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You are what you eat.

And who you hang out with, and the weather and the way your mother raised you, say gene researchers at McGill University.

Contrary to popular belief, recent research at McGill helps to prove your DNA alone does not determine who you are, says Michael Meaney, a neuro-biologist at the university.

While external factors like environment and experience don't alter our basic human genetic code, they do produce permanent changes to the way genes behave, says the McGill team.

"Our fate is not just sealed by our genetic inheritance, but by how the genes are sculpted by their environment," said Mr. Meaney's research partner, Moshe Szyf, a professor of pharmacology.

Researchers have mapped the billions of building blocks that make up human DNA and it seems every day they isolate another gene linked to specific characteristics or illness.

But scientists have known for some time that it is the chemical coating on the surface of genes that determines which genes in the cell will be activated and which will not.

Diet, maternal nurturing and even the weather can trigger changes to that chemical coating on the surface without changing the genetic code within.

Scientists at Duke University Medical Centre have likened it to putting gum on a light switch. The switch isn't broken but the gum blocks its function.

"We can no longer argue whether genes or environment has a greater impact on our health and development because both are inextricably linked," Dr. Randy Jirtle, a genetics researcher at Duke, said in a recent article on so-called epigenetics.

Mr. Szyf and Mr. Meaney were able to prove that a mother rat's early nurturing produced specific changes to the chemical coating in specific areas on the gene cell.

The changes affected the area of the brain responsible for stress response in the adult rats.

They then wondered if they could reverse the change by injecting the brains of healthy, well-adjusted rats with L-Methionine, a natural amino acid popular as a dietary supplement.

The chemical coating changed again.

"They became very anxious and had a very poor response to stress," Mr. Szyf said.

"It demonstrates that although our genes are sculpted very early in childhood. . . things can change later in life and the things that change could be as simple as some of the basic things we get from our food."

Although the research is in its infancy, it raises the prospect that drugs or diet could alter the genetic effects that predispose people to illness.

It doesn't mean we can totally change our genetic predispositions, Mr. Szyf said, but there are therapeutic possibilities.

"We hope one day we can have diagnostic tools so we can predict these things and follow it," he said.

"This is just the beginning."