I don't find that a nosy or rude or obnoxious question at all. :-) We're going with the theory of a genetic hiccup - we haven't had the tests done partly because of lack of insurance and also because it really doesn't change anything for us - it seems to have no other effect, and the doctors basically agree that there's not much that the tests would do except cause us to spend a lot of money.
(The technical reason for his blindness is a combination of factors: something called 'microopthalamus' (the pupils are abnormally small) and 'coloboma of the retina' (the center of the retina never fully formed past about month 5 in utero). It ends up being a condition which affects different people differently within a certain spectrum: which is why we believe Sean has a very small amount of vision in the lower portion and peripheral areas of his eye (nothing really useful as it happens) and Chris has light perception but nothing else.
Did that make sense?