I would not worry about this in terms of believability. I don't think humans have the possibilities to influence our biological lives in this aimed and controlled ways. I think of it as superstition. Now, having said this, that doesn't mean others will think of this as possible or even probable. That's the real problem, I think. People are believing all kinds of things which match their inner wishes. Transhumanism for me, is a naive and immature idea, resisting the thought of ones mortality and wanting it to be not true. That's laughable, in my view. 

Not for a moment I would ascribe such power, precision and other super qualities towards genetic manipulations. Those who do seem wanting to play god. 

One thought though remains: if people are of a mentality which thinks of all this manipulations as possible, will it become true because the power of thought influences it? I don't know... I wonder...