“Scientists now see inheritance as more fluid, and the barriers between genes and the environment as more porous.
It’s not all about the genes anymore; it’s about expressing genes, or turning them on and off. Cells commonly turn DNA off by dotting it with small bumps called methyl groups, or turn DNA on by using acetyl groups to uncoil it from protein spools.
And scientists now know that cells pass those precise patterns of methyls and acetyls on to daughter cells whenever they divide – a sort of “cellular memory.” (Indeed, scientists once thought that the methyls in neurons physically recorded memories in our brains. That’s not right, but interfering with methyls and acetyls can interfere with forming memories.)
The key point is that these patterns, while mostly stable, are not permanent: certain environmental experiences can add or subtract methyls and acetyls, changing those patterns. In effect this etches a memory of what the organism was doing or experiencing into its cells.
Unfortunately, bad experiences can be etched into cells as easily as good experiences. Intense emotional pain can sometimes flood the mammal brain with neurochemicals that tack methyl groups where they shouldn’t be.
Mice that are (however contradictory this sounds) bullied by other mice when they’re pups often have these funny methyl patterns in their brains. As do baby mice (both foster and biological) raised by neglectful mothers, mothers who refuse to lick and cuddle and nurse.
These neglected mice fall apart in stressful situations as adults, and their meltdowns can’t be the result of poor genes, since biological and foster children end up equally histrionic. Instead the aberrant methyl patterns were imprinted early on, and as neurons kept dividing and the brain kept growing, these patterns perpetuated themselves. The events of September 11, 2001, might have scarred the brains of unborn humans in similar ways. Some pregnant women in Manhattan developed post-traumatic stress disorder, which can epigenetically activate and deactivate at least a dozen genes, including brain genes.
These women, especially the ones affected during the third trimester, ended up having children who felt more anxiety and acute distress than other children when confronted with strange stimuli.
Notice that these DNA changes aren’t genetic, because the A-C-G-T string remains the same throughout.
But epigenetic changes are de facto mutations; genes might as well not function. And just like mutations, epigenetic changes live on in cells and their descendants. Indeed, each of us accumulates more and more unique epigenetic changes as we age. This explains why the personalities and even physiognomies of identical twins, despite identical DNA, grow more distinct each year. It also means that that detective-story trope of one twin committing a murder and both getting away with it – because DNA tests can’t tell them apart – might not hold up forever. Their epigenomes could condemn them.
Of course, all of this evidence proves only that body cells can record environmental cues and pass them on to other body cells, a limited form of inheritance. Normally when sperm and egg unite, embryos erase this epigenetic information – allowing you to become you, unencumbered by what your parents did. But other evidence suggests that some epigenetic changes, through mistakes or subterfuge, sometimes get smuggled along to new generations of pups, cubs, chicks, or children.”
From THE VIOLINIST’S THUMB by Sam Kean