Smoke is caused by the combination of the dominant Inhibitor gene with the recessive Non-agouti gene. The Inhibitor gene gives rise to a pale undercolour. The extent of this undercolour is variable and the smoke can be described as light, medium or dark depending on the amount of top-colour (tipping, known as "veiling" in smokes) and the extent of the pale undercolour i.e. how far the pale colour extends along the hair shaft. Occasionally a Smoke can be so light in colour that on visual inspection it is Shaded cat (e.g. a very light Black Smoke resembling a Shaded Silver), although true Shaded cats have the Agouti gene, not the Non-agouti gene.
In smoke cats, the undercolour varies from almost white to a bluish grey. Cats with darker undercolours may look self coloured, especially in black cats. It is possible to have a cat which is genetically a smoke, but which is visually solid black! The heavy tipping combines with a very narrow white hair base so that the smoke effect is lost. This variation in the relative amounts of undercolour and top-colour appears to be due to modifier genes and mirrors the way in which modifier genes act on shaded silvers to produce both shaded cats and chinchillas (and often in the same litter). Some smokes, known as overlaps, never develop the silvery undercoat and only prove to be genetically smokes when bred.
Smoke kittens sometimes show faint tabby markings (as seen in the photo on this page). Young smoke Persians look almost solid in colour except for a silver tracery (clown lines) where the agouti areas would appear in a tabby. Not all kittens have clearly visible clown lines, but those that do have them tend to develop more striking adult coats. In adults, the presence of clown lines is penalised. The smoke effect is more obvious in longhaired cats and is not restricted to black smokes. Other colours can be combined with the Inhibitor gene to produce blue smokes (black + dilute modifier + Inhibitor gene), chocolate smokes, lilac smokes and red smokes. Black, blue and red are the longest established and best known varieties. Some of the "newer" colours have appeared historically and were probably dismissed as "poorly marked" individuals in one of the then known colours.
The Red (Cameo) Smoke is produced by a combination of the Inhibitor gene and the gene for red. These have a red/orange tipping on a whitish undercolour. Because of the way in which red colour is inherited (Tortoiseshell and Tricolour Cats), the males are red, but the females may be either red or tortoiseshell.
The Non-agouti gene has no effect on the red colour as it is produced by a different pigment. This means that the difference in appearance between Red Smokes and Red Shaded Silvers/Red Chinchillas is not due to the cats having the Agouti or Non-Agouti gene as it is with the other colours. The Agouti gene affects non-red colours such as black, blue, chocolate etc so it does make a difference in whether the non-red areas of tortoiseshells turn out as smoke (Non-agouti) or as silver tabby, blue silver tabby etc (Agouti).
The difference between Red Smoke, Red Shaded Silver and Red Chinchilla appears to be due to the Inhibitor gene causing the pale undercolour and various modifiers dictating how far along the hair shaft the undercolour and top-colour extend respectively.
The Cream Smoke, Cream Shaded Silver and Cream Chinchilla are due to the presence of the dilution gene in addition to the red gene. In the tortoiseshells, this additionally turns a red/black tortoiseshell into a blue smoke tortie or a blue silver tabby tortie.
Shorthaired solid colour cats tend to have have ghost tabby markings which are particularly visible in kittens. As a result of ghost markings, Smoke shorthairs may appear to be Smoke tabbies. Hence the Black Smoke Egyptian Mau has visible spots on a Smoke background. According to Phyllis Lauder's book "The British, European and American Shorthair Cat" (1981), the Smoke factor can turn up in unexpected places. Lauder had a Blue Cornish Rex with underfur of the ash-white colour proper to a Smoke. When the cat was 2 years old, she noticed that the fur behind his ears looked silver. At the time, he could not be described as Smoke according to the standards for that colour (which demanded a light silver fril; and ear tufts and an ash-white undercoat tipped with blue), but he was nevertheless a Blue Smoke. The appearance of the smoke factor was not always welcomed by breeders. In particular, there were complaints that Red and Cream longhairs showed an undesirable degree of white in the undercoat.