Well, it has happened again. Someone has confused the concept of heritability. A recent Slate article discusses the difficulties of twin studies but screws up the concept of heritability in the first paragraph, saying that genes determine 50% of the likelihood that you will vote among other things. I’ll leave others to critique the rest of the paper and I’ll focus on trying to explain heritability.
Heritability is a measure of how much variation of a trait within a population is due to genes compared to variation due to environment. It is a POPULATION measure of variation and not a simple breakdown of determinism. It is a difficult concept so perhaps some details and examples will help.
Heritability is an abstract concept with a resulting number between 0 and 1. If a trait has a heritability of 1, it is purely due to genetic factors whereas 0 means that it is purely environmental. Most traits are somewhere between 0.2 and 0.8. If a trait has high heritability (closer to 1), it generally means that genetic factors strongly influence the amount of variation. Remember this is a measure of variation.
The heritability measurement depends on the population being observed or measured. Let’s look at height to start because we can think of examples of genetic factors and environmental factors that play a role. Scandanavians are often taller than Japanese people, bringing in genetic differences. Nutrition during childhood can impact height, bringing in environmental factors.
In a multicultural city like Vancouver, the variation in height across the population may be more influenced by genetic factors to the heritability would be high. If you had the same variation in a population with less genetic variation, the variation may be more impacted by diet or other environmental factors, giving a lower heritability.
In other words:
- varying genotypes may explain more of the variation than environment (high heritability)
- there may be more environmental influences impacting the variation (low heritability)
I hope the above makes sense – please leave a comment below if you have a better way of saying this or have a question.
Misconception: People twist a heritability of 75% (3/4 of the variation in a population is due to genes) to say that 75% of a trait is due to genes. This is not the same thing!
In the height example, if we say that height has 75% variability*, it means that 75% of the variation – from the shortest to the tallest – is due to genes. This is easier to take when we are talking about height but you can see the misconception better with intelligence. It is often misconstrued that X% of our intelligence is due to genes when it is saying that X% of the variation in intelligence across a population is due to genetics.
More advanced: you can apply heritability to an individual but not in the way that is often done. Going back to height…if we say that the average female is 5’6″* and that height has a heritability of 75%*, we can start to explain my deviation from the average. I’m only 5’2″ so we can say that 3″ of my height variation may be genetic factors – we don’t say that 75% of my total height is due to genetics.
So, are you confused yet? I’ve written the above as a start. I would appreciate comments on making it better – this is such a hard concept to explain in genetic classes.
I’m trying not to go on a rant about those who use heritability to suggest that one race is superior to another… maybe another time.
*I’m making up numbers for ease of calculation – these are not true averages or measures of heritability.