In my favorite fantasy world, the Discworld, Death – yes, the skeleton with the robe and scythe – has a granddaughter, Susan, and Susan has an unusual birthmark: when she gets angry, and her face flushes red, and three white lines appear on her cheek, almost like an afterimage of being slapped. Susan was raised to be sensible, so when she discovers that the origin of that Death once slapped her father, she takes exception to the science. Genetics isn’t supposed to work that way.
Except that, according to the latest understanding of biology, maybe genetics does work that way.
The first mind-blowing insight I introduce into a classroom discussion of nature and nurture is “epigenetics“, the notion that our genes get turned on and off. Yes, we inherit genes from our parents, and yes, all those genes comprising our DNA are sitting around in all of our cells. But those genes aren’t all working, all the time. Once this has been pointed out, it seems obvious. Somewhere in my DNA you can find the instructions for growing teeth – shaping molars and incisors, covering them with enamel, then prodding them to erupt from the gums. But those genes are not doing anything right now, or at any point in the last decade (thankfully, because I would hate to have even more brushing and flossing to deal with). Start thinking about all the experiences that might require some genetic support that only happen for certain, limited times – puberty, pregnancy, breastfeeding – and suddenly it becomes clear that genes are being switched on and off, or ramping up and tapering off production to meet environment demands, like any good protein factory should.
This means that experiences very early on in your life can have a dramatic effect based on which genes get turned on or off…dramatic enough that mice may look dramatically different than their parents and even their identical twins, perhaps helping us understand how one child might identify as a cisgender male while his identical twin identifies as a transgender female. Incredibly subtle experiences in the womb can flip the switch on a few genes and dramatically alter the path our “nature” will take.
But wait, there’s more.
The latest mind-blowing insight, that I am still coming to grips with myself, is that some of those epigenetic changes may be passed down to future generations. Even sperm and egg are not passive assortments of DNA; they may already have some genes turned on or off by the donor’s life experiences. When mice are trained to fear a particular scent (because it precedes an electric shock), their offspring and their offspring’s offspring (mouse grandchildren) were more sensitive to that very same odor – not afraid of it, exactly, but more likely to detect it and respond to it in some fashion. This was true even though mouse in vitro fertilization was used to make sure that the grandchild mice grew up with parents who had no reason to think anything of that scent. Somehow, the notion that “this scent could be important; detect it!” was being transmitted in the genes. How, we don’t know – which has some biologists incredibly skeptical about the whole thing, and raises the possibility of further mind-blowing insights for years to come.
One thing’s for sure, though: what we might learn about ourselves by getting mapping our individual genomes just got a lot murkier.