People are often a bit shocked to learn that, as a geneticist, I study depression. I often get asked in surprise, "oh, is there a gene for that?"
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Unfortunately, there isn't a simple yes or no answer. If depression had a Facebook profile, its relationship status with genetics would be "it's complicated".
Most people would have noticed that depression tends to run in families. If you have a parent with depression, you're something like three times more likely to develop depression, compared with other people.
When geneticists see disorders occurring in family groups like this, we tend to get a bit excited. People who are related share a lot of genetic material - so it suggests to us that there might be a genetic basis to those disorders.
To get a better idea of whether depression is all in our genes, researchers have turned to studying twins. Identical twins share almost 100% of their DNA, while non-identical twins share only about half. If our genes are responsible for depression we'd expect that pairs of identical twins would either both be depressed or not, while pairs of non-identical twins would be less similar, because they're less genetically similar.
Researchers studying twins and depression have found that there is a link. One study, looking at over 42, 000 twins, found that genes contribute to depression - but also that environmental effects play a big role.
So does all this mean there is a "depression gene"?
Even though we've figured out that genes play a role in depression, that doesn't mean that there's just one gene that's responsible. Instead, it seems that many different genes all work together to influence whether or not we develop depression.
Right now, we don't know all of the genes that may play a role, but we do have a couple of usual suspects.
One of the genes that's thought to be involved is a gene called SLC6A4. This gene makes a serotonin transporter. Serotonin is a neurotransmitter - a chemical messenger that helps regulate our mood and behaviour. Variation in the SLC6A4 gene can change how much of the serotonin receptor is produced, and influence how much serotonin signal can be sent and received.
Another candidate is a gene called BDNF. This gene makes a protein called brain-derived neurotrophic factor, which is important for making sure that our neurons grow properly. Some tiny changes in this gene can change how that protein works, and might make us more likely to develop depression.
Scientists are still trying to find out which other genes are involved. In a study published last year, researchers compared the DNA of thousands of people with depression to the DNA of thousands of healthy people. They found 44 different places where the two groups were different. The next job is to work out which genes are in those areas, and how they might be linked to depression.
Depression isn't all in our heads - but it's not all in our genes either.