A tabby cat is a cat with a dark striped or spotted pattern on a paler background. There are many different varieties of tabby pattern.
The old theory was that all tabby patterns were mutations on one Tabby gene, following a strict dominance hierarchy. This has now been disproven for some time. There are three identified genes which have a clear effect on tabby markings, plus some other theorized genes and polygenes.
The hair shaft illustrations on this page are all referenced from images submitted to me. If you would like to help further my research, you can submit images to my email. I am currently only collecting them for illustration reference and will not be sharing them or putting them on the site, if I ever do wish to do so I will ask.
The A locus, Agouti, is responsible for determining patterning in many different mammals. It does this by affecting whether black pigment or red pigment is made. In a tabby cat, the background color is created by these signals turning on and off, resulting in a hair with alternating bands of black and red pigment.
The hairs of a standard tabby cat look something like this. There is a dark band of the base, then there is a band of golden taking up half or so of the hair shaft, and the tip of the hair has multiple shorter bands of black and red pigment. This makes the background color look generally lighter and more reddish or golden in tone compared to the stripes, which are more fully pigmented.
The allele for solid (a) is a nonfunctional version of Agouti. It is recessive to the functional allele (A), which allows the tabby pattern to show. There may be some faint banding, which can result in “ghost markings”, but the hair remains overall fully pigmented. In all the pictures that I have received, there is some degree of fading toward the base, which can vary in intensity and may be more brown or more gray in tone. This same fading is present in the stripes of the tabbies.
It is also possible that there could be a yet-to-be-identified “extreme” nonagouti. This form of agouti is found in some rodents, notably mice and guinea pigs, and is a complete and true nonfunctional agouti. There is no banding or paler root whatsoever, and the hair is completely pigmented. Animals with extreme nonagouti have very sleek, glossy coats due to the effect of melanin on the structural integrity of the hair.
We also know of one additional form of agouti, found in hybrids derived from the Asian leopard cat. It is denoted Apb, for the scientific name of the Asian leopard cat, Prionailurus bengalensis. It causes the charcoal pattern in Bengals - in its homozygous form, it produces twilight charcoal, and when combined with nonagouti it produces midnight charcoal.
As mentioned on the Sex-linked Red page, red pigment does not tend to play nice with the Agouti gene. The hair shaft image above reflects the current data I have, and I intend to write a more in-depth article about the interaction between red pigment and agouti eventually.
The Mc locus, sometimes just known as the Tabby gene, determines whether the cat’s “base pattern” will be mackerel or classic. In truth, the “base pattern” of all cats is classic, which can then be modified in various ways. The Mackerel gene appears to have a function in “erasing” the base pattern, leaving behind vertical stripes rather than the blotchy classic pattern.
Spotted tabby has been traditionally identified as a dominant trait on a single gene, with the symbol Sp. This model can still be useful, but it has since been shown that there is a spectrum between spotted and mackerel, the intermediate of which is broken mackerel, and it appears to be determined polygenically. What this tells us is that unlike in the traditional model, where two fully spotted cats can carry non-spotted, under this model it is extremely unlikely for two fully spotted cats to have a mackerel kitten. I represent this in my calculators by using spsp for mackerel, Spsp for broken mackerel, and SpSp for fully spotted, however, there are degrees in between these as well.
We also now know that spotted can only modify mackerel tabby, probably because it uses that same “erasing” function as for mackerel. If the cat is mcmc, the protein needed for the erasing is not being produced, so the polygenes cannot work to have it erase more or differently. However, spotted tabbies may still appear to have their spots in a whorl shape. This is because classic is the fundamental underlying pattern that is being erased from. It is possible that a cat being homozygous vs heterozygous for mackerel (McMc vs Mcmc) could have some effect.
The Ticked gene inherited its symbol, Ta, from its symbol on the hypothesized Tabby gene. The A stands for agouti, because the ticked pattern is defined by most of the body having the agouti hairs instead of the solid hairs found in the tabby stripes. "Agouti tabby", however, is a subset of ticked where there is very little, if any striping on any area of the body.
There is a view that agouti tabby is caused by a cat being homozygous for the Ticked allele (TaTa), but I don’t believe this to be true. Agouti tabby is exclusively found in breeds that have been carefully selected for the pattern and crosses thereof, and if it were caused in this way, we would expect it to routinely pop up in any breed with the ticked allele present, at a rate of 25% when breeding any two ticked cats.
Instead, I believe it makes the most sense to view agouti tabby as being caused by a set of "Ticked Modifier" polygenes. This would still allow the pattern to pop up occasionally in lineages not bred for it, which does happen on occasion, but it would happen much more rarely. It also better accounts for how this pattern has been bred for over time in the lineages where it is present. Being homozygous for the Ticked allele could be one thing that would help influence it, but I don't think the Ticked gene can account for the pattern alone.
The unique markings found in Bengal cats and related breeds seem to be from the pattern sort of "spreading out". The particulars of the patterning have a large amount of variation, but generally it seems that spotted is modified to rosetted, and mackerel is modified to braided. The broken braided pattern as I have illustrated it is based on pictures of Toygers, which are always braided tabbies, which appeared more broken up than was typical. Generally, the pattern seems to break up into strings of leaf-shaped rosettes.
The records on the initial breeding of Bengals indicates that rosettes were not found in the F1 generations, and were achieved through breeding hybrid cats with "shadow spots". There is no indication as to what exactly this means, but there being a noticeable difference indicates that the modifier is either an incomplete dominant allele, or is polygenic. There are certainly unidentified modifier genes at play for determining the exact size and type of rosettes, so I'm inclined to say polygenes. I represent this in my calculators with the symbol Bm.
The following image consolidates all this information in an easily saveable format:
Grizzle is a unique pattern seen in the Chausie breed. It seems to be a variant of the ticked pattern, but much darker than usual, giving the overall appearance of a black cat with pale ticking. The description of the hair shaft is consistent with a normal tabby, just darker than usual (older descriptions describe them as "black tipped with white", but the current information on official breed resource pages supersedes these).
Based on this description of it as a melanistic mutation alone, the best guesses are a) that it is a recessive Agouti mutation, and would be dominant over solid but recessive or incomplete dominant with normal tabby, b) that it is a dominant Extension mutation, which would most likely make it hypostatic to nonagouti, or c) it is a modifier which interacts with Agouti somehow.
Based on the current available evidence, I think b is most likely. However, I can't tell for sure until I have more detailed information to look at. It is pretty much out of the question for it to be a dominant Agouti mutation - because of the particulars of how the mechanism works, melanistic mutations on Agouti are always recessive, completely or incompletely.
If anyone has access to pedigree information, please contact me. Though I'm sure this will be resolved in time through genetic testing, I would love to have a look and see what can be inferred from the current evidence.
The Agouti gene is ASIP, the agouti signaling protein. It interacts with MC1R (also known as Extension) and MSH, to create the pattern of banding found in a typical tabby. ASIP is responsible for turning off the black pigment, so when it is mostly or fully nonfunctional the bands all become varying degrees of black. I intend to write an article specifically going in-depth on this mechanism, eventually.
The Mackerel gene has been identified as LVRN. This same gene causes the difference between normal cheetah patterning and the King cheetah morph, which is part of how its function as a pattern “eraser” was deduced.
The exact locus of Ticked has yet to be identified, though it has been mapped to an area on chromosome B1.
Spotting has been firmly identified as a spectrum most likely caused by polygenes, but the other sets of polygenes mentioned are purely theoretical. The term "Bengal Modifier" and symbol Bm to talk about the effect was coined by me for use in my calculators, but the presence of such a modifier had been previously noted.
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