Yes this one gene causes you to have green eyes, but what else does it do?
The human being is a tapestry of millions and millions of threads all woven together in a complex knitting that we have yet to begin to understand. Merrily snipping threads out and weaving new ones in when we have no idea what we are doing is something that could easily end in tragedy.
How about accepting what God gives you and not trying to play God yourselves by "engineering" children? True happiness isn't having what you want, but wanting what you have.. I have one disabled child and one non-disabled child. Both are happy, succesful, God-loving and well adjusted children. I wouldn't change either of them for the world.. the disabled one is more mature and emotionally stronger for the challenges she's faced, the non-disabled one has a good heart and is greatful for the gifts she's been given by God. but She has had to learn that Good is more important than pretty or smart.
Hitler was looking for the master race, I don't see any difference in parents who want genetically "perfect" children..