By Sarah Hartwell (of Messybeast.com)
Estimates of the frequency of tortoiseshell and calico tomcats range from 1-in-1000 to 1-in-many-thousands though many are misidentified classic tabby-and-white cats. Contrary to myth, tortoiseshell/calico tomcats are not valuable in financial terms (from here on, I’ll use to term tortie to include tortoiseshell, calico and torbie cats). Though many are fertile, they rarely pass on the tortie colouration to male offspring. And as pedigree cats, there may be no colour class for them to enter at shows.
To understand what creates tortie cats, we must quickly look at the genes involved. The red gene is carries on the X chromosome. Normal male cats are XY and only need one red gene to make them a red (ginger) tomcat. Without a red gene, they will be black, grey, chocolate etc. Females are XX make-up. To be red, they need 2 red genes. If they only get one red gene they will be tortoiseshell females. If they get no red genes, they will be black, grey, chocolate etc. Cream is genetically “red + dilution gene” and behaves the same way. The scientific term for having patches of different colours is mosaicism.
At first, scientists believed that tortie male cats must have accidentally inherited an extra chromosome: an X with red on, an X without red and the Y chromosome to make them an XXY (Klinefelter) male. As such they would be sterile and usually have physical anomalies such as an oddly formed pelvis/hips, unusual fat distribution and possibly giantism. Many Klinefelter males are a mosaic (jigsaw) of XY cells and XXY cells because the developing embryo does amazing things to ensure a viable infant.
In the last 15 years, there have been many more studies of tortie tomcats and the XXY theory has been turned on its head. Many, or most, of the tortie tomcats studied were fertile. Tissue samples showed them to be normal XY males. Some of them had genetically impossible colour combinations such as black and grey patches or red and grey patches. It should be impossible to inherit a mix of normal colour and dilute colour patches. Though fertile, they only passed on one of those colours, not both, to their offspring. This warranted more detailed study and the surprising result was that many (or most) tortie tomcats are due to chimerism.
A chimera is an organism that contains tissues from two different organisms. Unlike a hybrid, these cells keep their own genetic identity. It’s like conjoined twins that have joined so fully they have become one baby. Because female cats might mate with several males, the embryos that fuse into a chimera might have different fathers.
Cat chimeras tend only to be noticed where they are “genetically impossible” according to normal Mendelian inheritiance. If a black XX embryo fused with a red XX embryo, the resulting tortie female kitten wouldn’t attract any attention. It’s mostly when the gender and/or colour combination is impossible that it gets noticed.
If a red XY embryo bumps into and fuses with a black XY embryo, the result is chimera that contains some tissues or organs that are genetically red and some that are genetically black. The resulting kitten has a mix of black and red tissues and appears to be a male tortie (white patches are caused independently by another gene). There are many variations on this e.g. a red XY embryo that fuses with a grey XY embryo gives the impossible combination of a red-grey tortie male! An XY embryo could also fuse with an XX embryo to give a male kitten; in this case, the kitten turns out male only if the XY embryo builds the reproductive system.
A third cause of tortie tomcats is somatic mutation. This is most easily explained as one or more large black birthmarks on a red or cream coat. It seems that during early embryo development, one of the cells might make a DNA mistake when dividing and this shows up in skin cells as one or more black blotches. This famously happened in a lion called Ranger born at Glasgow Zoo. Lions should never have the black colour, so when a lion with a black leg and black chest was born, the only reasonable explanation was a somatic mutation in the developing embryo.
Finally there are “tortie mimic” conditions. Late colour-change genes (caused by the extension gene) turn a genetically black cat into a reddish or golden colour cat. The colour change take several months to complete and while the change is spreading over the coat the cat may appear to be tortoiseshell. Some of those colour change genes stop short of changing the colour of the whole coat; the end result mimics tortoiseshell. Unlike other tortie tomcats, these “tortie mimic” tomcats will pass on their colour to offspring.