I recently picked up Nessa Carey’s The Epigenetics Revolution: How Modern Biology is Rewriting our Understanding of Genetics, Disease, and Inheritance. I plan to blog through the book as I read it, making posts on the study notes from a few chapters at a time. After I’m done reading (and blogging about) the whole thing, I’ll write a cumulative review of some sort. I have some understanding of how Dr. Carey writes, and I think this review scheme would be the most productive one for a book like this.
I’ve hardly read any other author who can so effectively break down complex topics in biology for consumption of an average-to-intermediate reading level audience. Dr. Carey is extremely adept at build-up- she takes her good time to gradually introduce novel concepts, making liberal use of innovative and intuition-boosting analogies along the way. Her explanations of facts and phenomena are peppered with references to relevant real-life events and general comments about scientific research. That sort of ‘couching’ may as well be the most balanced way to present information on a topic like this. All in all, this book is a nothing short of a stylistic masterpiece in this genre. My post series, on the other hand, would record the study notes only- which means having to strip away all the beautiful linguistic devices she makes use of and present the bare facts about epigenetics. That’s almost a crime.
The reason this disclaimer is necessary is because I don’t want anyone to get the impression that there’s nothing more to the book than my modest study notes. The book is an experience all on its own, and enthusiastic readers looking for their biology fix are highly recommended to get hold of their own copy.
One way to look at the book is as a sustained critique of what can be called DNA/genetic reductionism- the idea that DNA controls everything in our body, and that an exhaustive description of all biological processes can be given in terms of the way nucleotides are arranged. This idea is incredibly common among the scientific laity. There are definite appeals to this view, no doubt. It presents a Cartesian dualism-esque portrayal of how life functions- there’s a part of the cell with all the information (DNA), and other parts that do all the work (proteins). It’s a really simple, clear-cut way to sum up basic biology.
Reality, of course, is seldom as simple (and all the more so in biology). A very stark counterexample is presented by twin studies in Schizophrenia. Schizophrenia is proven to have genetic basis, and not surprisingly, when one of an identical twin pair has Schizophrenia, the other has a high chance of having it too. What’s puzzling is the lower than expected rate at which this correlation holds. Since identical twins are genetically- well, identical, and since Schizophrenia has a genetic basis, shouldn’t both twins always have this condition if any one of them does? But in reality, the identical twin of a Schizophrenic patient has a much lower chance of having this condition (50%) compared to what’s expected (100%). Clearly, genetic determinism can’t explain this. If genes dictated everything about our biology, then identical DNA would mean identical organism. That’s not what’s happening in this case.
Here’s a more elaborate case study. In post-WWII Netherlands, a generation of people were subjected to a really harsh famine, leading to the death of some 20,000. Tragic though it have been, epidemiological records collected at this time led to a fascinating scientific observation about the children who were born in this era. Consider the following two groups of children:
(A) This group of children were conceived during the tail end of the famine, and hence their first few months of pregnancy were spent in nutritionally dire conditions. However, food supplies arrived soon, and the mothers were relatively well-off during the last few months of pregnancy.
(B) This group of children were conceived just before the famine started, and so their first few months during pregnancy were normal. The famine rolled in soon, however, and the mothers spent their last few months of pregnancy deprived from adequate nutrition.
The children of these two groups showed some remarkable variations in their biology. Group A children, for example, consistently showed a higher rate of obesity, while those in Group B stayed small- and these effects stayed with both groups throughout their lives. To top it off, even the descendants of this latter group shared these effects.
Clearly, all of this indicates that an extra-genetic effect is being exercised on health, and even heredity. It seems during the early months after conception, nutrition or lack thereof puts a certain impression on the DNA, one that stays until death and may even pass on to progeny.
The above examples demonstrate the falsity of DNA/genetic reductionism, and lays the groundwork for introducing epigenetics. Simply put, epigenetics refers to the extra-genetic biological mechanisms which account for differences among genetically identical individuals.
One may be tempted to say that all these examples lead to is the rather obvious conclusion that both nature (genes) and nurture (environment) have a role to play, and there’s no reason to posit a biological mechanism when the environment can explain these effects just as well. That, however, leaves a few questions unanswered. It’s a genuinely bizarre fact that environmental impressions at early months of development not only stays with you for all your life, but can also pass on to the next generation. Which means while the source of this extra-genetic influence may be the environment, there’s a biological mechanism which translates the environmental effect to something innate to the organism (in addition, environment can’t account for all such deviations either- consider the twin study example cited earlier. Identical twins usually grow up in similar environments, and the difference isn’t drastic enough to account for the difference).
This gives us another useful way of defining epigenetics: it is the way of spelling out, in biologically precise terms, what “nurture” is. What does it mean to say that environment shapes a person’s health (and even offspring) in a certain way? Which mechanisms are at play here? That’s what epigenetics seeks to answer.