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

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 (h2) of a trait – the proportion of the trait variation that

is due to heritable (genetic) variation – in a give population

Of course, 0 ≤ h2 ≤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

Ways to estimate heritability (h2)

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?)

o For MZ twins raised apart, r indications heritability (h2):

E.g., r= .60 -> h2= .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)

E.g. r= 30 -> h2 = 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)

E.g., MZ r= .60, DZ r= .30 -> h2 = 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

## Document Summary

Researchers want to find the heritability (h2) of a trait the proportion of the trait variation that is due to heritable (genetic) variation in a give population. 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. 1) study many set of identical (monozygotic; mz) twins raised apart: mz twins share 100% of genes. 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). E. g. r= 30 -> h2 = 2(. 30) = . 60. 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.