We often think of the content of our DNA as the inevitable part of ourself, the things we can’t change no matter how much we try. Meanwhile, the other factor in who we are, how we look and how we act, our environment, is the opposite, changeable, open to manipulation and within our control. But actually the line between the two isn’t so clear. Our DNA is actually susceptible to change, and can have different effects on us, depending on how our environment and our DNA interact with one another. Although the DNA sequence itself doesn’t change as a result of this, the way it’s expressed does, meaning that certain genes can be rendered “silent” and so have no effect on us, or their effects can be amplified.
Epigenetics relies on molecules that can attach to our DNA, affecting how it is expressed and determining to what extent genes are turned “up or down”, like the volume buttons on a remote control, increasing or decreasing the impact of genes on ourself. Our lifestyle choices, things like smoking, drinking, diet and exercise can all contribute to this. There are two main ways in which DNA can be modified: by methyl groups binding to DNA bases, or by molecules binding to the histone proteins that our DNA is wrapped around. In the case of histone proteins, our DNA is usually tightly coiled around the proteins, preventing enzymes involved in decoding our DNA from being able to reach it. By attaching molecules to these histone proteins, the DNA strand can be moved, either loosening the coils and therefore allowing enzymes to reach it, or tightening them, preventing them from binding.
This modification of DNA is the reason our body cells don’t all do the same job. All our cells possess the same DNA, but they “know”, thanks to these epigenetic markers, that they need to do a specific job, for example, becoming a skin cell, or an eye cell or a neurone in the brain. Our lifestyle can change the action of this process, by determining which sections of DNA become methylated, and when: even things like stress or puberty can change the expression of our DNA, sometimes temporarily, and sometimes permanently. Recent research has even shown that like DNA, epigenetic markers might also be passed on to offspring.
In one experiment, mice were conditioned to fear a particular smell by being shocked every time they experienced it, and were then allowed to mate with females which had not been exposed to this same conditioning. When their offspring were then exposed to the same smell, they were more sensitive to it than other smells, becoming more easily startled during the time they were exposed. Later, even the grandchildren of the original mice responded in the same way, acting jumpy when exposed to that very same smell. When their neurones were investigated, all three generations were found to have similarities in structures which connected particular neurones in the nose to neurones in the olfactory bulb. These results indicated that somehow the fear of that particular smell was passed down to the conditioned mice’s offspring, and even their offspring.
Another indication of the effects of environment being passed down to offspring is in the case of the 1944 Dutch famine. Infants born during the famine were more susceptible to health problems, including being underweight and having a higher tendency towards schizophrenia. In fact, even the children of these infants were affected by these problems, despite growing up without famine, as the famine had ended by the time they were born. Here it seemed that the lifestyle of their parents affected the DNA of these children.
However, it is very difficult to study epigenetics. Environmental effects can also be passed on to offspring while they are in the womb: when this occurs, this is not due to epigenetics, but this can skew data, as scientists might think that we are seeing epigenetic inheritance in offspring, when actually it is just the impact of being in the womb. To avoid this, fathers need to be studied, rather than the mothers, to look at the transmission of the father’s DNA and find out if any epigenetic markers are passed on.
The idea behind epigenetics goes completely against the old dichotomy of “nature vs. nurture”, showing that in fact, the two can work together, and aren’t polar opposites as was previously thought to be true. This may also bring new ideas for the cause of physical and mental illnesses, as the answer to questions like “is depression due to the environment or genetics?” might actually be that the cause is a little bit of both. Perhaps some illnesses lie dormant in our DNA, but require environmental cues to switch them on, allowing the illness to express itself. The idea that our own lifestyle choices might affect our children in more ways than we previously thought is also a trigger for changing the way we think about inheritance. If this theory is correct, and epigenetic markers can be passed on, then lifestyle choices like drinking, smoking or having a stressful day-to-day life might not only affect ourselves, but generations to come.