2-4-Behavioral Genetics

Behavioral Genetics

Evidence of Genetic Influences on Human Behavior

  • Twin and Adoption Studies: These studies are crucial in exploring the effects of genetics and environment on behavior.

Genetic Relatedness

  • A crucial aspect of behavioral genetics is understanding genetic relatedness, which can be represented as follows:

    • Fathers: 50%

    • Brothers: 50%

    • Identical Twins (MZ twins): 100% genetic similarity

    • Sons: 50% genetic similarity

  • Additional relationships with their respective genetic similarities:

    • Grandfathers: 25%

    • Great-uncles: 12.5%

    • Uncles: 25%

    • First cousins: 12.5%

    • Half-brothers: 25%

    • Nephews: 25%

    • Sons: 50%

    • Great-nephews: 12.5%

    • Grandsons: 25%

    • Great-grandfathers: 12.5%

    • Great-grandsons: 12.5%

Inbreeding Considerations

  • How closely related must individuals be to be considered inbred?

    • 3rd cousins are generally considered "unrelated".

    • The common ancestor for this classification is typically a great-grandparent or a great-great-grandparent.

    • Genetic relatedness for various degrees:

      • 1st cousin ½ removed: 6.25%

      • 2nd cousin ½ removed: 1.56%

Schizophrenia and Genetic Relatedness

  • Risk for schizophrenia increases with genetic relatedness.

    • The relationship between genetic relatedness and schizophrenia risk can be summarized as follows:

      • Increased risk correlating with increased genetic similarity.

      • Despite the strong link, the risk is less than 100%.

Twin Studies

  • Types of Twins:

    • Monozygotic (MZ) twins are genetically identical at all loci.

    • Dizygotic (DZ) twins share, on average, 50% of their genetic loci.

  • Environmental Effects:

    • Differences between MZ twins can indicate environmental impacts.

    • If genetics plays a significant role, MZ twins should show more similar traits than DZ twins.

    • This comparison presupposes that both MZ and DZ twins are exposed to the same environmental conditions.

Equal Environments Assumption

  • The assumption that MZ and DZ twins brought up in the same family experience the same environmental variances.

    • There is a caveat: MZ twins may be treated more similarly than DZ twins.

    • MZ twins sharing the same amniotic sac (being in the same intrauterine environment) might experience different effects compared to MZ twins who have separate sacs.

Role of Genes and Environment in Schizophrenia

  • Do genes play a role? Yes.

    • Increased risk of schizophrenia is associated with greater genetic similarity.

  • Does the environment play a role? Yes.

    • The environmental impact explains why, despite 100% genetic relatedness, the risk does not reach 100% due to non-shared environmental influences.

Key Takeaway

  • Understanding genetic and environmental origins through twin and adoption studies is essential in behavioral genetics.