Genes and Behaviour

Theory

Nature of the Gene

• Genes are composed of DNA, which serves as a blueprint for the human body's structure and function.
• An individual's genome encompasses all the genes they possess.

Gene Regulation and Expression

• Gene expression is regulated, leading to differential expression.
• Possessing a gene for a behavior does not guarantee its expression.

Factors Influencing Gene Expression

• Genes can be activated or deactivated by internal signals (hormones, chemicals, other genes) and external signals (environmental events).
• The nature vs. nurture debate often intersects with gene expression.
• Environmental events can trigger hormones or proteins that regulate gene expression.
• The diathesis-stress model explains how stressful environments can trigger inherited predispositions, such as depression.
• Some genes can be permanently switched off through DNA methylation, a process known as epigenetics, which does not alter DNA but affects expression.

Importance of Studying Genes

• Many genes are linked to disabilities and mental illnesses.
• Genetics can increase the likelihood of inheriting diseases or characteristics, in interaction with the environment.

Study 1: Caspi et al (2003)

Aim

• Investigate why stressful experiences lead to depression in some individuals but not others.
• Examine the role of a gene related to serotonin and its contribution to depression.

Procedure

• Longitudinal study using questionnaires covering life events between ages 21 and 26.
• Sample included participants studied at multiple ages: 3, 5, 7, 9, 11, 13, 15, 18, 21, and 26.
• Participants grouped by 5-HTTLPR genotype into three groups based on allele combinations.
• Measured stressful life events and depression symptoms using Diagnostic Interview Schedule and informant reports.

Findings

• Individuals with the "s" allele had stronger interactions between life events and depression.
• Stressful life events predicted increased depressive symptoms and new depression cases in "s" allele carriers.
• Stressful life events also predicted suicide ideation in "s" allele carriers.

Conclusion

• No direct relation between the short 5-HTT allele and depression, but a relationship with stress and subsequent depression.
• Long 5-HTT allele may protect against stress-induced depression.

Evaluation

• Natural experiment not manipulated by researchers, reducing demand characteristics.
• Challenges in isolating gene actions and identifying exact genes responsible for depression.
• Correlations between genes, behavior, and environment observed rather than clear causation.

Study 2: Kendler et al (2006)

Aim

• Investigate heritability of major depression, gender differences, and temporal differences in genetic and environmental factors.
• Sample included 15,493 complete twin pairs from the Swedish Twin Registry.

Procedure

• Telephone interviews using DSM-IV criteria to assess lifetime major depression.
• Questions included shared and individual-specific environments.

Findings

• Higher concordance rates for major depression in women and monozygotic twins.
• Estimated heritability of major depression was 0.38, consistent with previous research.
• No significant differences in genetic and environmental factors across time cohorts.

Conclusion

• Heritability of major depression is higher in women and possibly sex-specific.
• Confirms reliability of European twin studies on heritability of depression.

Evaluation

• Difficult to isolate specific genes due to the interaction of multiple genes and environmental factors.
• Evidence from other studies (e.g., Caspi et al) reinforces strong correlation between genetics and behavior.
• Other factors like trauma and experiences also play roles in behavior, showing bias in deterministic approaches.

LAQ Plan

Theory

• Define DNA and Genetics.
• Discuss influences of genes on behavior and the diathesis-stress model.

Topic Sentences

1. Twin studies can help test genetic influences on behavior due to shared genes.
   • Reference: Kendler et al (2015).
2. Genes affect behavior in response to environmental triggers.
   • Reference: Caspi et al (2003).

Conclusion

• Synthesize understanding of genetic influences on behavior within environmental contexts.