Toxic chemicals that poisoned your great-grandparents may also damage your health, US research suggests.
Chemicals can change the way genes work
A team from Washington State University has produced evidence that some inherited diseases may be caused by poisons polluting the womb.
Research on rats indicates man-made environmental poisons may alter genetic activity, giving rise to diseases that pass down at least four generations.
The research is published in the journal Science.
The scientists exposed pregnant rats to two agricultural chemicals during the period that the sex of their offspring was being determined.
The compounds were vinclozolin, a fungicide commonly used in vineyards, and the pesticide methoxychlor.
Both are known as endocrine disruptors - chemicals that interfere with the normal functioning of reproductive hormones.
Rats exposed to the compounds produced male offspring with low sperm counts and poor fertility.
They were still able to produce young, however. When these rats were then mated with females that had not been exposed to the poisons, their male offspring had the same problems.
The effect persisted through at least four generations, impairing the fertility of more than 90% of male offspring in each generation.
The researchers found the damage was not caused by alterations in the DNA code, but changes in the way the genes work.
These epigenetic changes, as they are known, are caused by small chemicals that become attached to the DNA, modifying its activity.
Epigenetic changes have been observed before - but were not previously known to pass onto later generations.
Lead researcher Dr Michael Skinner believes they may contribute to diseases such as breast cancer and prostate cancer.
Both diseases are becoming more common, and Dr Skinner says that cannot be down to genetic mutations alone.
The researchers believe their findings suggest exposure to environmental toxins may play a key role in the evolutionary process.
Evolution may not be driven entirely by genetic mutations, as commonly thought.
Dr Skinner said: "It is a new way to think about disease. We believe this phenomenon will be widespread and be a major factor in understanding how disease develops."
However, Dr Skinner stressed more work was needed to corroborate the findings.
The levels of chemicals the rats were exposed to were very high - much higher than people normally ever encounter.
Professor Alan Boobis, a toxicologist at Imperial College London, UK, told the BBC News website the findings were interesting, but he said there was no need for people to be alarmed.
"This effect is likely to be concentration dependent, and these animals were exposed to very high levels of chemicals," he said.
"We need to find out whether this trans-generational effect is translated to much lower doses."