This squirrel, whose photo was sent in by reader Craig, looks as it if fell into some bleach. It is probably a mosaic for leucism, though it could also be a true albino in its nether parts (I’m guessing the former).
There are two possibilities here. One, perhaps less likely, is that the zygote of this animal was homozygous for leucism or albinism. (Leucism and albinism involve different recessive genes altering pigmentation. I’ve seen albino squirrels, and I know there are also white squirrels with leucism.)One of the “white” alleles could have mutated to its wild-type form in the embryonic head region during development, giving it a normal color in that part of the body.
Alternatively, the animal could have been heterozygous for either the melanism or leucism allele (one copy of a white-color allele, the other copy for normal color), but then one of its “wild-type” alleles mutated to the white form during development, making it white behind the neck.
There are probably other genetic possibilities at all, but I’m too tired to think of them.
Perhaps a reader can tell us which gene has mutated here, though I suspect you’d have to genotype the animal. If the head were white and the eyes were pink, it would certainly be an albino.