Nature or Nurture : it’s all in the (epi)genome

It’s an age-old debate : are we destined at conception to be the people we turn out to be, or are we products of our environment? Murderers, nobel-prize winners, world leaders, musical virtuosos – even your average law-abiding citizen from Witney, Oxfordshire: are they born or made? Most reasonable people would intuitively think that we are – to some extent at least- a mix of both. But what kind of mix? When you start getting down to a characteristic like academic performance it gets quite difficult to determine where on the nature-nurture continuum the trait lies. Science is helping us build a more concrete picture about the link between genetics and the environment, through a field known as epigenetics. We are starting to learn more and more about how the environment can lead to epigenetic changes – biochemical modifications which affect which genes are expressed (or “switched on”) and which remain silent, hidden away in the depths of our genome.

Take the effects of stress for example. Studies on mice have found changes in offspring’s response to stress depending on how caring the mother was – i.e. how much licking and grooming behaviours she carried out. This was found to be the result of epigenetic changes – high amounts of nurturing behaviour led to decreased DNA methylation. In turn, decreased methylation allowed for increased expression of the glucocorticoid receptor genes in the offspring, starting a chain of reactions which ultimately led to decreased cortisol production.

Conversely, low levels of nurturing behaviour resulted in increased cortisol release. Increased cortisol release has been well-established as a reaction to psychological stress. Thus, offspring were more likely to react to stress if they had not experienced as much licking and grooming from their mother. Interestingly, humans also display this release of cortisol in response to stress, and cortisol levels in humans have been demonstrated to be heritable. These mouse studies may therefore shed light on human reactions to stress.

Epigenetic changes have been related to a wide variety of diseases and conditions which are influenced by lifestyle and environmental factors, such as cancer and schizophrenia. Studies have even shown epigenetic changes to be maintained over generations. But isn’t it a given that changes that happen to parents during their life get wiped at every generation? Haven’t we all been ridiculing Lamarckism for over 100 years? A study published very recently in Science described how most epigenetic changes are wiped when the precursors to eggs and sperm, primordial gene cells, are made. But the study also helped explain how parental experiences can affect offspring, as sometimes the cells are not completely “reprogrammed”. These exceptions allow epigenetic changes to be passed on to the next generation – occasionally. The actual significance of this in evolutionary terms is still uncertain, but perhaps Lamarck wasn’t quite as off the mark as we thought he was…

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