At the same time, this is three hundred years from now. Genetic research will have changed significantly, and will probably have reached the point where it can safely change small things. You could write a predisposition for certain diseases, and since the future will most likely involve entirely socialized versions of current medical systems, it's not unlikely that major terminal or lifetime diseases will be written out. This doesn't mean you can't have cancer, it just means that it would happen less.

On the flip side, genetic changes that are mostly aesthetic would most likely not take off. Research would involve risks (early genetic design will involve plenty of 'Oops, we gave you a cancer.'), and it would be morally questionable. People won't exactly like it when the rich can turn their children into ubermensch, and the opposition would be pretty strong. You might see, say, the Chinese get into some limited amount of genetic testing, but as the world grows more and more globalized it seems like it would be a destabilizing factor that doesn't get you much bang for your buck.