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Lecture 6

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Department
Psychology
Course
PSYC 2P25
Professor
Michael Ashton
Semester
Fall

Description
PSYC 2P25 Oct. 22, 2012 Heredity and Environment  What proportion of the variation in a personality trait is due to inherited (genetic) variation? Environmental variation) o Why are people different in their personality? [variation]  Researchers want to find the heritability (h ) of a trait – the proportion of the trait variation that is due to heritable (genetic) variation – in a give population  Of course, 0 ≤ h ≤1  To find the heritability of a personality trait, we need to know whether (genetic) relatives have similar trait levels beyond any similarity due to their common environments 2 Ways to estimate heritability (h )  1) study many set of identical (monozygotic; MZ) twins raised apart o MZ twins share 100% of genes. If they’re raised apart, then similarity between them can’t be due to being raised together o Measure trait, find correlation (r) between twins’ levels (if twin A is “high” in trait, is twin B also?) 2 o For MZ twins raised ap2rt, r indications heritability (h ):  E.g., r= .60 -> h = .60  2) study many sets of fraternal (dizygotic; DZ) twins (or even non-twin siblings) raised apart o DZ twins share 50% of genes (same as for non-twin siblings). If they’re raised apart, then similarity between them can’t be due to being raised together o Measure trait, find r between twins’ levels, then double r to get h2 (need to double r because it represents the effect of our 50% genetic similarity) 2  E.g. r= 30 -> h = 2(.30) = .60  3) study many sets of MZ twins raised together AND may sets of DZ twins raised together, and compare them o Measure trait, find r between MZ twins’ levels, and r between DZ twins’ levels o Find difference between the r’s, then double it (difference in rs is due to the extra 50% genetic similarity of MZ twins; need to double it to get the full effect of heredity) 2  E.g., MZ r= .60, DZ r= .30 -> h = 2 (.60 - .30) = 2 (.30) = .60 o This method of comparing similarity of MZ twins with similarity of DZ twins is the most common way of estimating trait heritability o Note: that to find h2, you can’t just take the r between MZ twins raised together (or 2r between DZ twins raised together) because r might be due to [ADD FROM NOTES]  In example given above, the r for MZ twins together r(.60) was exactly twice as big as the r [ADD FROM NOTES]  First, it’s possible that the r for MZ twins together could be more than twice as big as the r for DZ twins together  E.g. MZ r= .70 and DZ r= .30 PSYC 2P25 Oct. 22, 2012  When this happens, it suggests that some of the heritable (genetic) variation is non-additive  Non-additive genetic influences occur when levels of a trait depend on combinations or “interactions” between genes, not just on effects of individual genes adding up  Examples: dominance/recessivesness (genes at same location -> effect of Aa is the same as that of AA, NOT halfway between AA and aa)  Epistasis (genes at different locations -> effect of Ab and aB is the same that of AB, NOT hafway between ab and AB)  Non-additive effects make relatives different from each other, except for MX twins  MZ twins have 100% of genes (and 100% of gene combinations) in common  DZ twins and siblings have 50% of genes in common, but LESS THAN 50% of combinations of genes in common  Therefore, when non-additive genetic effects are strong, MZ twins are MORE than twice as similar as DZ twins  Also, it’s possible that the r for MZ twins together could be less than twice as the r for DZ twins together 2 2  E.g. MZ r= .80 and DZ= .50; this h =gives h = 2 (.80 -.50) = .60  When this happens, it suggests that the similarity of both MZ twins and DZ twins is partly due to the common (shared) environment -> being raised in same household  Notice: that in the example above, both kinds of twins are more similar than h alone would suggest [ADD FROM NOTES] Note about womb environment effects 2  In (1) and (2), h might be overestimated, if there are womb environment effects  All siblings (including twins) develop in the womb of the same mother -> this might make them more similar to each other  And all twins develop in the same mother’s womb during the same pregnancy -> this might make them more similar to each other Common vs Unique Environment  Common environment -> shared, between family  Influences that make siblings in same family similar to each other but different from other
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