Stephanie Knowles hated change.

She liked stability and routine; the room to sound things out before she was fully involved in them.

In her early teens, a mini-revolution was underway in her so-far short life: she and her twin sister had moved with their family to a small country town, away from the city; high school was on the horizon, and the challenge of making new friends.

And of course, the greatest change of all: puberty.

“I just wanted [puberty] to desist,” Stephanie tells Insight’s Jenny Brockie. “I just wanted to serve it notice.”

So, she responded in the only way she knew: to take back control of her body. She did this by starving herself.

At 14, she was diagnosed with anorexia nervosa, and for the next 6 years travelled back and forth from hospital in attempts by her family to get her well again. She nearly died a few times.

For her twin sister, Brittany Watt, it was hard to see.

“It was the ultimate torture to watch. You love this person and you know how crap that must be and you just can’t do anything.”

Brittany herself was beginning to feel different.

Environmentally, she was watching Stephanie get much of the family’s attention; she’d managed to escape – albeit in the most extreme way – the world that had been causing them both pain.

“She was moved out of the situation that we were in, but I had to stay,” says Brittany. “[I thought], I’d just really like to be taken care the way Stephanie’s being taken care of.”

She developed bulimia.

On top of that, the same genes that had caused Stephanie’s predisposition to mental illness were stirring in her identical twin. At age 16, Brittany was clinically diagnosed with depression.

For both Stephanie and Brittany, their illnesses are unlikely to be just environmental.

Their experiences have been studied by Professor Tracey Wade of Flinders University, who has found eating disorders and depression have strong genetic components, where genes can account for 50 per cent of the likelihood someone will develop one of these conditions.

In anorexia, genes can account for 52 per cent of susceptibility. Wade has found that after puberty, this genetic predisposition can mean it takes less environmental factors to trigger an eating disorder.

As was Brittany and Stephanie’s experience.

“I think with our genetic predisposition to eating disorders and depression it was a perfect storm,” says Brittany.

Wade uses the analogy of an orchid and dandelion when explaining this shared genetic and environmental susceptibility.

Orchids have a genetic risk to certain environments, but can flourish in the right conditions with good management and awareness of that risk. Dandelions (the rest of the population), on the other hand, have a much higher chance of thriving no matter what the environmental conditions are.

For Stephanie and Brittany, awareness of this genetic risk – which they say will always be with them – has allowed them to put in place coping systems and return to better health.

Sharing genes: depression and eating disorders

Wade’s research, involving the Australian Twin Registry, has also found depression and anorexia come from the same group of genes.

“There’s quite a large overlap in the genetics between major depression and anorexia nervosa,” says Wade.

The overlap between the two is about 34 per cent, in terms of genetic risk, meaning that the conditions share about a third of their genetic heritage.

“The disorders also often share a lot of the same themes,” says Wade. “Feelings of self-loathing, feeling out of control, feeling not good enough and wanting to do things perfectly, but not feeling able to do that.”

For Michael Cameron, the genetic component of depression is of particular concern.

His identical twin brother, David, committed suicide at 16.

“It just came completely out of the blue,” he says. “There were no signs, he wasn’t into drugs, alcohol. He seemed like a happy enough kid.”

His brother’s death has triggered emotions he’d never experienced before, and it now has him wondering how susceptible he is to mental illness.

“Those questions definitely do loom over me. I do get depressed and get anxiety which I’ve never had before it happened.”

He admits, though, it hasn’t been so bad to instigate any suicidal ideations.

“I’m generally quite a happy, sort of confident person,” he says, adding that before his brother’s death he’d never had any mental health problems.

Circumstances aside, one explanation of this difference could be epigenetics: extra layers of molecules on top of our DNA that switch certain genes on or off without making any structural changes to our genetic sequence. For identical twins, who share 99.9 per cent of their genes, this can explain differences between them.

Wade says epigenetics will likely form a huge part of future research into mental health, particularly into eating disorders, identifying when to intervene and ‘switch’ helpful genetic action on, where perhaps it has been switched off.

This article first appeared on ‘SBS’ on 17 March 2016.