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What role do genes play in schizophrenia, depression, and mental illness in general?

-A curious adult from Maine

Not an easy question to answer! Bottom line is genes play an important but not decisive role in getting a mental illness. There are mutations or changes in DNA that can lead to an increased chance of getting a mental illness.

Genes are not destiny, though. No mutations that I know of give you a 100% chance of having a mental illness. So the environment plays a critical role as well.

Let's look at schizophrenia as an example. Schizophrenia affects over 2 million Americans, around 1% of the population. And studies have shown that it clearly runs in families.

So if schizophrenia runs in the family, genes must be involved, right? Not necessarily.

Let's use an example to show how someone might be fooled by something running in the family. Imagine there is a family that lives near a toxic waste dump. The whole family has high rates of cancer.

Is it because of their genes? Probably not. The cancer is probably the toxic waste dump's fault, not the family's genes.

To try to figure out what role genes play, scientists often do a twin study. Twin studies look at how often identical twins both have something compared to fraternal twins.

If something happens more often for both twins in an identical twin pair, then genes are involved. Why? Because identical twins share the exact same DNA.

The twin studies showed that if one identical twin has schizophrenia, then there is a 30-50% chance that the other twin will have it as well. If a fraternal twin has schizophrenia, then there is a 15% chance that the other twin will have it as well. (This is the same percentage as any brother and sister.)

So these results suggest that genes probably play a role. Of course genes aren't the whole story. If they were, when one identical twin had schizophrenia, there would be a 100% chance that the other twin would have it as well.

But how might genes play a role in schizophrenia? To understand this, we need to understand a bit how the brain works and what goes wrong in mental illness.

We collect lots of information about the world with our five senses. Too much. Special brain chemicals called neurotransmitters let us sort out and make sense of these signals. If something goes wrong with this system, then you can get a mental illness.

These neurotransmitters don't exist in a vacuum. For them to have an effect, they interact with other proteins called receptors. These receptors then cause all sorts of things to happen which then results in, for example, recognizing your mother's voice. Mental illness can happen if something goes wrong with any of these parts.

Let's look at a common symptom of schizophrenia, hallucinations. Normally when we see something, the brain interprets what we see with these neurotransmitters. If the neurotransmitters were to go into action without the visual cue, you'd "see" something that isn't there. Or if the receptor fired on its own, you'd get the same result.

Great theory but have any actual genes been found? One promising gene is the glutamate receptor gene (GRM3). One of the glutamate receptor's jobs is to respond to those neurotransmitters we talked about earlier. If it responds inappropriately, then you might have the beginnings of schizophrenia.

This mutant gene is almost certainly not the whole story. If it were, we could just track it in families and see who gets schizophrenia. But there are almost certainly lots more genes involved and scientists are busily trying to find them.

And let's not forget the environment. Because both twins in an identical twin pair didn't always get schizophrenia, the environment plays a role too. What the triggers are is still unknown. But whatever these triggers are, they play a critical role in mental illness.

Other mental illnesses are similarly complicated. By figuring out the genes involved, we can gain a greater understanding of how our brains work. And hopefully come up with better and more specific medicines to treat various mental illnesses.