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Lecture 4

PSYB30H3 Lecture 4: PSYB30 LEC 4

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Lisa Fiksenbaum

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PSYB30 LEC 4 - Genetics & Physiology January 25, 2017 The Human Genome ● Genome refers to the complete set of genes that an organism possesses ● Human genome contains 30,000—80,000 genes on 23 pairs of chromosomes ● Human Genome Project is designed to sequence the entire human genome—i.e., identify the particular sequence of DNA molecules in human species ● But identifying sequence of DNA molecules does not mean identifying the function of each molecule ● Most genes in a human genome are the same for all humans ● Small number of genes are different for different individuals, including genes that indirectly code for physical traits and for personality traits Controversy About Genes and Personality ● Behavioral geneticists attempt to determine the degree to which individual differences in personality (for example) are caused by genetic and environmental differences ● Highly controversial ○ Ideological concerns ○ Concerns about renewed interest in eugenics ● Modern behavioral geneticists who study personality are typically very careful about addressing implications of work and are sensitive to ideological concerns ● Knowledge is better than ignorance ● In addition, finding that a personality trait has a genetic component does not mean the environment is powerless to modify trait Goals of Behavioral Genetics ● Determine the percentage of individual differences in a trait that can be attributed to genetic differences and percentage that can be attributed to environmental differences ● Determine the ways in which genes and environment interact and correlate with each other to produce individual differences ● Determine precisely where in the "environment" environmental effects exist—e.g., parental socialization, different teachers to which children are exposed What Is Heritability? Overview ● Proportion of observed variance in group of individuals that can be explained or accounted for by genetic variance, OR ● Proportion of phenotypic variance that is attributable to genetic variance ● Environmentality = proportion of observed variance in group of individuals attributable to environmental variance Misconceptions About Heritability ● Heritability CANNOT be applied to single individual ● Heritability is NOT constant or immutable ● Heritability is NOT a precise statistic Nature-Nurture Debate Clarified ● No such debate at the individual level ● Influence of genes and of environment is only relevant for the discussion of group-level variation Behavioral Genetics Methods Selective Breeding—Studies of Humans' Best Friend ● Can only occur if a desired trait is heritable ● Selective breeding studies of dogs ● Cannot be ethically conducted with humans Family Studies ● Correlates the degree of genetic overlap among family members with the degree of similarity in personality trait ● If a trait is highly heritable, family members with greater genetic relatedness should be more similar to one another on the trait than family members who are less closely genetically related ● Problem: Members of a family who share the same genes also usually share the same environment— confounds genetic with environmental influences ● Thus, family studies are never definitive Twin Studies ● Estimates heritability by gauging whether identical (monozygotic or MZ) twins, who share 100 percent of genes, are more similar than fraternal (dizygotic or DZ) twins, who share only 50 percent of genes ● If MZ twins are more similar than DZ twins, this provides evidence of heritability ● Calculating heritability—many formulas, simple one: Two times difference between correlation ("r") for MZ twins and DZ twins, or 2 (rmz — rdz) ● Two assumptions of the twins method ○ Equal environments assumption ○ Representativeness assumption Adoption Studies ● Positive correlations on traits between adopted children and adoptive parents provide evidence of environmental influence ● Positive correlations between adopted children and genetic parents provide evidence of genetic influence ● Adoption studies are powerful because they get around the equal environments assumption—genetic and environmental causes are unconfounded ● Assumption that adopted children and their adoptive and genetic parents are representative of the general population—questionable ● Problem of selective placement of adopted children ● Design that combines strengths of twin and adoption studies = twins reared apart Major Findings from Behavioral Genetic Research Personality Traits ● Summaries of behavioral genetic data yield heritability estimates for major personality traits (extraversion, agreeableness, conscientiousness, neuroticism, openness to experience) of about 20-45 percent
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