When I was a kid we had an an all-white female cat who produced two litters of kittens each year, like clockwork. It horrifies me now* but that was a different time. She was an indoor/outdoor cat, litter boxes were unheard of, and in ten years I only remember her going to the vet for a rabies shot once when there was a rabies scare in the area. Her kittens were almost always white or dark tabby. (We had an intact all-white male cat but, judging by the number of cats who appeared during mating season, he wasn’t always the father. Or at least not the only father.) We were always told that as long as the white kittens were born with a black spot on their head (which always disappeared when they got older) they wouldn’t be deaf. Without the black spot they would be deaf. For our kittens, that was true. We did have a few deaf ones over the years, always pure white from birth.
A lot of people believe that all heterochromia cats are deaf, but this article says no and supports the “black spot” theory. In any case, Cricket is definitely not deaf and maybe the bit of black on her head will still disappear.
As for heterochromia in cats, as long as it starts when the cat is a kitten it’s genetic. Apparently, genetic heterochromia can occur in cats of any fur colour though it’s more common in the whites. However, if an adult cat has a change in eye colour, it’s likely a reason to visit the vet.
*though my mother always managed to find homes for them. The occasional tabby went to the farm down the road. Not that “farm”! A REAL farm and we would go visit them (and the cows) in the barn.