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Lecture 11

Lecture 11-12 (Genetics).docx

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Department
Psychology
Course
PSY230H5
Professor
Ulrich Schimmack
Semester
Fall

Description
Genetic and Environmental Influences Genetic and Environmental influences on personality  Humans have different personalities according to personality measures and cross-situational consistency studies  Stability in personality over time (longitudinal studies)  Environmental factors can influence variation in personality o i.e. parents may socialize their kids differently  Nurture- hypothesis that parents influence their children’s personality o Alternative- genes contribute to personality o Nature- personality traits and behaviour vary across humans could be influenced by genetic variation across individuals  Behavioural genetics- scientific discipline that examines influence of environment and genes on behavoiur o Most of the existing knowledge about causes of personality comes from behavioural genetics studies  Possible to study genetic influence son personality directly by getting DNA samples from people and examining how variation in DNA across individuals related to variation in personality traits, behaviour, and life outcomes  Molecular genetics- modern approach of studying personality on basis of actual DNA samples  Most important evidence comes from studies that looked at influence of genes and environment indirectly by comparing individuals who vary in their genetic relatedness (i.e. twin studies) o Twin studies show that genetics have influence on personality Behavioural Genetics- The Indirect Approach  Phenotype- variation in specific observable characteristic (i.e. eye colour)  Genotype- information in an individual’s DNA  Alleles/polymorphisms- different versions of genes o Alleles- variations of functionally identical genetic information at same location on a chromosome (i.e. brown, green, or blue eyes)  Environment- environment factors that vary across individuals  Population- well-defined group of individuals o Twin studies and related studies in behavioural genetics try to answer the question of how much the variation in a phenotype in a specific population is due to variation in genotype and how much is due to variation in environmental factors  Neither the genotype nor environment are directly measured; unobserved causal factors that contribute to phenotype  **causal effect of genotype on phenotype is called heritability (h)  **called heritability coefficient because we know people inherit their genes from biological parents (square a coeffeicent for causal effect to get amount of explained variance in dependent variable)  thus, h squared provides information about amount of variance in phenotype that is explained by variation in genes that contribute to this phenotype  **square root of heritability effect (h squared) called heritability coefficient  0% heritability coefficient- environmental factors, 100%- genetic variation Twin Studies  examining how similar twins are in phenotypes  Monozygotic twins (MZ) result from split of same fertilized egg during early stages of development of fetus; two fetuses develop that are genetically identical  Dizygotic twins (DZ) result of two eggs in uterus that are fertilized by two different sperms; thus, the developing fetuses share only 50% of their genes o Not different from normal siblings genetically; born at same time and raised under more equal environmental conditions that siblings  Biggest issue in twin studies is recruiting large number of MZ and DZ twins  The closer the correlation is to 1, the more similar twins are in their phenotype Equal Environmental Assumption  Assumption that MZ and DZ twins differ only in their genetic similarity but not in similarity of environmental factors that cause variation in phenotype  May be false Assortative Mating  Existence of random mating for trait being studied  Spouses don’t choose each other based on similar characteristics  Assortative mating- people marry people who are similar to them  Strong Assortative mating leads to false conclusions about heritability if standard assumption of 50% genetic similarity for DZ twins is made  Very little Assortative mating for the Big Five personality traits o Exception- openness to experience (moderate degree of assortment)  Spouses usually have similar IQ levels but correlations tend to be small (.20 to .40) o Correlations for physical attractiveness tend to be higher o Strongest correlations found for conservatism and religiousness  Can be tested easily by examining correlation between spouses’ personality traits Estimating Heritability from Twin Data  MZ twins- identical fingerprints- suggests strong heritability of fingerprints  Additive genetic influences- implies that similarity in phenotypes increase in proportion to similarity in genotype  Proportion of genetic similarity between MZ to genetic similarity of DZ twins (2:1), additive genetic assumption makes prediction that the same proportion also is found for similarity in phenotype o Therefore, if MZ twins are twice as similar as DZ twins genetically, they should also be twice as similar in h2 = 2 x (MZr – DZr)
 [MZ r = correlation between MZ twins, DZ r = correlation bet
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