You are obviously not a parent.
Giving your kids the best chance of success takes strong lessons of morality, honesty, character building and hard work ethic not hocus pocus 'your genes tell you you will be good at x'.
Focusing on one child over the over is the perfect way to make one successful over the other child. Is it worth it to destroy one of your own offspring at the cost of another's chance of success just because 'the gene test' says so?
No.
Besides, scientists haven't been able to cure most of the world's deadly diseases in all our years of existence. Seems kind of sneaky that we can all of a sudden jump to gene manipulation with any success...