Researchers are focusing on the differences between identical twins, rather than the similarities, in an attempt to rethink the nature versus nurture debate, and unlock medical mysteries including cancer.

Researcher Tim Spector of King's College, London, studies twins in the U.K. to learn what their differences can teach us.

"Unlike the press stories that show twins often die within a few months or each other, or a few hours ... they're often dying years apart of completely different diseases," said Spector, one of the experts interviewed in a new documentary. "So something must be altering those genes in these clones to make one die much earlier than the other."

Toronto-based filmmaker Leora Eisen, an identical twin, faces that reality head-on in her documentary Two of a Kind. She wonders why she remains in good health while sister Linda struggles with leukemia. The documentary airs on Thursday at 8 p.m. ET on The Nature of Things with David Suzuki.

By studying identical twins, scientists such as Spector are learning how genes interact with life experiences to shape future health. The field is known as epigenetics.


Two of a Kind

Tune in to CBC's Documentary Channel on Sunday at 9 p.m. ET. for a separate and distinct point-of-view feature film examining the psychological ramifications of twinship, also entitled Two of a Kind. The channel is offered free this month by many providers.


"It broadly describes a mechanism by which chemical signals can switch on or off genes," Spector said. "It's not the gene that counts — it's how you use them."

Spector, a professor of genetic epidemiology and head of one of the largest twins labs in the world, said the question is no longer "nature or nurture." 

"Now we know enough about it to be able to say it's clearly a mixture of both."

In his TwinsUK lab, identical and fraternal twins both go through medical tests and DNA analysis.

"If you compare the two sets, you can tell what proportion of their behaviours, their traits, their disease, is due to genes or is due to their lifestyle or upbringing."

Differences in twins begin early

University of Melbourne geneticist Jeff Craig said environmental factors that affect people start early — while they're in the womb, in fact. 

TwinsUK

Maren and Kris Hallenga are volunteers in scientific research at TwinsUK lab. (Courtesy Leora Eisen)

Identical twins have epigenetic "marks" at birth, Craig said.

"They're receiving the same nutrients and the oxygen coming from the mother, but we think it's the size of the placenta and how big the umbilical cords are that causes one twin [sometimes] to get a bigger share of the mother's goodies, and the other twin to get a smaller share, and this we think can program their health," he said.

Spector said twin studies could help explain the cause of many diseases, including cancer.

"Within 10 years, epigenetics will be a commonplace part of medicine, used in all kinds of cancer treatments, and it's going to really make predicting individuals who get disease a much more precise art," he said.