Notes on Behavioral Genetics

Adoption Studies: Compare adopted children to both adoptive and biological parents to disentangle genetic vs. environmental influences. These studies are particularly valuable in separating the impact of shared genes from shared environments. By examining traits and behaviors in adopted individuals, researchers can infer the relative importance of nature versus nurture.
- If adopted children resemble adoptive parents more, environment (e.g., parenting style, socioeconomic status) has a stronger influence. This suggests that the environment plays a critical role in shaping the observed traits.
- If adopted children resemble biological parents more, genetics has a stronger influence, indicating a significant genetic contribution to the trait in question.
Twin Studies: Compare identical (monozygotic) twins (100% shared DNA) with fraternal (dizygotic) twins (approx. 50% shared DNA). Twin studies are premised on the understanding that if a trait is primarily influenced by genetics, identical twins should exhibit a higher degree of similarity compared to fraternal twins.
- Higher concordance rates (likelihood both twins share a trait) in identical twins suggest a genetic component. Concordance rates are calculated to determine the extent to which twins share specific traits or conditions. Significant differences in concordance rates between monozygotic and dizygotic twins provide evidence for genetic influence.

Genes and environment interact and combine to influence behavior; it's not just nature versus nurture. This perspective recognizes that genes provide a blueprint, but the environment determines how that blueprint is expressed. The interplay between genes and environment is dynamic and complex.
Example: Genetic predisposition for insulin resistance + carb-rich diet = weight gain. Individuals with a genetic predisposition for insulin resistance may only develop type 2 diabetes if they also adopt a diet high in carbohydrates. This illustrates how environmental factors can trigger or exacerbate genetic predispositions.
Caspi et al. study: Genetic risk factor for depression (variant of gene affecting serotonin) only increased risk when paired with negative life experiences. This study highlighted the role of the 5-HTT gene, which affects serotonin transport. Individuals with a short allele of this gene were more likely to develop depression if they experienced stressful life events, showcasing a gene-environment interaction.

Environments can alter gene expression (epigenome) without changing the genes themselves. Epigenetic changes can influence how genes are read and expressed without altering the underlying DNA sequence. These changes are often reversible and can be influenced by various environmental factors.
Identical twins can diverge due to different life experiences altering their epigenome. Although identical twins share the same DNA, differences in their life experiences can lead to variations in their epigenetic profiles. These epigenetic differences can result in phenotypic differences between the twins over time.
Example: Gene expression of astronaut Scott Kelly differed from his identical twin Mark after a year in space. This study demonstrated that spaceflight could induce epigenetic changes, affecting gene expression. These changes were observed in Scott Kelly compared to his earthbound twin Mark, providing insights into the impact of extreme environments on the epigenome.
Environmental inputs (e.g., diet, stress) can change the epigenome. Factors such as diet, exposure to toxins, and chronic stress can lead to epigenetic modifications. These modifications can affect gene expression and influence various health outcomes, demonstrating the profound impact of the environment on our biology.