See, a cat’s coloring is determined by a set of eight different genes. They’re what tell a black cat to be black, or an orange cat to be orange. And then there are modifier genes — these can cause a cat’s fur to be patterned in a certain way, or mask the color altogether, like you’d see in the white bib-and-spats on your classic tuxedo kitty.
But Siamese and similar cat breeds have a special modifier gene called a Siamese allele that mutates the color gene. It inhibits pigment — in other words, it causes albinism.
But that modifier signal only gets through to a kitty’s fur if it’s above a certain temperature.
This modifier starts sending out its “stop the color!” message around 100.4-102.5 F (38-39.2 C), which is a cat’s standard body temperature. Anything lower than that and the mutation is blocked and the color gene can then do its color thing.
Because a cat’s body is cooler around his ears, paws, and tail, that’s where the color begins to kick in. In essence, these cats are walking heat maps.