He was blind from birth, but I don’t think they ever specifically said it was genetic. Not totally sure.

Th closest we got to an explanation was in All good things when his optic nerve DNA regenerates, curing his blindness.

So…mutated DNA maybe? It seems generic, but there is new evidence every day for epigenetic causes for all sorts of things.

I think it’s safe to say ultra rare generic disorder that pops up essentially randomly and can’t be screened for.