What has attracted even more attention in the media is cases of identical twins who were separated at birth and brought up in different families. When reunited the twins often seem to be uncannily similar, even down to trivial matters such as the kind of dog they have or the names they give to their children.
People usually feel uncomfortable when they hear of such cases. Most of us are not members of a twin pair but the implication seems to be that we are much more determined by our genes than we like to believe. But is that true? Spector is a geneticist who has made a special study of twins, and anecdotes based on his findings figure prominently in the book. His researches have led him to question the prevailing assumptions about genetic determinism and here he explains his reasons.
Spector believes that the differences between twins are as important as their similarities; the twins themselves often insist on their own individualities. These differences can be quite profound; there are even rare examples of identical twins who have different eye colours. But how can this be, given that identical twins have exactly the same genetic inheritance?
The answer is epigenetics. This is an old idea that has recently acquired new importance thanks to discoveries that help to explain the underlying mechanism. The human genome contains about 25000 genes, or possibly fewer (a surprisingly small number). The same genes are present in all the cells of our bodies, yet these cells differ widely; there are about 200 types. So only certain genes must be switched on (expressed) to give rise to muscle cells, nerve cells, skin cells and so on, while the rest are switched off. How this happens is a fundamental problem in embryology that is still not properly understood.
Gene expression does not occur only during the development of the embryo. It continues throughout life and this is what helps to explain the differences between twins, as well as how all our characteristics are shaped not only by our genes but also by our life experiences. Once epigenetic change has occurred it is often long-lasting. What is even more remarkable is that it may persist over the generations; the experiences of your parents or even your grandparents may influence your own characteristics and health. This 'soft inheritance' means that the much-derided ideas of Lamarck—the inheritance of acquired characteristics—may not be as wrong as is widely believed.
Spector identifies two mechanisms by which genes are switched off. One is methylation, in which "at certain sites (usually cytosine bases) of the gene's DNA, small chemical methyl groups … floating around the cell attach themselves to it, rather like sticking an olive on a cucumber with a cocktail stick. This has the effect of stopping the gene producing a protein." The other mechanism is histone acetylation, "which alters DNA folding"; no further details of this are given.
Having established the importance of epigenetic soft inheritance, Spector turns to a detailed consideration of the relative importance of nature and nurture in various contexts that have been the subject of media attention. There are chapters looking at happiness, talent, religiosity, criminality, mortality, cancer, sexuality, and marital fidelity, mainly in the light of twin studies. All these contain plenty of surprises; Spector does a good job of debunking much of the popular 'wisdom' that has become attached to these topics. The general impression one gets is that the role of inheritance, while certainly far from negligible, has usually been over-stated.
Although the book is mostly concerned with human genes, Spector makes the important point that we all carry a huge variety and quantity of bacterial genes on our skin and especially in our gut. Recent research suggests that these may be more important than we generally think. They may affect our weight and cause or prevent allergies, ulcers, or cancer. We still know little about this but it provides an additional reason for caution in the use of antibiotics unless they are strictly necessary.
At the end of the book Spector concludes that epigenetics allows us to "rewrite irreversibly" at least four key genetic doctrines. First, we now know that DNA is not the single most essential determinant of who we are. Second, our genetic inheritance is not fixed and unchanging but can be modified. Third, a single environmental event can plant a lifelong memory in our cells that is not obliterated when cells divide. Fourth, the experiences of your parents or grandparents, such as famine or diet changes, can influence you and your health.
Spector provides a very readable survey of important recent research in epigenetics. In fact for me he leans a little too far in the direction of readability and popularisation. Children are almost invariably referred to as "kids" and the phrasing is sometimes loose to the point of being misleading, as when mitochondria are described as "the body's energy cells". I think that "cells" in this context is based on the analogy of electric battery cells, but the unwary reader may think that mitochondria are another type of body cell like muscle or nerve cells rather than a component of those cells. (To complicate matters further, the mitochondria were originally free-living bacteria that in the remote past were incorporated into another cell to give us eukaryocytes, but is that what Sector has in mind?)
I also found the author's optimism to be somewhat excessive. Epigenetics does not eliminate determinism as effectively as he implies. Yes, you can change your genes by what you do. But we still have little knowledge about what we can or should do for the best effect. And if we are influenced by what our parents or grandparents did and experienced, does this not simply exchange one kind of determinism for another?