A new twist has been thrown into the classic debate of "nature versus nurture" through the budding field of epigenetics, which has found that nurture can alter the genetic nature of both an individual and the person's descendents.
Epigenetics researchers investigate the ways that environmental factors - pollution, emotional stress, physical trauma - can affect the way people's genetic blueprint is expressed through their physical and emotional development.
"Decades ago, we looked at genes as being the hard-wired plan for how the body develops and functions," said Dr Steven Dowshen, chief medical editor of KidsHealth at the Nemours Center for Children's Health Media and a paediatric endocrinologist with the Alfred I. duPont Hospital for Children in Wilmington, Del.
"That still is the case. However, we didn't understand until we knew more about this concept of epigenetics how environmental factors can change how those genes work."
What happens to you today affects your grandchildren
Doctors studying epigenetics also have found evidence that a person's current environment can affect the health of their progeny, with today's events echoing decades down the family tree.
The word "epigenetics" provides a clue to the concept, as the Greek prefix "epi" means "over" or "above." Researchers have found that environmental factors essentially can flip an on or off switch in a person's genetics, affecting not only that person's development but also how genetics are transmitted in the person's eggs or sperm.
"Epigenetic mechanisms don't alter the actual DNA structure, but they do alter the DNA molecule in a way that modifies the amount of biologic information that will be transmitted by the gene," said Rachel Yehuda, a professor of psychiatry and neurobiology and director of the traumatic stress studies division at the Mount Sinai School of Medicine in New York City.
"Imagine if you're listening to beautiful music and somebody mutes it. The music might still be playing, but it won't be heard. Or you can amplify that music."
Evidence of an epigenetic influence on human health and development include studies that found:
Hormonal differences in children born to mothers who had suffered extreme emotional and physical trauma. The differences make the children more susceptible to such mood disorders as anxiety and depression. The changes have been observed in second- and third-generation offspring of Holocaust survivors, as well as in the children of women who were pregnant on Sept. 11, 2001, and were evacuated from the World Trade Center, Yehuda said.
Extended longevity in people whose grandfathers suffered from malnourishment or starvation as children. This came from a landmark Swedish study that found that children raised in years when the harvest was bad produced grandchildren who lived longer than children who had plentiful food during their formative years, Dowshen said.
An effect on offspring from such behaviours as smoking and overeating. Dowshen said that such behaviours can predispose a person's children to systemic diseases, including diabetes and obesity.
Epigenetic effects aren't necessarily generational, however. There is evidence that trauma and stress may affect a person's psychological health by fiddling with the genetics that regulate body chemicals, Yehuda said.
Medical benefits of epigenetics
"We think that epigenetics may be very informative in helping us understand why environmental events like trauma may be so transformative," she said. "When people undergo watershed life events, they say they are changed by them. What does that mean? This might help explain that."
The medical benefits of an epigenetic view of health and human development are not some pie-in-the-sky notion. Doctors already are putting this view of genetics to work in the treatment of patients.
"We're probably using epigenetics already in, for example, giving folic acid to pregnant women to prevent neural tube defects that cause spina bifida," Dowshen said. Folic acid influences the way a woman's DNA is expressed in her offspring, reducing the chances of the baby developing the devastating birth defect.
Epigenetics also have led doctors to encourage pregnant women to eat well, avoid alcohol and smoking, and eliminate as much stress as possible in their environment, he added.
In the future, findings from epigenetic studies could uncover ways to treat depression, cancer and much more by manipulating a person's genetics, Dowshen and Yehuda said.
"When treating depression, instead of treating the chemicals that flow through our neurons using antidepressants, we might be able to tackle more immediate causes for these chemical imbalances," Yehuda said.
People with a family history of cancer may be able to avoid developing the disease through epigenetic therapies that inhibit the expression of cancer-causing genes, Dowshen said. Manipulation of those genes might even lead to a cure for cancer one day.
In the meantime, epigenetics experts say, people today should realise that the lifestyle they lead will affect not only their own health but will probably have an impact on their children and grandchildren as well.
"Environmental factors like eating a healthy diet and physical activity and exercise are very likely influencing not just that individual but several generations emanating from them," Dowshen said. "It's likely that leading a healthy lifestyle will turn out to be understood to have some very direct effects on subsequent generations."
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