Following on aeh's excellent point, even if there were a perfect one-to-one mapping of "undesired" traits to specific alleles, genetics aren't deterministic. There is currently a very limited understanding of the role of epigenetics in gene expression.
I'm reminded of the old joke about econometricians. Three professors are tasked with studying a black cat that has been placed in a windowless box without moving the box: a mathematician, a macroeconomist, and an econometrician. The mathematician tries first and is quickly driven mad by the ludicrousness of the task. Next, the macroeconomist tries and claims he has devised a series of graphical models that map the movements of the cat. Finally, not to be outdone, the econometrician studies the box for a short while, then declares that he has caught the cat by the scruff of the neck.
