My guess is that DRD is under diagnosed because of reduced or low penetrance in some cases. As far as I can tell, for the majority of our lives, no single person in my immediate family would fit the diagnoses, but as a family, we apparently do, especially if someone knows where to look and what to look for. I suspect my extended family on my grandma's side has it as well, but I doubt any family with it would be diagnosed unless they happened to have someone who presented with it in an obvious way.
The urgency you're detecting in my posts is because in my experience, 10+ doctors and 4 neurologists missed it and/or mistook it for other diseases. It was especially hazardous for my grandmother because they were insisting on doing the medical version of taking her out back and shooting her even though their treatment of her in medical and custodial care settings was causing the very symptoms they claimed were due to other disorders. And yes, I do not want someone else to go through that, especially since having other relatives who also respond to carbo/levo seems to be critical in helping establish a diagnosis when there isn't someone in the family who presents in a typical way.
If you want, I can modify my post, but like I said before, I strongly suspect the presentation of ADHD with other characteristics, which are admittedly not symptoms of either disorder, increases the odds that someone has DRD or some other neurological disorder that's also contributing to the presentation of their ADHD.
If you have some compelling evidence that there is no interaction between DRD or similar and ADHD, I'm happy to see it, but from personal experience that is not the case.
I'm here because I'm learning to write software, but I wouldn't pretend to understand the paper or mathematic theory behind, say, Paxos because I recognize some terms in it. I know my limitations, and all I ask is that you consider them, as well. The problem with raising awareness as you are is that it's difficult to objectively remove yourself: Segawa's is _incredibly_ rare, making all of the events in your family difficult to explain. Scientifically, transferring your experiences to others is a very tall order without much further study (and I hope you, and your family, are being studied, specifically because the genetics of Segawa's are poorly understood). What people portray as medical professionals swinging the hammer on "the information that could make us irrelevant" is actually trying to stand fast to the exact same scientific discipline that you, and every member of this audience, does in _every single other context_.
Your speculation about the relationship between ADHD and Segawa's is so far off base that a second-year medical student could probably explain why; you just happen to have been diagnosed with both. Left unspoken: it is quite possible to have both, because they present and manifest in different ways.