As Suzanne Methot discusses in her book Legacy: Trauma, Story, and Indigenous Healing, a person's DNA is not set in stone at birth. She points to epigenetics, or the study of the changes in an individual's DNA expressions, to show that childhood traumas, such as neglect, as well as "severe stresses" in adulthood, can actually change the expression of a person's DNA. These "molecular scars" can then be passed on to descendants, leading to intergenerational trauma.
The fact that we now know that trauma and stress affect not only the mental but the physical structure of a person in ways that can be inherited adds urgency to the need to treat mental illness and foster good mental health through physical as well as mental therapies. Traumas often show in bodily forms, such as alcoholism, substance abuse, and, according to Methot, such symptoms as headaches, chronic pain, and intestinal diseases (these physical symptoms can also manifest without any relationship to trauma).
Helping people to deal with their bodily as well as mental health can help prevent or ameliorate epigenetic transfer of trauma to future generations. Knowing that trauma is epigenetic helps us understand, too, that unhealthy behaviors replicated in younger generations are not an inherent "flaw" in a particular population but the result of trauma that can be dealt with.