Heritability Introduction Take a trait like extraversion Think

Heritability: Introduction

Take a trait like extraversion. Think of all the people you know, including characters you have read about or seen in the media, and imagine all the observed (phenotypic) individual differences in extraversion among them. The heritability of extraversion is the extent to which all of these observed individual differences in extraversion can be traced back to individual differences in genetics.

Definition: Heritability is The extent to which phenotypic variance is predicted by genetic variance. Because “variance” is a statistical term for “individual differences, ” the definition can be reworked as: The extent to which phenotypic individual differences are predicted by genetic individual differences.

• Heritability is often denoted as h 2 • Heritability is a number between 0 (genes to not influence individual differences) to 1. 0 (all individual differences are due to genetics) • Monomorphic traits (where everybody is the same, like number of heads) have a heritability of 0 (because there are no individual differences) but can still be influenced by genes.

Definition: Environmentability is The extent to which phenotypic variance is predicted by environmental variance. Because “variance” is a statistical term for “individual differences, ” the definition can be reworked as: The extent to which phenotypic individual differences are predicted by environmental individual differences. Usually denoted by e 2

Environmentability is subdivided into two quantities: 1. Common environmentability = c 2 (AKA shared or family environmentability). The extent to which environmental individual differences make relatives similar. 2. Unique environmentability = u 2. The extent to which environmental individual differences make relatives different.

Basic Quantities h 2 + e 2 = 1 c 2 + u 2 = e 2 c 2 =. 15 h 2 =. 40 e 2 = 1 – h 2 =. 60 u 2 =. 45

Heritability is: 1. An abstract statistical concept—it tells us nothing about the genes that contribute to the trait. 2. A population statistic. It measures the average effect of genes across a population and does not apply to an individual. E. g. , an h 2 of. 80 for height does not mean that 80% of a person’s height is due to genes. 3. NOT an invariant biological property. It can change over culture, history, and the lifespan, and depends on the range of environments (see next slides). 4. No reason for therapeutic nihilism. Even traits where h 2 = 1. 0 can be changed by the environment (e. g. , PKU)

Heritability is NOT an invariant biological property: Examples • h 2 can change over the lifespan—e. g. , for intelligence and antisocial behavior, it is higher in adulthood than in childhood • h 2 can change over culture—e. g. , ALDH-2 deficiency in immigrant adults versus their children • h 2 can change over the time—e. g. , reading ability in feudal society versus today

Statistically, h 2 is the square of the correlation between hypothetical genetic values G E 1 h 1 e P 1 Theoretically, if GWAS could uncover all of the genes contributing to a trait, h 2 would be the square of the correlation between PRS (or GPS) and phenotypic scores

Estimating h 2: Two Generic Methods: (1) Genetic Epidemiology • Unmeasured genotypes • Use correlations between informative relatives e. g. , twins, adoptees (2) Molecular approach • Measured genotypes (almost always SNPs) • Several approaches NOTE: the actual techniques are advanced, see slides and text for grad students if interested