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Lecture 6

PSYC 2P25 Lecture Notes - Lecture 6: Heritability, Trait Theory, Epistasis

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Michael Ashton

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Lecture 5- Heredity and Environment
What proportion of the variation in a personality trait, among people in a given population, is
due to:
o Inherited (genetic) variation
o Environmental variation
Researchers want to find the heritability (h2) of a trait the proportion of trait variation that is
due to heritable (genetic) variation in a given population
Of course, 0 </= h2 </= 1 (h2 is a proportion)
To find the h2 of a trait of a personality trait, we need to know whether (genetic) relatives have
similar trait levels beyond any similarity due to their common environments
Ways to estimate heritability (h2) …
1- study many sets of identical (MZ) twins raised apart
MZ twins share 100% of genes. If they’re raised apart, then similarity between them cant be due
to being raised together
Measure trait, and find correlation ( r ) between twins levels (if twin A is “high” in trait, is twin B
For MZ twins raised apart, r indicated heritability: e.g. r = .60 h2 = .60
2- Study many sets of fraternal (dizygotic; DZ) twins (or even non-twin siblings) raised apart
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
Measure trait, find r between twins levels, then double r to get h2 (need to double r because it
represents the effect of only 50% genetic similarity)
o E.g. r=.30 h2 = 2(.30) = .60
Interlude: Womb environment
In (1) and (2), h2 might be overestimated, if there are womb environment effects
Siblings (including twins) develop in the womb of the same mother this might make them
more similar to each other
Twins develop in the same mother’s womb during the same pregnancy this might make them
more similar to each other
3- Study many sets of MZ twins raised together AND many sets of DZ twins raised together, and
compare them
Measure trait, and find r between MZ twins levels, and r between DZ twins levels
Find difference between rs, 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
(note that any effects of being raised together should apply to both MZ twins and DZ twins, and
shouldn’t influence the difference in rs)
E.g. MZ r= .60, DZ r= .30 h2 = 2 (.60-) = (.30) = .60
This method of comparing similarity of MZ twins with similarity of DZ twins is the most common
way of estimating trait heritability
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