The Australian actor Chris Hemsworth recently took a genetic test that flagged him up as being at a higher risk of developing Alzheimer’s disease. We spoke to Sir Peter Donnelly, founder of CEO of Genomics PLC and professor of Statistical Science at the University of Oxford, about what genetic screening can tell us about our health – and what we can do to stay healthy regardless of our genes.

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What happens when someone has one of these tests?

Usually, they would involve taking a biological sample from the individual. Typically, either a blood sample, which is a bit more common, or a saliva sample. Then that would be sent to a laboratory which would take the DNA out of the sample and then analyse it for the piece of genetic information that the test is trying to find.

What is DNA?

DNA is the chemical material which contains all of the information our cells use to do their stuff - to make the proteins that make them function and to build up tissues and organs. We get one copy of our DNA from our mother and one from our father.

The totality of the DNA is called our genome - that's just a word for all of the DNA. The DNA itself is a long chemical made up of different components. You can think of it as a long list, and at each position there's one of four possibilities that happen to start with letters, A,G,C and T.

In total, we get 3 billion letters of DNA from our mother and 3 billion letters of DNA from our father. Each of us, in every cell in our body, we've got 6 billion letters.

What is a gene?

A gene is a piece of our DNA where the letters contain explicit instructions that help our cells make a protein. They can differ in length. A gene might be just a few hundred or thousand DNA letters long, but some genes are much longer than that.

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If we look at the totality of our DNA, only about 1 per cent of it is the genes. The rest used to be called junk DNA before we understood what it was doing. We now understand that it has other information. For example, information which tells a particular gene when it should be making a protein.

All of our cells have all of our genes. There might be some gene which is making a protein that is really important in the retina but you absolutely don't need that protein in your tongue. And so there are instructions in the DNA that will be able to tell that gene, I want you to make this protein if you're sitting in a retina, but the same protein in your tongue won't be turned on.

How big an influence can a single gene have on our bodies?

We've got about 20,000 genes in total. In Chris Hemsworth's case, he's been talking about one particular gene. We know some things about what that does, but there are many mysteries, as there are with lots of human biology. We all have two copies of that gene – one from our mother and one from our father. The issue is that there can be slight differences between the copies.

Lots of these differences don't make any difference at all to us, but some of them kind of have consequences. Sometimes those consequences can be really severe. In conditions like cystic fibrosis, for example, where if you inherit a mutated copy of a gene that doesn't work the way it's meant to, you can end up getting really sick. Sometimes, if you have one copy of a gene which doesn't work, you're fine. Because, the one from your mom doesn't work, but the one from your dad is fine.

Hemsworth has been told he's around 10 times more likely to develop Alzheimer's due to this genetic factor. How significant is that?

There are some diseases where if you inherit the genetic change, you will get sick. There are other examples, and this is one of those, where if you inherit a particular genetic change, you can be more likely, sometimes quite a bit more likely to develop the disease.

For most of the common conditions, like heart disease and diabetes and many of the common cancers, genetics is a big part of the risk. But it's not one change or two changes. It's millions of positions which each contribute a tiny bit to that risk. So his example is in the middle where he is probably about ten times more likely to develop disease. And although we're all very aware of diseases like Alzheimer's, it's quite rare.

There's a big difference between relative risk, which is how much more likely you are than someone else to get the disease, and absolute risk, which is about whether you'll actually get it. So the important point is it doesn't determine that he will or won't get the disease. It just increases the risk for him.

What would you say to people who would rather not know about their risk as it would only make them anxious?

It depends a bit on the disease and it depends a bit on how big the impact is. So in the case of Alzheimer's disease, the gene that's called APOE that was checked for Chris Hemsworth, that has quite a big impact on his risk of getting the disease. At the moment, there's not very much you can do about it.

I think different people will take different views. Some would rather know and some would rather not know. And that's absolutely up to the individual. But I think Alzheimer's research is progressing really quickly. So it might well be the case before too long that indeed there are things that you could do and there might be drugs to help reduce the risk or catch it early enough to slow down its progression. I think that it’s a slightly different question for different diseases.

Will we ever see a day where we have entire populations routinely having tests like these?

I think that will happen. First of all, it should be up to individuals. No one should be forcing individuals for this. But for most common diseases, genetics is a risk factor. And if we knew about it, instead of just saying “here are the ten or 20 diseases you should be most worried about. And here's some generic advice” we could be saying in your case, you are particularly high risk of heart disease.

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We can actually tell you this when you're in your twenties, so you should work even harder on diet and lifestyle. Maybe it would be appropriate to go on drugs to reduce your cholesterol a bit earlier in life. We could do that because we have that special information about you.

About our expert: Sir Peter Donnelly

Peter Donnelly is Professor of Statistical Science at the Wellcome Centre for Human Genetics, University of Oxford. He is also the co-founder and CEO of genetic healthcare service provider Genomics PLC.

Read more about genetics:

Authors

Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.

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