Bodie has a small patch of black hairs on one back foot - not enough to technically qualify as a tortoiseshell, but enough to keep him off the show bench if he were a pedigree cat. And if he were genetically a tortie, then as a male he'd be 1) known as a "mosaic", and 2) in all likelihood infertile, due to the way the O gene is carried.
It's such a small spot it barely shows, just a number of black hairs speckled through the background colour on his foot. It's interesting, seeing the way it's displayed. Tabby usually manifests as the agouti gene A switching off the black in bands (usually on the tip) of each hair shaft, in a striped pattern, giving the rich yellow background commonly seen between the black stripes on a tabby cat. And then throw the O gene into the mix - and the O masks the action of B, meaning that the background (which had little B to start with) remains its lovely yellow, but without the darker base to the hair, while the dark stripes, which were predominantly B, turn a glorious orange colour. So by rights if he's going to have any black on him you would expect the black to be restricted to the base of the hair if the hairs were within the background colour. Instead the base of the hair is yellow, with the black manifesting as tipping.
The fact that the black exists at all is a mystery. It's quite rare, as the O gene is sex-linked ie it is carried on the X chromosome. Female cats have two of these, so it is possible for females to be tortoiseshell with one X displaying the O gene (which masks, but does not remove, the B or b in whichever form it's carried)while the other X causes the B / b to display. Male cats are XY, and if they carry the O gene on their X chromosome they will be orange as they don't have another X to fight for dominance in colour. So for a male to display black as well as orange (which breeders call "red") he must be genetically XXY, which usually results in sterility.
Tortoiseshell cats usually display a gentle intermingling of the colours, in a pattern sometimes referred to as "brindling" or "speckling" - but throw white into the mix, and all of a sudden your intermingling becomes defined patches of colour. Interesting, no? White masks both colour and agouti, as proven by tortie and white cats (known in the USA as "calico") which display clear tabby markings in their black and orange patches. (and tortie-tabbies are usually referred to as "torbies". Just so you know *wink*).
And if you want to really confuse yourself - the gene a (or non-agouti) masks the tabby pattern, but the O gene acts on a, A, B and b to mask the effects of these genes. So in effect you can have a tortoiseshell who displays tabby markings in its orange patches (where the O gene has overridden the non-agouti gene as well as the black) but no tabby pattern in the black (where the B or b has overridden the agouti).
Which, by extension, means that all red Persians will be phenotypically agouti and, one could argue, genetically agouti as well, even if the A is not present....