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This paper contains four parts. The first focuses on The Human Genome Project (HGP). The concept of a "normal sequence" seems problematic. The second part reviews what is meant by "genetically-caused" and by "a (several) gene(s) for a trait". Genes work in concert, and it is likely that many genes together influence most complex traits. A difference in traits between two individuals, however, can be due to a difference in a single gene, as is the case for several thousand rare "genetic diseases". Such diseases were the original focus of gene therapy. Could gene therapy also be applied to more common complex traits? Diagnostic problems and the unsuccessful attempts to find a single gene difference in patients with manic depressive disorder, schizophrenia and Alzheimer's disease are discussed. Part three reviews future genetic research into complex traits and what kind of treatment can realistically be expected. It is concluded that improved diagnostics is the most likely outcome, not therapy. Part four raises arguments from the emerging Darwinian approach to medicine and states that it is far too early to even think of gene therapy. The needed knowledge about genes, behaviour and disease will not come from the one-sided genetic effort represented by HGP, and the resources should instead be invested in several different approaches to these complex issues.