# Heritability Coefficient

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

Heritability is a measure of how much of the variation you see in a certain trait can be attributed to genetic variation. In this lesson, we will discuss what heritability means and how it is calculated.

## What is Heritability?

Say you are the champion weightlifter at school. You take your date home to meet your parents (who are both almost as big and beefy as you are).

'Wow,' says your date. 'Are you so big because you work out, or did you get that from your parents?'

Intuitively, you already know the answer- both.

But suppose you and your date want to quantify your answer. That is when you might use heritability.

Heritability is how much of the variation seen in a certain trait within a population can be attributed to genetic variation, as opposed to environment. A high heritability means that for all the variation of a certain trait in the population, a large portion is caused by genetic differences. A low heritability means that for all the variation of this trait, more of it is caused by the environment. For example, a recent study showed that several major mental illnesses, including schizophrenia, bipolar disorder, and ADHD, were 17-28% heritable, meaning that genetic variation accounted for about 17-28% of the risk of illness.

Heritability is not the proportion of a trait that is caused by genetic factors. Heritability can change dependent on the environment- even heritability for a single trait within a species. For instance, in a society where most people have adequate medical care and enough to eat, you may expect to be tall if your parents were tall, and short if your parents were short. But if your parents experienced disease and famine as children and you did not, you might be tall even if your parents were short. When environmental variation increases, heritability decreases.

The geneticist Richard Lewontin illustrated this point with the following example. Imagine you plant two trays full of seeds. You put one tray in good lighting, with adequate water and fertilizer. You give the other tray poor soil with no fertilizer, barely enough water, and poor lighting. The heritability of traits you see within each tray is close to 100%; the differences you see between each individual plant within a tray are probably due to genetics. But when you compare the two trays, it is clear that environment played a large role in how your plants look.

## How to Calculate Heritability

Broad-sense heritability is represented as H2 and indicates all the genetic factors that go into phenotypic variance in a population. It can be calculated as:

H2 = Var(G) / Var(P)

where Var(G) is the variance in genotype and Var(P) is the variance in phenotype in a population. In other words, Var(G) is the genetic differences in your allele of interest (say, one that contributes to plant height) while Var(P) is the difference in phenotype (how tall a plant is).

Narrow-sense heritability is the variance due to the additive effect of alleles. Because you are diploid, you get one allele from each parent, and it is how those alleles interact- not what they do individually- that determines your phenotype. Narrow-sense heritability is represented as h2.

It can be calculated as

h2 = Var(A) / Var(P)

where Var(A) is the additive or average effects of the allele.

Why do we care about narrow-sense heritability? Natural (or artificial) selection can only act on phenotypes, not alleles. Often, we talk about narrow-sense heritability with respect to artificial selection- when humans select for desired traits. For instance, if I'm breeding cows to give more milk, I care more about how much milk my cows give than I care about the presence of a certain allele at a certain locus.

If milk production has a low heritability in my herd, my breeding program might not be very successful. There might just not be enough genetic variation for me to improve milk production in my herd, or it might be the case that there are other non-genetic factors that are causing low milk production. If milk production has high heritability, breeding may be able to solve my problem.

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