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.