This is an issue that I spend a lot of time thinking about. Being sort of closet theoretical evolutionist, I wonder just how poorly equipped our species is becoming with each medical/genetic advance we discover and then apply.
Your question implies the direct application of genetic engineering and manipulation to the human race - which is an area where there is a lot more potential for than has actually been applied, thus far. I propose that our plight has been cultivated by the application of much more mundane medicine - to be honest, mundane might not be the best choice of words here because many of these advances are truly grandiose. However, for every modern advance that has come down the way, we surely have done nothing but weaken the genetics of our species.
Think about every time an antibiotic is administered? Yes, we are helping that person overcome an infection that he/she was otherwise ill equipped to rid themselves of via their own immune system. But look where that’s got us. After only a few generations of using – and grossly misusing – antibiotics, these same organisms we were able to fight off have developed resistance to. Now, not only are these bacteria back weeding out the less genetically fit folks among us, there are literally orders of magnitude more of such people susceptible to infection because they have been able to propagate these deficiencies through procreation. In this instance, saving thousands of people of lives over the years may ultimately cost millions of other theirs. You have to ask yourself: Which situation is better.
What you have to truly realize is that is it more correct to think and apply our medical abilities on the individual. We are able to heal many diseases and disorders that, even as recent as 50 years ago, would have never even been detected, let alone treatable. But there are a litany more that, even though they aren't curable, they are able to be treated and controlled to the point that become nothing more than chronic inconveniences - whereas in prior times, these diseases and disorders would have been lethal. Every time we have been able to sustain a person's life beyond that which the technologies of prior years could have, we are allowing those people the opportunity to procreate. Because we don't treat these disorders and genetic susceptibilities to diseases at the genetic level, the offspring of these individuals still carry these "deficiencies.” And thus pass them along for future generations. This is great for the coffers of the medical establishment – acting as ready-made population in need of their services, but it is definitely polluting our gene pool with lesser-fit alleles that would have been eliminated by the process of natural selection. If medicine wanted to truly live up to its credo of: To practice and prescribe to the best of my ability for the good of my patients, and to try to avoid harming them; and to never deliberately do harm to anyone for anyone else's interest, maybe the medical field should take in consideration what they are doing to the species – something that Hippocrates could not have even contemplated let alone include in his oath.
Where we humans miss the boat at is thinking that we are always one step ahead of nature. What we are realizing is that’s probably a huge miscalculation. We need to take a step back and reconsider our objectives. Is medicine’s and science’s ultimate goal to help individuals within the species or to help the species?