Every generation of Jews, it is thought, must learn the trauma of the Holocaust anew from parents or community.

But a new study has provided the strongest proof yet that some of the trauma can be passed along genetically, and that other genetic changes people accrue during life may also be transmitted to their children.

The study, by researchers at New York’s Mount Sinai hospital, looked at the genes of 32 Jewish men and women who survived a Nazi concentration camp, witnessed or experienced torture or hid during World War II, and the genes their children.

“The gene changes in the children could only be attributed to Holocaust exposure in the parents,” Dr. Rachel Yehuda, the head of the team of researchers, told the Guardian.

Yehuda, a psychiatrist and neuroscientist, and her team’s work is the clearest example in humans of the transmission of trauma across generations through “epigenetic inheritance” – the idea that genetic changes caused by the environment over a lifetime can be transmitted to offspring.

Genes contained in DNA are thought to be the only way to pass biological information from parent to child. But environmental influences – like smoking, diet and stress – modify genes all the time via chemical tags that attach themselves to DNA, switching genes on and off.

Recent studies suggest that some of the epigenetic tags might somehow be passed from parents to their children.

In their study, published this month in the journal Biological Psychiatry, Yehuda and her team focused on one region of a gene associated with the regulation of stress hormones and known to be affected by trauma.

They found tags on the same part of this gene in both the Holocaust survivors and their children. The correlation did not show up between the control group and their children.

Further genetic analysis ruled out the possibility that the epigenetic changes were a result of trauma that the children had experiences themselves.

“To our knowledge, this provides the first demonstration of transmission of pre-conception stress effects resulting in epigenetic changes in both the exposed parents and their offspring in humans,” Yehuda told the Guardian.

Other studies have less robustly linked the genetics of parents and their children.

How exactly parents could be passing the epigenetic tags to their children remains a mystery. Tags on DNA were thought to be wiped clean soon after conception. But recent research has shown that some slip through to leave their mark on the next generation.

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