by Truda M Straede BSc Ph.D.
Copyright T.M.Straede 2000
Tabby cats appear so because of two interactive genes - the pattern and the agouti or ticking gene. All cats have some form of tabby pattern, but it is only expressed when the ticking gene causes the pattern to show as solid coloured hairs against a salt and pepper background of ticked or banded hairs. Australian Mist are all patterned cats, A-, by definition. As the generations pass generally they are AA, though the very rare occurrence of a non-patterned cat, aa, indicates that some still carry the non-agouti recessive, Aa.
According to classic theory, the tabby pattern is divided into three types, the abyssinian or ticked Ta, the mackerel, or wild type T, and the blotched or classic tb. Ta>T>tb, so that an abyssinian pattern cat can carry mackerel or blotched, the mackerel can carry blotched, and the blotched is homozygous recessive, tbtb.
You will note that there is just one small problem with this hypothesis - where are the spots? This problem kept me occupied for a long time, but finally I came to form a new hypothesis, that there are only two tabby patterns, ticked/abyssinian Ta and classic tb. Ta>tb still, but there is another set of independently inherited pattern modifiers, with at least two forms, Pm, which breaks the pattern into spots, and pm which leaves it intact. Pm>pm, so a spotted cat is PmPm/pm tbtb. The blotched tabby is still homozygous recessive, but now it is a double recessive pmpm tbtb.
I have envisaged these two genes as interacting as a pair of geometrical waves on the sides of the cat - tb as concentric ripples in a pond, Pm as a linear wave front perpendicular to the spine. The diagram , drawn from a classic baby kitten illustrates the method of interaction suggested - if you trace the bar patterns onto a transparent sheet and lay them over the classic patterns, and only colour in the area where the black, or non agouti pattern hairs coincide, you will see for yourself that the expression of the spots/ bars can be very variable, accounting for the rather odd shaped spots often observed, especially on cats with a few very large spots. I believe that this mechanism works by both tbtb and Pm turning the agouti gene on and off - but it only stays off- hence solid coloured hairs, when both pattern genes say off.
The enormous variability in the expression of the blotched gene, from the 'bull's eye of the show British Classic Tabby, to the almost complete obscuring of the concentric ring pattern in the most desired of all Marbled Bengal patterns, suggests that a large number of potential spot patterns are possible from interaction with the Pm gene. It is also probable that the width of the Pm bars is quite variable, so that spot sizes can also be quite small, or quite large. It is likely that the interaction of a very fine barred Pm with a certain form of the blotched pattern also underlies the best mackerel pattern. It is certainly very obvious from casual observation of the domestic tabby population, that most cats are neither spotted, nor mackerel, often half and half, and that it is often possible to see the underlying blotched pattern influence. One of my neighbours had a cat which was blotched on one side, and half and half spotted and banded on the other side!
To summarise :
classic tbtb pm pm
spot tbtb Pm Pm/pm
I have one set of direct evidence that there are two separate genes influencing the spotted/mackerel tabby complex of coat pattern.
As I was not satisfied that
hypothesis was really any more than just that, and in view of the long
established nature of the current hypothesis, I kept my eye open for an
opportunity to provide a more definitive test. Finally the opportunity
came ( in 1986), with a ticked tabby (Ta) Sarvita,
had produced a classic kitten - according to classic theory she was Tatb
- but was that all? By mating her to a cat who was the recessive by
theory - a classic tabby tbtb (called Charly
and by my hypothesis the double recessive, tbtb
, I might be lucky enough to gain support for my hypothesis.
According to classic theory
Charly Brown tbtb x Sarvita Tatb should produce
1 ticked tabby Tatb : 1 Classic tabby tbtb
According to my hypothesis
Charly Brown tbtb pmpm
Sarvita Tatb ? pm
might produce ticked, classic AND something else tabbies.
Sarvita's parents were a pure bred Abyssinian sire and a pure bred Burmese dam, from the latter gene pool I had already derived spotting and mackereling, and the dam was ghost spotted, even in maturity.
The resultant progeny were four, 3 ticked and one a very broad banded mackerel tabby . It would appear that Sarvita was best characterised as tbtb Pmpm according to my hypothesis, while according to classic theory she couldn't exist at all. Whilst it would be desirable to repeat such crosses, this was not possible at the time due to the constraint of the cost of producing kittens which were of no value to the breed. (This was when spots were the only pattern allowed, and the classic gene an unwanted recessive in the 'Spotted Mist') .
A Second Test
Now that marbling, based on the classic (blotched) gene is one of the desired patterns of the Australian Mist, I am using the new Foundation Cross, Abyssinian x Marbled (classic) Australian Mist as an opportunity to repeat this test. The expected progeny by classic theory will be Tatb, and by my Hypothesis Ta tb ? pm. As I have both male and female Marbled (classic) Australian Mist breeding stock, I will have an opportunity to perform useful back crosses which might give litters with ticked, spotted/mackerel and marbled (classic) kittens in the litter. Of course, it is quite possible that this particular Abyssinian stud does not carry Pm, and that I will be none the wiser ... but such an opportunity was not to be missed.
The results of mating
progeny from this first cross were followed, litter by litter,
the next few years, and added no further support to my hypothesis.
Probably the Abyssinian used did NOT carry the Pm gene, as
suitable test litters were bred from 5/9 offspring, some having more
than one litter. Over recent years, the idea that tabby patterns are
more complexly inherited than according to classic theory has certainly
been agrowing view amongst breeders and cat geneticists. With the
growing availibility of DNA testing, it is probably only a matter of
time (and funding) until the true individual genetic components of all
tabby patterns are revealed.
to A Work in Progress