what I found on the web

Yes, apparently tortoise shells (of which the calico is merely a subtype) needs a brown gene on one chromosome and a ginger gene on the other, and this seems to be possible only for females. (Except that one vet says that 1 in 3,000 tortoise shells are male anyway. Not sure why, but that’s what he says.)

[ADDENDUM: The small number of male tortoise shell cats are apparently the ones with XXY chromosomes rather than just XY, a rare event. This enables them to get a color pattern from the X chromosomes that is normally restricted to females. So I am told by someone who has read a bit about at genetics.]