Researchers have deciphered the full DNA makeup of a single human being, a remarkable achievement which has, unfortunately, driven the crescendo of misplaced genomic enthusiasm to new heights. “This human’s life, decoded” was the front page headline in the Globe two days ago. Predictions of individualized gene-based therapies for a host of diseases are again being voiced, an expectation sure to be frustrated by scientific reality.

It’s not that genes do not matter–- they certainly do; it’s only that they do not and cannot determine even simple behaviours, let alone the infinitely complex and sacred process that is a human life. Nor can genes explain most illnesses or address possible cures for them.

Far from being the autonomous dictators of our destinies, genes are controlled by their environment, and without environmental signals they could not function. In effect, they are turned on and off by the environment; human life would be impossible if it wasn’t so. Every cell in every organ in our bodies has exactly the same complement of genes, yet a brain cell does not look or act like a bone cell, and a liver cell does not resemble or function like a muscle cell. It is the environment within and outside the body that determines which genes are switched on, or activated, in which cell.

There is a new and rapidly growing science that focuses on how life experiences influence the function of genes. It’s called epigenetics. As a result of life events, chemicals attach themselves to DNA and direct gene activities. The licking of a rat pup by the mother in the first hours of life, for example, turns on a gene in the brain that helps protect the animal from being overwhelmed by stress even as an adult. The same gene remains dormant in rats deprived of such grooming. Epigenetic effects are most powerful during early development and have now been shown to be transmittable from one generation to the next, without any change in the genes themselves. Environmentally induced epigenetic influences powerfully modulate genetic ones.

How a gene acts—that is, what protein messengers it will produce, if any—is called gene expression. Gene expression is determined by inputs from the environment which reach the DNA by means of receptors on the cell membrane. It’s the membrane, not the DNA, which serves as the real brains of the cell. Every cell membrane holds many thousands of receptors for many types of messenger molecules. Cells receive input and direction from the brain and the body and from the outside by means of messenger-receptor interactions. It is now clear that “the early environment, consisting of both the prenatal and post-natal periods, has a profound effect on gene expression and adult patterns of behavior,” to quote a recent article from The Journal of Neuroscience. One example is related to alcohol consumption: in monkeys a certain gene that predisposes to alcoholism is expressed, or active, in peer-reared animals but not in well-nurtured young monkeys brought up by their biological mothers.

Among the many influences on gene activity throughout the lifetime is stress. A crucial  part of human DNA are telomeres, long strands at the ends of chromosomes which protect our genetic material, much like glue prevents the end of shoelaces from fraying. As we age, our telomeres shorten and by the end of life their length is greatly curtailed. Mothers of children with chronic illness have been found to have shortened telomeres that represent as much as ten years of aging as compared with their biological contemporaries. The greater was their perception of the stress of caregiving, the “older” was their DNA.

Thus, when it comes to illness, health, behaviours and life patterns, genes can predispose but they cannot predetermine. Because no two people are subjected to exactly the same input from the environment, not even the brains of genetically identical twins will have the same set of connections, nerve branches, and active chemical pathways.

Why, then, are narrow genetic assumptions so widely accepted for everything from aggression and illness to addictions? Our preference for simple and quickly understood explanations is a major factor, but a deeper source may be that we human beings don’t like feeling responsible—as individuals for our own actions, or as a society for our many failings. Genetic explanations take us off the hook and, worse, divert attention from many of the actual sources of human experience.

“ ‘It’s all in the genes’ [is] an explanation for the way things are that does not threaten the way things are,”  the writer Louis Menard pointed out  a New Yorker article. “Why should someone feel unhappy or engage in antisocial behaviour when that person is living in the freest and most prosperous nation on earth? It can’t be the system! There must be a flaw in the wiring somewhere.”

Succumbing to the urge to absolve ourselves of responsibility and seeking easily digestible rationales for complex phenomena—for life itself—our culture has too avidly embraced genetic fundamentalism.