A very user friendly introduction to heritability

Heritability is an important and often misunderstood concept (assuming you’ve even heard the term before). It is one of the fundamental concepts, if not the fundamental concept, in quantitative genetics.

 

The technical definition of heritability is this:

 

“It is the proportion of phenotypic variation attributable to genetic effects.”

 

Sound simple enough?

 

If you’ve never done any genetics, or statistics, then this definition is probably pretty meaningless to you. In this article I will be unpacking this definition and explaining some of the finer points, and misunderstandings surrounding heritability.

 

So everybody is different right?

 

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Very different.

 

The population of earth, and various sub-populations of different types all have some degree of variation within them. One popular and simple example is human height. Everybody is a different height.

 

What makes us differ in height from one person to the next?

 

Intuitively most of us would say our genes. Slight differences in the genetic makeup of different people, influence slight differences in how tall or short we all are. So we would say that variation in our genome, contributes to variation in our height.

 

Genomic variation is due to genetic mutations. We all have numerous mutations in our genome that make us all slightly genetically different. These mutations affect innumerable different aspects of our biology, including the genome itself. This leads to phenotypic differences between individuals.

 

Your phenotype is literally what makes you, you. Your phenotype is the sum of your unique traits. Height is a trait.

 

You may or may not be surprised to learn that your ‘phenotype’ is not just influenced by genetics, but by your environment also.

 

Take our body mass index (BMI) as another example. Most of us would agree that, to a reasonable extent, our BMI is influenced by diet and exercise. Differences in individual people’s diet and exercise contribute to environmental variation in a given population. Our body mass is also influenced by our genetic differences, our genetic variation. These differences, both genetic and environmental, contribute to the differences in BMI between individual people.

 

For the purposes of statistics absolutely anything that influences our phenotype, that is not tractable to our genome, is considered environmental variation. This includes experimental error. Our environment includes things like our friends, home/school/work environment, sleep cycles, the actual environment; anything. Scientists don’t like to rule anything out.

 

Even our height is influenced, to some extent by our environment.

 

In fact most traits that makes you who you are, are going to be influenced to some extent, by your genome, and to some extent by your environment. The contribution of either the environment or our genome will vary from trait to trait, and even from population to population.

 

Estimating how much of the variation between individuals is due to their genetic differences, and how much is due to environmental differences is a central question of quantitative genetics.

 

In other words, what is the heritability of a given trait? That is, to what extent is the phenotypic variation (the differences within a population) influenced by our genetic differences?

 

Understanding heritability as a proportion

 

A common misunderstanding regarding heritability is that it represents a proportion of variation. This has important implications.

 

First of all, we need to understand that heritability is a measure of the genetic influence on the variation of a particular trait. It is not a measure of how much genetics influences a given trait in one individual. It requires a population of individuals. This also means that it is population specific. The heritability of a given trait in one population will not necessarily be the same in another population.

 

Take body mass again. Suppose you took 30 people, and raised them all on a desert island, and they all consumed exactly the same food, in the same amounts, at the same time. Suppose they all got exactly the same amount of sleep and at the same time. Suppose everything about their lifestyle was identical, or almost identical. Then how much variation would there be in their environment?

 

None right? For all of them, their environment is virtually the same. If there is no difference, then by definition, there is no variation.

 

Suppose further that even though their environment was completely identical, there were still sight differences in their overall body mass. Then you could reasonably conclude that whatever that difference was, no matter how small, can be attributed to their genetic differences.

 

Thus the heritability would be 100% (or very close to it) for that population. Remember though that if we tried to measure it, then it would come out to less than 100% because of experimental error.

 

So you see, heritability can vary depending on the environment a given population is in, and of course, the genetic variation also.

 

We can also see from this that even if the total amount of variation is small, the heritability can still, in theory, be anywhere from 0 – 100%.

 

Because heritability is a proportion, it can be influenced by changing the genetic effects or by changing the environmental effects, or when the phenotypic variation in the population changes.

 

Heritability bars

 

Take a look at the extremely crude diagram above that I made all by myself. You can see that by decreasing the environmental variation, the heritability will increase. Note however, that just because the heritability has increased doesn’t mean the total amount of genetic variation has necessarily changed.

 

Heritability requires variation

 

Hopefully you can also see that because it is a measure of variation, then variation must exist in order to measure heritability.

 

I explained that if there are no environmental differences, then there is no environmental variation. The same goes for the trait itself.

 

The implication of this is that if there is no variation in a trait, then it cannot be ‘heritable’. Take noses for example. Virtually everyone on earth has one nose. No one has two noses. I suppose there would be an extremely rare few who have ‘no’ nose (maybe they lost it in an accident or something more sinister). For all practical purposes however everyone has one nose.

 

Now, if you were to ‘measure’ the trait ‘nose number’ for a human population, you would find there is no variation. Everyone has one nose. This means that if you tried to estimate the heritability for ‘nose number’ it would be zero. If there is no variation in the trait, then there is no heritability.

 

And this makes sense right? There might be enormous genetic variation in a population, but everyone has one nose, and so all that genetic variation contributes nothing to ‘number of noses per person’ in a given population.

 

So even though it is the information in our genome that gives rise to having a nose, the number of noses any one individual has is not heritable, because all the individual genomes give rise to only one nose per person.

 

If however, we wanted to look at nose length, or shape, colour, hair number, etc. Then we know that there would be variation, because whilst everyone has one nose, everyone’s nose looks different from everyone else’s.

 

So heritability is concerned only with traits that vary within a population.

 

Heritability does not measure the total genetic influence

 

This is where heritability gets a little more obscure, and more misunderstood.

 

Heritability cannot actually tell us the absolute amount that genetics influences a trait. Heritability is a measure of the genetic influence relative to the overall variation in the trait, and also the environment. A trait may only have a marginal genetic component, but if there is no environmental variation (as per our island example) then the heritability will still be very close to 100%. By contrast, even if a trait is very strongly influenced by our genetics, but there is an enormous environmental variation, or the influence of the environment is also great, then the heritability can still be quite low.

 

More Heritability bars

Looking at the diagram above again. The variation in the second trait is ‘half’ that of the first, but the heritability is still equal for both traits, because the genetic effects are still ~0.5 of the total, or about 50%.

 

Hopefully you can now see heritability is a highly population dependant estimate. If you change the variation in the genotype (or if you change the genetic effects), the phenotype, or the environment, you change everything.

 

Why is heritability important?

 

So you might be wondering if it’s so sensitive, then why is it so important and useful?

 

One obvious answer is that understanding the relative proportion of genetic effects on a trait has implications for broader research questions, including where to direct funding.

 

From a clinical point of view, if a particular trait has a low heritability, then it doesn’t make sense to spend excessive amounts of money on genetic research for that trait, it might be better to invest in understanding the environmental component better. Of course scientists and other curious minded people would be happy to do it out of pure interest, but from a health and funding perspective, when it comes to spending millions of dollars, it needs to offer a reasonable return on investment. The reverse is also true as well, if a particular disease has a highly genetic influence, then it makes sense to invest heavily in understanding the ‘genetic architecture’ of that trait (diseases are traits too).

 

Heritability is also important for natural selection, and for similar reasons, selective breeding. The more heritable a trait is, the more potential there is for selective breeding, or for natural selection to influence variation within a population. If a trait is not heritable, then it cannot be influenced by natural or artificial selection.

 

So heritability is a highly population specific estimate of the proportion of phenotypic variation attributable to the genetic effects.

 

The next question is how do we estimate the heritability of different traits?

 

That is the topic of a future post.

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