Deep in the DNA of Marietta Drucker sits one mutated gene amid the tens of thousands that account for, among other things, her blue eyes, blond hair and wide toothy smile.
That gene, one of only a handful of recognizable landmarks in the chemically complex and shadowy map of the human genome has a name, BRCA, and a destiny -- to spawn a potentially deadly form of breast or ovarian cancer.
Six years ago Drucker, then 76, was diagnosed with stage three ovarian cancer, news she believed was a "death sentence" and "about as bad as it could get."
"But then," she says, "it got worse."
Within weeks of Drucker's diagnosis, her two daughters tested positive for the breast cancer gene, an inadvertent and unwelcome gift from their mother.
"I felt devastated. I felt sick about it. How could I give the people most precious to me in my whole life -- my two daughters -- this awful, awful gene," says Drucker, now cancer free after surgery and chemotherapy, from her home in St. Petersburg, Fla.
Rationally, Drucker knows she cannot be blamed for her daughters' cancer and surgeries.
Emotionally, she says, it does not matter.
"The guilt is with me every day. I absolutely blame myself. I was and I still am beside myself that my girls had to go through this hell, that they should have to go through life being worried," she says.
Guilt, evolutionary psychologists believe, is an emotion as old as human societies. It is a "negative emotion," which nevertheless often compels us to altruistic action, allowing us to empathize with and then help those in need.
But it can also be debilitating, filling people with a deep sense of shame that shuts them off from others.
"Our guilt mechanism did not develop at a time when we had a scientific understanding of disease or genetics," says John Tooby, co-director of the Center for Evolutionary Psychology at the University of California at Santa Barbara, who studies how humans developed a guilt instinct.
"We rationally understand we are not to blame, but the guilt mechanism does not," he says. "It's comparable to the way you might be paralyzed by fear when on a cliff, even though you rationally understand you have to climb down to safety."
In families stricken with genetic diseases, feelings of guilt -- even the fear of potential guilt -- plays out in myriad ways. Guilt sometimes prods healthy family members to get their sick relatives treatment, and sometimes causes them to withdraw and distance themselves.
It is not just parents who feel guilty about inherited diseases. Healthy siblings often feel intense shame and self-loathing that they were spared the degenerative or deadly disorder that affected a brother or sister.
Expectant parents who learn their unborn child is sick confront feelings of guilt whether they decide to have the child or abort. And adult children diagnosed with inherited diseases are often reluctant to tell their parents in fear of the guilt they might feel knowing they have upset a parent.
Understanding guilt as it applies to families with genetic diseases is also helping researchers better understand the emotion, which has classically been explained as an internalized sense of morality.