The cover story of the January 18, 2010 issue of TIME Magazine is entitled, Why Genes Aren't Destiny. You can read it here: http://www.time.com/time/health/article/0,8599,1951968,00.html
The article discusses important new research into the role epigenetic 'marks' play in switching genes on or off, and in regulating their expression--as author John Cloud puts it, telling them whether "to speak loudly or whisper." Salk Institute Biologist Joseph Ecker makes the analogy that if your genome is like computer hardware, then the epigenome is its software. The epigenome consists of cellular material that sits atop the genome, and unlike the genes themselves, which in evolutionary terms, change very slowly over time, the chemicals in the epigenome are relatively reactive and respond quickly to environmental stressors, including nutritional factors and chemical exposures.
Moreover, it seems that changes in the epigenome may be carried over immediately from one generation to the next, as the sort of "inheritance of acquired characteristics" disdained by Darwinian evolutionary theory.
Autism was traditionally held to be a genetic disorder, or disorder of the genome itself. Researchers long pinned their hopes for the development of effective therapies on discovery of 'broken' genes, which have yet to be identified. That straightforward genetic model, however, failed to explain the rapidity of the increase in the prevalance of autism, the heterogeneity of its symptoms, its differential distribution between boys and girls and the fact that in sets of identical twins, whose genetic make-up is the same, one might have autism and the other, not.
Many now argue that autism is more properly regarded as an epigentically-modulated disorder, and that to continue to look for broken genes is a bit like concluding that a lightbulb has burned out without first having checked whether or not the switch has been turned on. That genetic switch has finally been identified. Epigenetics appears to offer great explanatory insight into autism, particularly into how and why it manifests so differently between individuals. Stay tuned...