Nature vs Nurture: Genes and Behavior

• Nature and nurture both influence behavior, with current research focusing on their interaction.

What Are Genes?

• Genes are hereditary transmission units, initially referring to traits and later defined as DNA sections coding for proteins.
• Genes are organized into chromosomes, DNA strands in a double-helix.
• Messenger RNA (mRNA) communicates DNA code copies to cells, which affects traits.
• Humans have 23 chromosome pairs, inheriting one of each pair from each parent randomly.
• Sex determination in mammals depends on X and Y chromosomes; females have XX, males have XY, inheriting one X from the mother, with the sex being determined by the father.
• Humans share 99% of the same DNA, with variable portions influencing individual differences.
• Degree of relatedness is the probability of sharing genes.
• Monozygotic twins share 100% of their genes, developing from a single fertilized egg.
• Dizygotic twins share 50% of their genes, developing from separate eggs, like regular siblings.
• Comparing traits in monozygotic vs. dizygotic twins helps determine genetic influence, assuming a constant environment.
• Higher trait presence in monozygotic twins suggests genetic influence.
• Example: Schizophrenia likelihood is 27% for dizygotic twins, 50% for monozygotic twins, suggesting a high genetic influence, but also environmental influence.
• Genetics contributes to traits, but environmental context is important as genes are expressed within an environment.

A Role for Epigenetics

• Epigenetics studies environmental influences on gene expression without altering DNA sequences.
• DNA is analogous to a play script; the environment affects the expression without changing the underlying DNA.
• Epigenetic marks, chemical DNA modifications, can switch genes on or off.
• DNA methylation adds a methyl group to DNA, switching off the methylated gene.
• Histone modification adds chemical modifications to histone proteins, which package DNA, turning genes on or off.
• Experiences can create epigenetic differences in monozygotic twins, making them no longer identical as they age.

Relevance of Epigenetics

• Epigenetics is relevant to the brain, psychology, and everyday life.
• DNA methylation and histone modification play a role in learning and memory.
• Epigenetic changes are linked to stress responses.
• Holocaust survivors showed reduced DNA methylation of the FKBP5 gene in offspring, linked to stress response regulation.
• Epigenetics plays a key role in the long-lasting effects of early experiences.
• Rat studies showed that pups of high LG mothers were less fearful and had lower stress hormones and increased hippocampal serotonin.
• Increased serotonin from high LG mothers decreases DNA methylation of the glucocorticoid receptor gene.
• Low LG mothers showed increased DNA methylation of the glucocorticoid receptor gene, leading to an inability to respond calmly to stress.
• Studies show a role for epigenetics in the persisting effects of childhood abuse in adult men.
• Children in disadvantaged neighborhoods showed greater DNA methylation in genes linked to inflammation and smoking.
• Epigenetics is increasingly important for psychological disorders with early life stress as a risk factor.

The Role of Environmental Factors

• Genes set the range of possibilities, but individual characteristics are determined by environmental factors and experience.
• Heritability measures behavioral trait variability among individuals accounted for by genetic factors, ranging from 0 to 1.00.
• Heritability of 0 means genes do not contribute to individual differences, and 1.00 means genes are the only reason for individual differences.
• Most human behavior has moderate heritability, between 0.30 and 0.60, indicating both genetic and environmental contributions.
• Heritability of 0.50 for intelligence means half of the variability is due to genetics, and half is due to environmental influences.

Important Points about Heritability

• Heritability is an abstract concept and tells nothing about specific genes.
• Heritability is a population concept and tells nothing about an individual.
• Heritability is dependent on the environment, and is meaningful only for the environmental conditions in which it was computed.
• Heritability is not fate and tells nothing about the degree to which interventions can change a behavioral trait.