What can twins teach us about genetics?

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Look into the eyes of a twin one day.

You may not know it, but you will have found an intriguing window into humankind, a kind of flesh-and-blood Rosetta stone that holds the promise of curing disease and easing mental illness.

That's no exaggeration - and it's much more than the seemingly supernatural twin discoveries that occasionally flash into the public conscious. Stories about separated identical twins who have lived startlingly similar lives, capture the imagination, as do ruminations about near-mystical forms of unspoken communication between twins.

But the science goes far deeper - and with double births on the rise, it's worth looking at how much we have already learned from twins, and what we continue to learn. There is, after all, an argument to be made that twins have been among the most important contributors to our genetic understanding of ourselves. They are what Robert Plomin, Professor of Behavioural Genetics at King's College London, calls "the workhorse" of an entire field of genetic research.

If that doesn't sound particularly flattering, then let's delve into the fascinating results of that work. By contrasting identical twins - who are genetic photocopies of each other - with fraternal twins, whose genes are different but upbringings very similar, it's possible to pinpoint what's genetic and what's not. Thanks to those studies, we know that intelligence has a strong genetic component - or, as the scientists say, it's very "heritable." So your smarts are largely coded into your DNA - learning disabilities are 85 per cent heritable; language, 75 per cent; mathematics, 70 per cent. Personality, on the other hand, is only about 45 per cent heritable. In other words, you may have your parents to thank for your test grades, but not your temper.

And that's only scratching the surface.

In the Vietnam War, twins helped pinpoint traits that make elite Marines and pilots. Today, as Canadian soldiers return from Afghanistan, UBC researchers have turned to twins to better understand what contributes to post-traumatic stress disorder. (It's partly heritable and, in a bitter twist, those with sensation-seeking genes - as in, those most likely to end up in traumatic situations - are also most vulnerable.)

Some of the most fascinating lines of modern twin studies involve so-called epigenetics, the examination of how our behaviours and environments literally rewrite our genetic codes. Those studies may one day provide researchers insights into how genetic switches for conditions like schizophrenia are turned on - and then how to develop treatments.

"If we know what the gene action is, we can develop a medicine to counteract it," said Kerry Jang, a UBC professor of psychiatry who has studied thousands of twins over two decades. And for those traits that aren't heritable, "on the psychology side, if we can identify the environments that cause or protect you from mental illness, we can use that in therapy to train people to avoid those situations."

In fact, twins are so important to answering these questions that Mr. Jang has applied to the Social Sciences and Humanities Research Council for funding to launch a Canadian national twin registry. If he's successful, he hopes to populate it with 5,000 twin pairs from across the country, creating a database of research subjects that other scientists can query. The list of possible inquiries is virtually endless.

"For example," Mr. Jang said, "is voting NDP genetic or not?"

Our own twins remain roughly 17 years and 50 weeks from being allowed to decide whether they harbour left-wing sympathies. Still, when I look into their faces today - Marcus, with his placid gaze; Callum with his scrunched-up cheeks that can't quite pull back into a smile - it's an extra thrill to think that for all the things we have yet to teach them, they may actually have far more to teach us.