In fact this has already been done since Homo Sapiens existed. People choose their partners (or the partners of their offspring) based on features they consider desirable, and hence desirable genes were passed to later generations in greater numbers and eventually dominated the population. Undesirable individuals carrying undesirable genes were marginalized, and many eventually vanished from the population.
Modern technology lets us identify which genes code for which characteristics and either splice them out of the DNA directly, or identify embryos with these genes and terminate them.
This technology, and it successors will have a more profound effect on humanity than any other technology we have seen to date. It will explode the popular notion that intellect and nature are environmental, it will force communities to address what humanity is, and what a human is.
Even if we as a community decide that certain types of genetic experiments shouldn't be done on humans, the technology will be developed anyway.
Few people would disapprove of a technology which allows children with significant deformities to be cured before they are born, so the first wave of experimentation will allow this. It will only take one crippled person shouting at a politician 'why didn't you allow the research which would have prevented this from happening to me' for such legislation to pass. Any democratic legislation which prevents this will not last longer than 16 years.
The next wave will be culling children which have gross abnormalities - which is already happening ('this fetus has Downs Syndrome, you can't be serious about taking this pregnancy to full term'). Then we will see selection of a number of possible offspring, based of the mixture of genes ('this zygote would be left handed, but this one would have a heated temper, do you want to use one of these, or should we start again?') Then we will see the cut-and-splice of designer babies, which will contain the most desirable genes of the parents, with genes from others mixed in, where neither parent had a version which they felt was good enough.
Along the way there are likely to be some dreadful mistakes made. The defects and gross abnormalities in the short term will make the technology nonviable, but the technology will improve, and all of these things will become a reality.
See