Wednesday, 4 September 2013

Due to genetics, or, simplicity and determinism

I typed "due to genetics" (complete with the quotation marks) into google news, and this is what I learned: 

Due to genetics, you might have a higher risk of heart attacks.

Due to genetics, you can get dark circles under your eyes.

"Happiness is 50% due to genetics"

Due to genetics, some athletes are simply better at doping without getting caught. (This article was amusing for all the pictures of athletes who did get caught doping, as if somehow this proves their point)

Your enlarged pores may be due to genetics.

Other articles that came up on this search discussed autism, breastfeeding, stretch marks, and Larry Summers' views on women in science (if you're not familiar with them and you just have too much good feeling for humanity, google it). Clearly a lot of our health, in every sense of the term, is presented as being "due to genetics".

Genetics is certainly important, and an understanding of it should be a priority for science educators. It's one of the areas of science that directly impacts people's understanding of themselves and their health. I worry, though, that the dominant perception of genetics is one that is far simpler and more deterministic than the reality. To quote the heart attack risk article linked to above, "When it comes to certain diseases, preventing the onset might be almost impossible. Due to genetics and other factors involved, certain people have a greater risk of developing certain health conditions." Genetics here is simple: genes=heart attack. It's also deterministic: preventing heart attack "might be almost impossible". Genetics, though, is neither simple nor deterministic.


First, simplicity: genetics is complicated. As genetics prof John H. McDonald points out, a large number of "canonical" examples from high school biology, from eye colour to earlobes to hitchhiker's thumbs are simply wrong. Two parents with blue eyes can, in fact, have a brown eyed child.

Why is genetics so complicated? It's a question that is still a subject of academic research. One answer, though, comes from looking at what DNA actually does in a cell. What DNA does is surprisingly simple: DNA codes for proteins. That's it. That's the only thing it does. Each three base pairs in your DNA code for one amino acid, and a string of amino acids forms a protein. Everything else that DNA is responsible for is the result of those proteins interacting with other parts of the cell, such as other proteins, lipids, DNA itself, etc.

In order to go from a relatively simple mechanism (DNA-->proteins) to extremely complicated outcomes (the huge variety of cells in your body, which work together to form your nervous system, your immune system, and you) requires a lot of feedback loops. What I mean by that is that DNA codes for proteins, which then interact with the DNA to affect the way it codes for other proteins.

The interaction between your DNA and your environment, in the broad sense, is composed of many, many of these feedback loops. DNA interacts with proteins inside a cell, which interact with proteins embedded in the cell wall, which interact with proteins outside the cell (and also with the lipids that make up the cell wall), which interact with proteins on other cell walls, in a chain of microscopic links that stretches from deep inside you to the surface of your lungs, skin, or eyes. Is it any wonder this chain of interactions is hard to understand in simple terms?

What these complicated interactions mean, among other things, is that you can't in general apply average results to an individual. So if gene X causes people on average to be 50% more likely to die of a heart attack than people without the gene, that won't hold true across all sub-groups. More concretely, a vegetarian who is also an avid jogger with gene X isn't necessarily 50% more likely to die of a heart attack than a vegetarian jogger who lacks gene X. It could be larger or smaller; the 50% on its own doesn't tell us. The complicated interactions of DNA with the environment--keeping in mind that what you eat and what you do are part of "the environment"--means average results may not apply to a particular group, or worse: they may not apply to any group, and only be relevant when everyone is added together. Still useful, perhaps, but not to you as an individual.


Genetics is also less deterministic than it's made out to be. Part of this is because it's complicated. Another part is because you inherit more than DNA from your parents.

I don't just mean that metaphorically; I'm not saying here that you also pick up a variety of behaviours and ideas from them, though that's obviously true as well. No, I mean that you inherit, in a biological sense, more than just the information encoded in the base pairs of your DNA.

People have known that this is technically true for a long time. The building block of a human isn't just a piece of DNA; it's a cell, with a cell wall and organelles and structural elements and an environment. This much has been known for a while. What's changing is our appreciation for how that early environment can actually reverberate through a child's development to affect their life as an adult. (The grammarians out there should note that a child's development also effects the life of an adult.)

The interactions DNA has with the cell environment not only change the way it behaves in the short term, they can also change the DNA in ways that are passed on to daughter cells. DNA methylation is one such way. Here a particular piece of a molecule called a methyl group is attached to part of the DNA. Not only does it change the way the DNA codes for proteins, it's a modification that gets passed on when the cell divides--even, potentially, when the cells in the reproductive system divide, combine, and go on to form a new organism.

What this means is that environmental factors can alter genes for several generations before dying out. Since both the alteration and the time it takes for it to go away depends on the environment, trying to describe a deterministic role to genes is mistaken at best.

The field that looks at stuff is epigenetics. It's the field that studies the way the environment influences genetic expression, and it's currently a buzzword in science reporting. In many ways it is still a controversial field. What's clear, though, is that the environment does influence expression, even if we're still not sure exactly how it all pans out.

The amount of data available about both individual and group genomes is huge, and only growing larger. Soon people will be asked to make health decisions not only on current diagnosis but also on what their genes say about likely future scenarios. We need to keep this in mind, though: Genetics is complicated. It's not deterministic. Pretending it is only undermines our understanding and our health.

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