A dense, long, but important summary of the limitations and complexity of the scientific genetics of mental illness...
There are many reasons for the limitations of genetic research in psychiatric illness.
The complexity of the genetic processes is much greater than previously thought. Mutations are most often in the large regions of regulatory DNA. Regulatory RNAs produced in one spot can, in fact, influence many different genes in different location and in different ways. Studies with cancer show that not one, but many mutations are necessary to create specific cancerous cells. And these mutations are different in each individual, which makes treating cancer much more complex. This same situation may be occurring with a psychiatric illness where multiple different mutations are needed to produce an illness and each case might be different.
Even more significant, perhaps, is the fact that genetic networks have multiple layers of regulation that are only vaguely understood. These layers all operate simultaneously and interact in unknown ways. The shapes of DNA have great impact. The place of the DNA in the nucleus is a factor. Tags on both the histones and the DNA itself are critical in determining which DNA can be used in specific circumstances. Many different epigenetic tags have now been described for both histones and DNA. Research shows that alternative RNA splicing is greatly enhanced in humans and in particular in the human brain. It is now clear that RNA alternative splicing produces a large number of unique proteins that have been instrumental in the development of the human brain. Also, there are many thousands of other factors, both proteins and RNAs that regulate all aspects of the DNA transcription and RNA translation process.