College of Cat Genetics
by Patricia Turner
The point of discussion for this seminar is that of the Tabby (Lynx) Colourpoint Shorthair or Tabby Point Siamese. Replies sent in to the question posed in the May/June issue have made it clear that although most students understand that a dominant gene cannot be inherited from parents not exhibiting its effect (other than in certain unusual circumstances), they do not understand how this relates to the breeds of cat they actually keep.
I will, therefore, attempt an explanation of the gene complex, or genotype, of the Tabby Point before explaining further. Just as one has a recipe for a cake or a pudding, so one has a recipe to produce a variety of cat. The ingredients producing the Tabby Point are basically these:
Possibly the action of the agouti gene needs further explanation. The agouti gene is the one producing yellow ticking and banding of the individual hairs of the cat and its degree of action varies with the part of the cat's body concerned. On the belly and brisket the amount of yellow pigment is increased and the amount of tipping decreased so that the cat is lighter in colour on the underside. Yet in some areas the effect of the agouti gene is almost completely suppressed and these areas are those of the tabby pattern. The size and shape of the areas in which the effect of the agouti gene is suppressed depends upon the tabby pattern inherited so that agouti cats with Abyssinian tabby pattern will show almost uniform agouti and agouti cats with the blotched tabby pattern (also known as marbled or classic) will show large clearly defined areas of solid colour.
It can be said that the agouti gene allows full expression of the taby pattern by lightening the areas around it with yellow pigment. The tabby pattern will still be present even when the surrounding areas are not lightened by agouti. In fact, this is what happened when the original mutation for non-agouti occurred. By removing the yellow pigment bands from the hairs in the areas surrounding the tabby pattern, the impression of a solid black cat was given. But, looking closely as such a solid black cat, the tabby patterns can still be seen. This is particularly true during kittenhood and in strong sunlight. The replacement of the agouti gene by that for non-agouti serves only to remove the yellow pigment banding from some hairs. It has no effect on the basic tabby pattern.
I have mentioned the fact that the agouti gene allows for full expression of tabby pattern and described the manner in which this occurs, but there are other genes which allow tabby pattern to show to a certain extent. These are the tabby genes for melanin inhibitor (producing tabby markings in smokes), sex-linked orange (producing tabby markings in reds), and genes which lighten coat colour (including Siamese).
This fact explains why the standard varieties of Siamese--Seals, Blues, Chocolates and Lilacs--invariably show tabby markings at some stage in their lives. The tabby pattern is always there and the dilution of colour produced by the genes for Siamese gives us glimpses of it, particularly at the sides of the mask and on the tail. Ringed tails in Blue Points were a problem to many breeders long before the advent of the Tabby Points. The evolution of the Tabby Points provided uninformed breeders with a seemingly logical cause for the "fault".
There is a lot of truth in the saying that pure breeding for a colour is best. In an ideal world, the breeder specializing in Seal Points would be advised to rid his stock of colour recessives if he could do so but not for the reason that, with colour recessives, it would necessarily show the effects of its hidden gene complex. Thus it is also true that, in an ideal world, the breeder of non-agouti Siamese would be best advised to keep his stock free from agouti, but not true to say that is any of his cats had an agouti Siamese (a Tabby Point) in the pedigree there would be Tabby Point "throwbacks" in the kittens or that any tabby markings on the face would be the fault of this Tabby Point. In each case the fault would be in lack of selection. Breeders of the non-agouti Siamese varieties select for dense even pointing while breeders of the agouti Siamese varieties select for clearly defined tabby markings. As Tabby markings are inherited independently of agouti it is clear that the presence or absence of agouti in a non-agouti pedigree is immaterial if selection of breeding stock has taken into account the need for dense points in the Seal, Blue, Chocolate and Lilac colours.
In an ideal cat fancy, the breeders of Seal Points would own stock pure for Seal, the breeders of Blue Points would own stock pure for Blue, and so on. But the Siamese breed is still evolving and probably always will be. Certainly there are as many Siamese varieties around the corner as there are already on the show bench. At the present time, and in most countries of the world, it would be a retrograde step to breed only like to like for generations as to do so would fix the problems currently present in the breed colours. Type might not be the only quality that was lost.
A recipe for a Tabby Point can be written as follows:
Such a cat will be pure breeding for seal, carrying no colour recessives of brown (b) or dilution (d) and will also be pure breeding for the uniform agouti (Abyssinian pattern). The presence of AA indicates that it is true breeding for agouti, too. If selective breeding with Abyssinian tabby patterned Siamese could produce the cats with clear ringing on the tails this might be the ideal formula for the Seal Tabby point Siamese.
The recipe for a Seal Point can be written as follows:
Such a cat will be pure breeding, too. The difference lies at the agouti locus only.
To sum up, I will repeat that a Seal Point with a Tabby Point in its pedigree will breed in the same way as a Seal Point with no Tabby Points in its pedigree. If any Tabby Point bred Seals (or Blues, etc.) show tabby markings, the remedy lies not in the eradication of the Tabby Point from the pedigree, but in better selection for dense points among the non Tabby Points in the pedigree.