# eli5 Heritability of traits

56 views
0

I’m really having trouble wrapping my mind around the concept of heritability.
These numbers are obviously wrong, but let’s just use them for convenience:
Let’s pretend that scientists say that 80% of height is due to my genetics. If I’m 100cm tall, I understand that the scientist is NOT saying that 80 of my cm is due to my genetics.
I also understand that it doesn’t mean that if there are 100 people in my population, it doesn’t mean that 80% of them are only as tall as they are because of their genetics.
Does it mean that 80% of the reason I am as tall as I am is because of my genetics?

I started wondering about this when someone mentioned something to the effect of, “If your dad was a drinker, it runs in the family.” and also things like, “you’ll probably go bald because your mom did”. I understand that heritability is a term used for population studies, but I’m not sure why it doesn’t apply to individuals as well.

And if you read this and think “hoooo boy, OP’s gotta go ALLL the way back to square 1”, I’m more than happy to click a link and read!

Thanks a lot for your help!

In: 2

> Let’s pretend that scientists say that 80% of height is due to my genetics. If I’m 100cm tall, I understand that the scientist is NOT saying that 80 of my cm is due to my genetics.

> I also understand that it doesn’t mean that if there are 100 people in my population, it doesn’t mean that 80% of them are only as tall as they are because of their genetics.

> Does it mean that 80% of the reason I am as tall as I am is because of my genetics?

Frame it this way: Everyone falls somewhere on the distribution of heights that humans can have. What determines where any given person falls on this spectrum? Well, there are lots of factors such as diet (i.e., were you malnourished). In a statistical model you can isolate the impact of each of these various factors and calculate what percentage of the variation across people can be attributed to each. The hypothetical scientists are stating that 80% of the variation in height comes from differences in genetics between people.

> I understand that heritability is a term used for population studies, but I’m not sure why it doesn’t apply to individuals as well.

Yeah, these types of percentages, as outlined above, are about describing populations, not individuals. Of course there are ways to look at individual’s risk of various conditions and what not due to genetics quantitatively, but this usually involves less complicated traits than height.

Usually the numbers you see are percent of the variance in the trait (in your case, height) explained by the gene. In more technical terms, it’s the [coefficient of determination](https://en.wikipedia.org/wiki/Coefficient_of_determination), which you’ve probably seen as r^2 in a basic statistics class but has more general applications.

Given a collection of things, the variance (which is the square of the more familiar standard deviation) describes how spread out that data is. Higher variance means more spread out, lower variance means less spread out. Height, for example, might have a variance of (say) 100 cm^2 (so a standard deviation of 10 cm), though that’s probably low for real data.

If you have some prediction, like a best-fit line, you can instead measure how spread out the real values are *from your predicted values* instead of from one another. This value measures how much your model *isn’t* explaining. This results in a lower number than the original variance. Say it’s 40 cm^(2) in a model predicting height. Then you’d say that 40% of the variance is *not* explained, i.e., 60% of it *is* explained.

(The reason this value is always lower than the original variance is that the original variance is basically just this with a model of “assume everyone’s average”, and best-fit models never do any worse than that.)

Some of our physical features are determined directly by our genes. Eye color, for example, is directly coded in our DNA, which instructs cells how to come together and make proteins and pigment and other necessary building blocks. This DNA is a combination of our parents’, and so the genes that determined their eye color influence ours as well.

However, some traits are a mix of genetics and outside influences. This includes height, weight, and likelihood of getting various cancers, among other things. While our DNA has a large part in dictating these features, they are also affected by our lifestyle and environment.

To go back to your requested subject of height- if you give a child the bare minimum of calories and nutrients necessary to survive for their formative years, it is unlikely that they will grow to be very tall. Their body didn’t have the resources needed to fund more growth, with all of the energy being directed to keeping that child alive. Some medications slow growth, and some environmental chemicals will affect this as well. The amount of exercise the child gets will also affect muscle and skeletal growth.

However, (using your numbers) if 80% of height is determined by genetics, that means that if you compare the heights of many different people, about 80% of the variation in their heights can be predicted by their genetics. Many different genes (coded by inherited DNA) combine to give you a rough estimate of the height that their body can achieve, while about 20% of that variation can be predicted by their diet, exercise, and environment. The genetic effects are weighted much more heavily in determining height than environmental and lifestyle effects.

It depends on how they chose to quantify it exactly, but likely what it means is that 80% of the variability in height across individuals is determined by genetics. The rest then would be determined by environmental factors like diet.

To simplify, suppose that all humans were between 170-180 cm tall, and all heights within this range were equally common. If height were 100% genetics, that would mean that (in theory) you could tell exactly where in that range an individual would fall just by looking at their DNA. “Oh, your DNA in this particular gene goes AACATTTAAGGGTCCAA? Well then that means you must be 177.45 cm tall.”

If 80% of height variability can be explained by genetics, that instead would mean (in this simplified example) that your genes only determine your height to within 2 cm (eliminating the other 80% of the range of possibilities). So e.g. by looking at your DNA, scientists might be able to determine that you are between 172.2-174.2 cm tall. The remaining 2 cm (20%) of variability then is determined by environmental factors.

Real-life variability is more complicated than this, but that’s the basic idea.

>I understand that heritability is a term used for population studies, but I’m not sure why it doesn’t apply to individuals as well.

Well, it *does* apply to individuals.

/u/lollersauce914 has a solid explanation on the nature of your 80% figure: namely that once you separate and control for different sources of variation, genotype hypothetically accounts for 80% of the total variation.

This is something that you can only realistically figure out from large population studies, but the biological mechanisms are very much at play in every single individual. Height is complicated because it’s influenced by many genes and environmental factors, but with sufficiently detailed understanding of their interplay, you totally could take that general knowledge and extrapolate back to individual cases.

This is the basis of much of modern life sciences & healthcare; figuring out exactly how biological systems work (we call that “mechanisms”) and then engineering precise interventions to steer things in a more desireable direction.