Why cats look the way they do

Turing’s last paper before he died was called The Chemical Basis of Morphogenesis. It’s a strange one. There’s no mention of computing, no breakdown of any specific organism, and the experimental support is, charitably, absent. What it has instead is a beautiful mathematical argument: a sheet of cells, two chemicals diffusing, certain conditions on the reaction kinetics, and spontaneously, without anyone telling it to, the sheet develops a stable pattern.

For decades this was treated as a curiosity. Then, slowly, biologists started catching up. The genes that produce tabby stripes in cats turn out to involve exactly the kind of activator-inhibitor coupling Turing described. Recent work on the Dkk4 gene in feline embryos has even pinpointed the activator: a signalling molecule that, at the right developmental moment, kicks off a reaction-diffusion process across the embryo’s skin. The pattern that emerges is locked in for life.

This is why your cat looks the way it does. Not because the genome stores a picture of stripes, but because it stores the parameters of a pattern-forming process — feed rates, diffusion rates, kill rates — and the stripes happen by themselves, in the embryo, in a few hours, from chemistry alone.

The dramatic part: small changes to those parameters produce wildly different coats. A bit more inhibitor decay and you get pure spots. A bit more activator and you get marbled mackerel. A tiny shift either way and you get a uniform colour — which is what most of the larger cats outside the leopard lineage actually have.

In Felinotype, the (f, k) pair you see in the card is doing precisely this. Each hashed seed picks a different point in that parameter space and runs the simulation. There is no template — the same code that produces a thin striped tabby produces a marbled bengal, by changing two numbers.

The species names in the breed are mostly real (Felis silvestris, Felis lybica, Otocolobus manul); the varieties are invented in a Linnaean style. The actual taxonomy of cat coats has things like agouti, non-agouti, blotched tabby, spotted tabby, ticked tabby — they are all the same gene operating with slightly different reaction kinetics.

What’s worth noticing: the same equation also describes spots on giraffes, stripes on zebra fish, the arrangement of feathers on a chick, and the bands on a tropical seashell. The pattern-formation problem turned out to be one of nature’s favourite tricks, and Turing saw the whole shape of it before there was any evidence at all.