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Genetics of Silver & Smoke
by
Patricia Turner & Roy Robinson





The precise heredity of silver, chinchilla and smoke has been the subject of debate in the fancy for a number of years especially between the more discerning cat breeders, who felt that their experience contradicted the current explanation of recesive inheritance of silver and smoke, and geneticists, who based their opinion on the published evidence.  This article presents breeding results which show that the old explanation will have to be revised.

But firstly, a little history.  In 1933 Clyde S.Keeler and Virginia Cobb published the results of crosses between smoke and black longhairs and silver, smoke longhairs and Siamese (i.e. smoke x black longhair, smoke x Siamese, etc.) which indicated that the gene for silver was inherited as a recessive to full colour but dominant to Siamese.  Since Siamese is known to be inherited as a recessive to full colour (i.e. black, blue, etc.), these results implied that full colour (represented in the crosses as tabby or black), silver or smoke and Siamese were a series of three alternative genes.  In support of this conclusion, silver and Siamese were likened to the Chinchilla and Himalayan of the rabbit which are known to be inherited in this manner.  Anyone who is familiar with these varieties of rabbit cannot fail to be impressed by the similarity of appearance of silver and chinchilla and Siamese and Himalayan.  The difference between silver and smoke cats is that silver is agouti while smoke is non-agouti.  There is no disagreement on this point.

The main reason why debate has been so protracted is that no-one seemed willing to shoulder the bother and expense of making the crosses which would solve the problem.  We have made several tentative crosses since 1968 but in 1970 a definite breeding plan was outlined and put in hand.  Two Chinchillas and two silvers were mated to Siamese, producing over 8 silver or shaded-silver (shorthaired) kittens.  On maturity, these were mated to Siamese producing the following:
 

Silver or Smoke
Full Colour
Smoke Siamese
Siamese
17
13
2
27

This backcross generation is decisive and deserves to be examined in detail.  On the Keeler-Cobb explanation, the 15 kittens of the two center classes should not have occurred.  In more detail these are 2 black, 1 blue, 1 lilac, 1 blue-cream, 2 blue tabby, 6 brown tabby and 2 Siamese smoke.  Now, it could be argued that the first 7 kittens are in reality smoke in which the white undercoat has either failed to appear or is indistinct.  This is true and is recognized.  However, the 6 brown tabbies were unmistakeable. The two smoke Siamese were also unmistakeable in that they were Seal except for the white undercolour of the smoke and distinctive tabby facial markings.  Only two of these have been definitely observed but others may well have been bred.  It is well known that young Smoke kittens are not always easy to classify.  The majority of the Siamese in the last group are agouti and we have found it impossible to distinguish the young kitten that is the Siamese equivalent of the Shaded Silver from an ordinary agouti Siamese (Tabby Point).

We have concluded that the above breeding results are consistent with the heredity of a dominant gene which is independent of Siamese.  Those breeders who have been maintaining that silver and smoke are inherited as a dominant are thus vindicated.  However, it has not been possible hitherto to show that the gene is independent of Siamese and this has now been accomplished..  We have symbolized the gene by I (denoting inhibition of hair melanin).  The action of the gene is to partially inhibit the formation of melanin in the coat--the effect being most pronounced for those areas which are least pigmented.  That is, the basal portion of the hair (to produce a white or very light undercolour) and the agouti areas between the tabby pattern (to produce white or very light areas with dark ticking).  The tabby pattern persists as concentrations of heavily ticked hairs over a white undercolour.  This is the silver and it is easy to perceive why it should be confused with a chinchillated animal.

A chinchillated animal is one in which all (or most) yellow pigment is removed.  It so happens that the agouti areas most strongly affected by the action of the I gene are those which manifest most of the yellow pigment in the coat.  The mongrel silver often shows a suffusion of yellow on the head and back but this has been eliminated from the exhibition silver by selective breeding.  Small wonder that the silver resembles a chinchillated animal but the resemblance is misleading genetically.

In the smoke, the white undercolour persists but the action of the non-agouti gene is to extend the black ticking to all parts of the coat.  The points become very dark or black and the amount of white undercolour is reduced.  There is considerable variation of undercolour as a matter of fact.  In some animals the undercolour is obvious, with only the tips of the hairs being pigmented.  In others, the undercolour is merely lighter than usual and the cat may, superficially, be mistaken for an ordinary black.  Indeed, there are indications that some smokes so closely resemble blacks that even experienced observers are confused.  The occurrence of smokes that are apparently black explains the misleading fact that two blacks may produce smoke kittens if either parent has a smoke sire or dam.

The chinchilla cat is an interesting breed, particularly in relation to the silver.  That the two are related is obvious.  In fact, for a short while in kittenhood, the chinchilla looks like a silver but quickly loses the tabby pattern, becoming almost white except for fine black tipping.  It has long been accepted that the chinchilla differs genetically from silver by the possession of modifying genes increasing the white undercolour.  There is no reason to change this supposition except perhaps to make it more precise.  The chinchilla is the extreme expression of the I gene, where the pigmentation has been confined to the tip of the hair.  The long hair probably enhances the effect although shorthaired chinchillas are known.  The amount and/or quality of the tipping is variable as shown by the occurrence of the shaded silver (in the USA particularly).  It is possible that the I gene is semi-dominant so that II is chinchilla and Ii is shaded silver.  We hope to investigate this interesting possibility in due course.

Is it possible to reconcile the breeding data reported by Keeler and Cobb with the present hypothesis?  There is only one cross reported by these authors which would seem in direct disagreement and that is the mating of two black cats which produced 5 black and 1 smoke kitten.  However, this is not decisive since smoke can resemble black and one parent may have been a smoke.  In the initial crosses, a smoke mated to a black gave 5 black and mated to a tabby gave 4 tabby.  Usually the sum of these two litters (9 kittens) would be sufficient to establish statistically (but not genetically) that smoke is inherited as a recessive but, strictly speaking, the black kittens cannot be counted (because one or more could be dark smoke) and the 4 tabbies by themselves are not numerically sufficient.  It is perhaps pertinent to note that Keeler and Cobb remark that one of the smoke kittens produced during their experiments was "darker than the rest and gave evidence of the silver factor only upon the appendages and about the eyes."

There remains the possibility that Keeler and Cobb did actually discover a silver gene which is recessive to black.  This idea implies that two genes for silver exist in the cat.  Well, it is possible but improbable in our view.  Only the future can decide this question.  Keeler and Cobb experimented with cats of North America while ours are British stock with remote ancestry featuring imported chinchilla cats.  But this difference is immaterial for several American breeders have been just as adamant as the British that silver or smoke are inherited as a dominant.  The same opinion has been expressed by a breeder in South Africa.

We would like to acknowledge the practical help given in this breeding project by Mrs. A. Law, Upton, Yorkshire and Mrs. S. Scott, Withyham, Sussex.