6.2 Genes, environment and development
Caspi et al.’s (2003) results suggest how gene–environment interactions (sometimes written as GxE interactions) might help explain the incidence of some disorders such as major depression, though the precise mechanisms involved (i.e. exactly how the two interact) are not known. However, understanding of genetics has recently undergone a profound paradigm shift in the light of evidence that the environment can affect the working of genes in ways that were thought impossible in the recent past.
To have an effect on the human phenotype a gene must be used to make a protein, a process known as gene expression (paragraph 2, Box 3). Scientists have recently shown that gene expression can in effect be switched on and off through epigenetic mechanisms (paragraph 3, Box 3).
It is important to note that the genes themselves (that is, the DNA, and the codes it provides for making proteins) are not changed by the action of epigenetic mechanisms – the genes remain exactly the same. Many common environmental factors or experiences are now known to trigger epigenetic mechanisms, including diet and exercise.
What is it that changes, then, when epigenetic changes occur? Think of a factory with lots of machinery for manufacturing different kinds of chocolate. If one of the machines is switched off, then that kind of chocolate is no longer produced. The machine is still there, unchanged, and it has not lost the capacity to make this particular chocolate, but it cannot make the chocolate because it is switched off. In this analogy, the machine is the equivalent of the gene or the genetic code for making a particular protein, and the epigenetic change is the equivalent of the switch – it can switch genetic machinery for making particular proteins, on or off.