The truth is that humans, like all other animals, have been selective breeding all the time, it's called evolution. We select our mates based on their phenotypes (like aggression, intelligence etc). It was more so in the past than now. People used to abandon infants that are born "defective" (we now have abortions). Parents selected the mates for their children because they are believed to be better judges of "good" phenotypes. It was customary to ensure that the family of the respective mate has no members with mental illness. Yet, after millennia of selective breeding, we still have all the crazies among us. The fact that those "bad" genes still exist must mean that they confer some benefits to the carrier.
The interplay between genes, RNA, proteins and other macromolecules is so complex that I doubt we can pin down a mental illness to a single gene or safely remove a mutation that causes mental illness from the gene pool without reducing other collateral beneficial traits.
The benefits that natural selection causes to propagate are those which improve the organism's reproductive opportunities. Those "benefits" aren't necessarily all that great when you've got a civilization. To take it to a ridiculous extreme, if a gene exists which makes you kill all the other men in your village, then impregnate all the women, that gene will be pretty popular in the next generation.
That ridiculous extreme must be quite a popular gene, because it still exists: It's called war and mass war rape.
Yet, we are not all warmongers and rapists (I hope). So civilisation is already putting its slow but steady evolutionary pressure onto our genes. Let's hope that it's putting our gene pool onto the right track.
I simply don't share joonix' optimism that genetic screening can eradicate "unbeneficial" traits within next centuries without wrongly removing "beneficial" ones, before we have a better and complete understanding of our biological makeup.
This is also a bit simplistic I think. I don't think most abortions are because of a physical defect or deformity, or at least, many abortions are not. As well, in many cultures that selected mates for their children, it usually did not exactly involve phenotype checking, but usually other kinds of economic and cultural reasons. Also, the fact that "bad" genes still exist does not mean they confer some benefit, that's a very panglossian view.
The interplay between genes, RNA, proteins and other macromolecules is so complex that I doubt we can pin down a mental illness to a single gene or safely remove a mutation that causes mental illness from the gene pool without reducing other collateral beneficial traits.