My DC don't have a rare genetic condition. But I do - a very serious one. Mine's inherited from my parents (recessive, so they don't have it themselves). I have never for a moment thought that they are to blame or that they didn't make me properly.
Medical conditions are a part of life. Lots of people have them. As a society we still have a disastrous attitude towards disability - that perfect health is the 'norm' and disability is a deviation. It's not. It's part of the norm. It's normal to have a disability, it's normal to have a child with a disability. What shouldn't be normal is the huge barriers that we as a society put in disabled people's way - badly designed buildings, vicious or thoughtless attitudes, inaccessible public transport etc. etc. That's what's wrong.
I think that for you, and for everyone else who is suddenly brought up short by a disability that affects them or someone they love, it is a massive shock because you are - or the person you love is - suddenly part of that abnormal Other you've been seeing as 'different' all your life. It's easier to throw yourself into wanting to 'cure' them, than it is to confront the (even) bigger task of changing society step by step.
Please don't think I mean this unsympathetically, as I am very aware of how you feel and have watched my deeply caring parents feel the same way. But as I get older and my disability progresses I am more and more aware of how hurt I feel that everyone, everywhere, sees me as Other.
Well, not everybody. DH and the DC don't.
The biggest gift you can give your DC is to embrace the normalness of who they are, medical condition and all. That's different from loving them, which of course you do.