Someone pointed out to me on another list that the random mutation of having eyes of two different colors might occur about as often as a random mutation for channeling magic does. Only I don't think that people with odd colored eyes produce odd-colored eyed offspring, whereas wizards nearly always produce wizards.

Even Squibs seem to produce wizards if the now discarded example of little Mafalda Prewett is to be considered.

The only thing that makes any sense at all is for it to be recessive. But I have difficulty accepting it being a single-gene effect even then. There is just too much variation, not to mention oddball traits like Parselmouth and the abilities of a "true" seer.