, the advantages the genes offered also changed.  Because of modern medicine, these traits will be retained in the population, as these issues most likely can be treated.  Also, carriers will continue to contribute these traits to the next generation.

 

3. If heterosis is acting on the schizophrenia alleles, what might you expect will happen to these alleles over the long term ? (Think fitness tables...)

If heterosis is acting on the schizophrenia alleles, you would expect that over a long period of time the heterozygote genotype will become fixed in the population with the homozygote dominate and homozygote recessive genotypes going to an equilibrium based on the frequency they are at during the time heterosis is selected for.  This means that the schizophrenia alleles will always be in the population.  There will always be a small population of people who are affected by schizophrenia because currently only one percent of people are affected. 

4. This paper, and much of evolutionary psychology, is panselectionist or ultradarwinist.  What does that mean?  What other mechanisms of evolution might be at play here?

A panselectionism is the belief that the only mechanism of evolution was natural selection and that all genes are inherited (“Neo-Darwinism”).  An ultra-darwinist is a person who believes in Darwin’s idea of natural selection, but thinks that it is the only mechanism of evolution (Cain 220).  This paper emphasizes these ideas greatly, putting great consideration on natural selection in the evolution of the schizophrenia genes.  The paper briefly mentions that sexual selection may have caused schizophrenia to appear in the population.  Exaptation is also mentioned, as some of these genes may have started being used for other functions like speech and art.  Other mechanisms of evolution that should be considered when analyzing these traits should be mutations, genetic drift, and gene flow.  There might have been disadvantageous mutations that caused these alleles to become present in the population.  Founder affect might have occurred, bringing together a population with these alleles that would increase the frequency of inheriting schizophrenia susceptibility genes.  Gene flow might have then integrated these alleles into larger populations over time. 

If you want to read more, check out this link:  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2632450/

Works Cited

Cain, Clifford Chalmers. "Darwin's Pious Idea: Why The Ultra-Darwinists And Creationists Both Get It Wrong - By Conor Cunningham." Reviews In Religion & Theology 19.2 (2012): 220-221. Academic Search Complete. Web. 2 Apr. 2014.

"The Heritability of Schizophrenia." Biological Basis of Mental Health. Open Educational Resources, n.d. Web. 2 Apr. 2014.

"Neo-Darwinism." Princeton University. N.p., n.d. Web. 02 Apr. 2014. <http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Neo-Darwinism.html>.

"Schizophrenia and Genetics: Research Update | Psych Central." Psych Central.com. N.p., n.d. Web. 02 Apr. 2014. <http://psychcentral.com/lib/schizophrenia-and-genetics-research-update/0008736>.