“This image shows a DNA molecule that is methylated on both strands. Methylation is a type of epigenetic change in which a methyl group gets added to part of a DNA molecule and prevents it from being ‘read’ and therefore expressed. Christoph Bock, Max Planck Institute for Informatics/Wikimedia Commons (CC BY-SA 3.0)
"It’s just genetics." How many times have you heard someone use that statement as an explanation for traits and behaviors they deem out of their control? If you, like me, are absurdly directionally challenged, maybe you’ve blamed your inability to read a map on the structure of your DNA. Or maybe you point to your dad’s permanent sweet tooth as scientific evidence that your own predilection for sugar is clearly secured in your genes. These silly examples are pretty low-stakes in the grand scheme, but they do make you wonder: does our DNA seal our fate, or is something else at play when it comes to how our genes express themselves?
If you guessed that your genetic destiny is based on more than just your DNA, you nailed it. But before we can get to how and why that happens, we should start with some basic definitions. For starters, what the heck is DNA anyway? Those three little letters stand for deoxyribonucleic acid, the master molecule of every cell. DNA contains the essential info that’s passed on to every successive generation, and even the slightest change to its sequence (or order) can have serious consequences.
There are four fundamental base types that make up DNA: adenine, cytosine, guanine and thymine, (A, C, G and T), and humans have around 3 billion bases, total — believe it or not, over 99 percent of them are identical in all people. Within those billions of bases are about 20,000 genes, otherwise known as units of heredity. Some genes provide instructions to make molecules called proteins, which carry out life functions, and others don’t. Overall though, genes are considered the biological players that influence the regulation and maintenance of stuff that happens in your body, like the building of bones, the movement of muscles, and the beating of your heart.
All of this makes it sound like your body is permanently set for action from the moment you’re born. But that’s not the case. One important element we haven’t talked about is how the environment or external factors influence the way cells read your genes.
"Epigenetics doesn’t actually change the sequence of DNA — that stays the same," Cynthia M. Bulik, Ph.D., FAED, Distinguished Professor of Eating Disorders in the Department of Psychiatry, School of Medicine, at the University of North Carolina at Chapel Hill, says via email. "But it refers to changes that affect how the genes are read or whether they are expressed or not."
What kinds of changes? All kinds, according to Bulik. "For example, one type of epigenetic change is DNA methylation," she says. "It is when a methyl group gets added to part of a DNA molecule that prevents it from being ‘read’ and therefore expressed. No protein will be made from that gene because it was basically silenced."
What Causes Epigenetics?
"Epigenetic changes can be caused by environmental factors that we are exposed to — smoking, what we eat, trauma, other environmental exposures," Bulik explains. "The cool thing about it is even though the sequence of DNA doesn’t change, these ‘epigenetic modifications’ can still be passed down through generations. So they are heritable. We pass down not only our DNA but also these instructions of how our DNA is read."
Bulik, who is also the founding director of the UNC Center of Excellence for Eating Disorders and the co-director of the UNC Center for Psychiatric Genomics, has a favorite illustration of epigenetics in action. "I think one of my favorite example is sex determination in turtles," she says. "We have known that temperature determines whether a turtle will become male or female (sexual fate), but researchers have shown that the process underlying how this works is actually epigenetic. Temperature affects a gene that controls the ‘epigenetic’ state of another gene, which is the master sex determining gene. Unlike humans, where sex differences lie in our X and Y chromosomes, in turtles, genetic differences don’t determine gender — instead, environmental cues (in this case, temperature) influence epigenetic mechanisms that turn gender-determining genes off or on."
The Present and Future of Epigenetics
There’s been a lot of talk about epigenetics in the news and its potential to influence the future of health, since everything we eat, where we live, who we interact with, when we sleep, how we exercise, etc. has the potential to cause chemical modifications that turn genes on or off over time. The field of epigenetics is growing rapidly as researchers investigate the ways external factors can influence things like the risk of developing certain chronic diseases and behavioral disorders, and how prevention and treatment strategies could be tailored to address those issues. But according to Bulik, there are some misconceptions around its possible promise.
"People seem to think that epigenetics will be the answer to everything," she says. "If we don’t get an immediate answer with genetics, they rush right over to epigenetics instead of being patient and realizing that we sometimes need really large sample sizes to identify genes that affect disease risk. Epigenetic research is hard as the conditions have to be very constant across studies in order to be sure that you have a replication. They are complementary technologies and can both be used to answer important questions about the causes of disease."
As for Bulik’s current and future work in the field of eating disorders — conditions largely influenced by both genetic and environmental factors — epigenetics will play a critical role. "We are doing a study of identical (monozygotic) twins who are discordant for anorexia nervosa (one has the illness and the other does not)," Bulik says. "Since identical twins basically have identical DNA sequence, it is not the DNA sequence that caused one twin to develop the illness and the other not to develop the illness. So, what could cause this difference? One thing would be environmental factors: one twin went into gymnastics and had a coach who made them diet and body shamed them if they gained weight leading her to go on severe diets, whereas the other twin played violin in an orchestra and none ever commented on her body side or shape. But another possibility is that there are epigenetic changes between the twins that could influence disease risk. So we will be looking at differences between the twins to see if differences in gene expression might be related to why one became ill and the other remained well."
Now That’s Bananas
Yes, you share about 99 percent of your DNA with other humans, but you also share about 85 percent with mice, 40 percent with fruit flies, and 41 percent with bananas.