In case you weren't worried yet about inadvertently damaging your children's and grandchildren's DNA, scientists in Japan have demonstrated precisely how that might be possible by stressing out some fruit flies.
You might think that once you've contributed sperm or egg to your offspring, its genetic destiny is set: you're free to mess up the kid psychologically or raise it exclusively on gluten-free Cheetos, but you can't do any harm to its DNA. You'd be wrong, though. Scientists have learned that factors in our environment can change our genes and our children's genes--without altering the letters of our DNA. This phenomenon is called epigenetics and it's mostly a mystery.
Some epigenetic effects have been linked to diet; for example, human mothers who diet during pregnancy might predispose their children to be heavier. For this study, scientists looked at the effect of stress on fly embryos. Specifically, they kept the embryos at an uncomfortably warm temperature for an hour at an early point in their development. This is called heat shock, and although your mother is unlikely to have briefly baked you while you were in utero, the effect of heat shock is the same as the effect of other stressors on the body.
Packed tightly into your cells' nuclei, your DNA is coiled and coiled and coiled again, like a disastrously tangled phone cord (remember phones with cords?). The fruit fly's DNA is the same way. But the coiling isn't random. Areas of DNA that are being used are more loosely coiled, your genome's way of bookmarking its most-read pages. The tighter clumps of DNA aren't easily read by our cells' machinery, and might include genetic gibberish or redundant information.
The Japanese researchers found that stressing the fly embryos disrupted the clumping of their DNA, which in turn changed what genes were read and used by the embryo during its life. And the effect went further than that: After the embryos grew into adult flies and mated, their own offspring inherited some of their mis-coiled DNA. This was true whether the parent flies were male or female. But by the next generation, the flies' DNA had reverted to its original packing style, the mistakes of the past generations somehow corrected.
The flies in this study were physically stressed. In this case, they were overheated as embryos, but that bodily stress could also have come in the form of infection or starvation. Emotional stress is a kind of physical stress too--in the long term, it's known to be harmful to our bodies. So if humans are anything like flies (don't laugh! The protein responsible for clumping flies' DNA is the same as a protein we have), physical or emotional stressors in our lives could re-package the DNA of children we don't even have yet.
It's also possible that inherited changes in how our DNA is clumped could be responsible for certain diseases. This could lead to exciting future studies--not to mention fruit flies with a good reason to be stressed.
Clarification: As far as this study goes, the answer to the question "Is your stress affecting your future grandkids?" is "Possibly, if you are a pregnant woman." Since you're providing the environment for your embryo, your unborn child is like the stressed fly embryo and your future grandchildren are like the fly's children (though undoubtedly cuter). The paper doesn't address other mechanisms of epigenetic change that might apply to people other than pregnant women. But even for non-pregnant women, I would guess that your current lifestyle influences the kind of environment you'll provide for a future embryo.
Seong, K., Li, D., Shimizu, H., Nakamura, R., & Ishii, S. (2011). Inheritance of Stress-Induced, ATF-2-Dependent Epigenetic Change Cell, 145 (7), 1049-1061 DOI: 10.1016/j.cell.2011.05.029
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