IB Docs (2) Team
Revision: Genetics
The role of one or more genes on behaviour
After completing this lesson, you should be able to:
- Explain how one or more genes may affect behaviour.
- Explain one or more studies of how one or more genes may affect behaviour.
- Discuss the role of genes in behaviour.
Conceptual understanding
In both an SAQ or an ERQ, you are expected to first describe and/or explain the theory. For describe, this means that you are identifying and defining the different components of the theory. Explain asks you to be specific about how the theory actually works - that is, identifying assumptions made by the theory and/or explaining how the components of the theory interact.
Candidate gene: a gene whose function is known that is hypothesized to be associated with a particular disorder or behaviour.
Diathesis-stress model: attempts to explain a disorder as the result of an interaction between a predisposition and stress caused by life experiences.
Gene expression: the process by which the instructions in our DNA are converted into a functional product; in psychology, this is when a gene is "turned on" or "off", resulting in a behaviour.
Genetic vulnerability: An underlying inherited susceptibility - that is, the theory that you may have genes that may make you more likely to have certain traits if those genes are exposed to the appropriate environmental stressors.
Genomewide Association Study: an examination of genetic variants in a large sample of individuals to see if any variant is associated with a trait.
Linkage analysis: Matching genetic variations (polymorphisms) with the frequency of a behaviour
Polymorphism: a genetic variation (mutation) resulting in the occurrence of several different forms or types of individuals among the members of a single species.
Transgenic mice: mice that have had a single gene changed or removed
Checking for understanding
The following statements are based on the page: The Human Genome Project. For each statement, decide whether it is true or false. If the statement is false, explain why it is false and correct the statement.
1. Genomewide association studies use a deductive approach to test the link between a gene and a behaviour.
False. GWAS are inductive. Researchers do not decide what genes to look for before carrying out the research; they use a Manhattan plot to see the frequency of polymorphisms related to a behaviour compared to a control group.
2. In a twin study, the results were that MZ twins had a concordance rate of 42% for bipolar disorder, DZ twins had a concordance rate of 36% and siblings had a concordance rate of 32%. This shows that bipolar disorder is inherited.
False. When we see numbers like this, it is difficult to know what conclusions to draw. It will partly depend on the size of a sample. Remember that although DZ twins are not identical, they may share a lot of the same genes. They could, for example, both have the genes for bipolar disorder. That could explain why the concordance rates are similar for MZ, DZ, and even siblings. It doesn't mean that we can clearly rule out that bipolar disorder is genetic.
3. Psychologists argue that a single gene may lead to a behaviour.
False. Psychologists argue that a single gene may play a role in a behaviour, but behaviour is the result of the interaction of several genes. This is what makes genetic research difficult; it is not really clear how genes interact to produce behaviour.
4. Genetic arguments are deterministic.
Sort of. Although it used to be assumed that if you have the gene for something that you would "get it," this is no longer the prevailing attitude. The argument is still deterministic in that if you have the gene, your risk of developing the behavior or health problem is significantly higher than the general population, but it is not your destiny. In fact, knowing that you have the gene may mean that you can engage in behaviour to potentially lower your risk.
5. Most genetic research is correlational.
True - at least in humans. Researchers may manipulate the genes in animals - such as the OB mouse or breeding knock-out mice that may lead to an increase in aggression (Cases et al, 1995) - is experimental. This allows a researcher to draw a cause and effect conclusion. However, it is not really possible to generalize to humans.
6. A study in which a pair of MZ twins are separated at birth - where one stays with the birth family and the other is adopted - is an example of a natural experiment.
True. But it is important to remember that it was not the researcher that manipulated the IV!
Understanding research
For an SAQ, you need to be able to describe/explain one study. This means that you have to be able to describe the aim, procedure, and findings of the study. To explain it, you must link it to the theory or concept in the question. In an ERQ, you should know two pieces of research. In addition, to describing the aim, procedure, and findings, you need to be able to evaluate the research. The use of correct terminology when describing a study - for example, correctly identifying the research method - is an important aspect of the top markband.
Kendler et al (2006) investigated whether major depressive disorder may be inherited. They also wanted to see if there were significant gender differences in the heritability of major depression.
The sample was made up of 15,493 complete twin pairs. In order to gather their data, the researchers used a team of trained interviewers to carry out telephone interviews. The interviewers assessed lifetime major depression by using modified DSM-IV criteria. 8056 twins met the criteria for a diagnosis of major depression at some point in their life.
In addition to this information, the interviewers also asked questions about the twins' "shared environment.”
The researchers found a 44% concordance rate for MZ female twins and 31% for MZ male twins – compared to a 16% concordance rate for DZ female twins and 11% for DZ male twins.
The results indicate that the heritability of major depression was significantly higher in women than men. In addition, the correlations were significantly higher in monozygotic than in dizygotic twins. The estimated heritability of major depression was 0.38. They also found no correlation between the number of years that the twins had lived together and lifetime major depression.
The aim of Caspi et al’s (2003) study was to determine whether there is evidence for a gene-environment interaction for a mutation of the serotonin transporter gene, 5-HTT. The serotonin transporter is involved in the reuptake of serotonin in brain synapses. There is a mutation of the 5-HTT gene in which there are short alleles.
The researchers studied a sample of 847 New Zealand 26-year-olds. All were members of a cohort that had been assessed for mental health on an every-other-year basis until they were 21. They were divided into three groups based on their 5-HTT alleles: Group 1 had two short alleles; Group 2 had one short and one long allele; Group 3 had two long alleles.
The participants were asked to fill in a "Stressful life events" questionnaire which asked them about the frequency of 14 different events - including financial, employment, health, and relationship stressors - between the ages of 21 and 26. They were also assessed for depression.
People who had inherited one or more short versions of the allele demonstrated more symptoms of depression and suicidal ideation in response to stressful life events. The effect was strongest for those with three or more stressful life events. Simply inheriting the gene mutation was not enough to lead to depression, but the genes' interaction with stressful life events increased one's likelihood of developing depression.
Checking for understanding
Answer the following questions with regard to the studies summarized above.
1. Why is the finding in Kendler's (2006) study that there was no correlation between the number of years that the twins had lived together and lifetime major depression important?
2. What is meant by the term "gene-environment interaction?"
3. What role could cognitive biases play in Caspi et al's (2003) study?
4. To what extent can we generalize the findings of Kendler et al's (2006) study?
5. Risch et al (2009) carried out a meta-analysis of attempted replications and found that the results of Caspi et al 's (2003) study were not able to be replicated. What does this say about the findings?
Critical thinking
Remember that critical thinking is an essential part of the ERQs, but not of the SAQs. There are two key ways in which critical thinking is assessed. First, you need to be able to evaluate the research that you use in your response. Secondly, you need to address the command term. The command term may ask you to evaluate the theory or discuss the theory. Evaluate means that you must address both strengths and limitations of the theory. Discuss is the broader command term allowing to discuss either strengths or limitations of the theory, why it is difficult to study the theory, or the implications of the research.
- Genetic arguments help to explain why behaviours run in families.
- Research in genetics has helped researchers to understand the role of the interaction between biological predisposition and the environment. However, research is correlational in nature and causality cannot be determined.
- Genetic research may be used in preventative health approaches.
- Genetic explanations are a reductionist approach to explaining complex behaviours.
- Although in some behaviours candidate genes have been identified, it is not always clear how the gene actually leads to the behaviour.
- Although some experimental research is being done with animals, it is difficult to generalize the findings to humans.
Exam tip
On the exam, they may ask you a question focused on "one gene." Although the question may also read "one or more genes," especially with regard to the SAQ, you should be ready for one gene. In that case, you could use the study by Caspi above. However, depending on what you have studied, you may consider using other examples. For example, Hamer et al's. study of the Xq28 gene and sexuality, Shmelkov's study of the SLITRIK5 gene and compulsive behaviour; Quervain's study of the alpha adrenoreceptors gene, Meany's study of the GR gene, Case's study of the MAOA gene, or Scott-van Zeeland's study of the EPHX2 gene and anorexia.
Testing for understanding
A psychologist carries out a twin study and finds that there is a 43% concordance rate for depression in monozygotous twins. Applying your understanding of epigenetics, why is it not 100% if they have exactly the same genes?
Epigenetics has taught us that we don't just have to have a gene, but that gene must be expressed in order for a behaviour - such as bipolar disorder - to exist. What does this mean about the "deterministic nature" of genes? In other words, if I have the gene, is it my fate to develop bipolar disorder? Explain why or why not.
You have been asked to go to the pre-IB biology class and explain the concept of the "gene-environment interaction" - that is, epigenetics. Using Caspi's study, how would you explain this concept?
Revision: Evolution
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