In the range of cat coats, black tortoiseshell is one step up from black. I am discussing all the cat coat colors progressively from black to white. This is the second post.
What makes for the black tortoiseshell appearance? Tortoiseshell is black and orange in various degrees of mix from a lot of black and not much orange to a lot of orange and not much black and the range in between.
There are in fact 3 things to discuss here: What turns the black pigmentation to orange? (Pigmentation means: Coloration of tissues by pigment. The pigment is minute granules in the hair and skin that absorb and reflect light to varying degrees to create different colors). What dictates the mix of orange to black? Why are tortoiseshell cats almost always female?
1. Black to Orange
This happens through the actions of a gene called the O gene. This gene causes the black pigment to become reddish. Red is also, orange, in the cat fancy.
2 and 3. The Mix of Orange and Black and female torties
The answer to this is linked with the answer to why tortoiseshell cats are female. The O gene mentioned above that changes black pigment to red is carried on a sex defining Chromosome, the X Chromosome. You may know that our sex and the sex of cats is dictated by the sex Chromosomes X and Y. Males have XY make up and females XX.
As the O gene is carried on the X Chromosome, only one O gene is required for a male cat to be ginger (orange). The female requires 2 copies of the O gene for her to become ginger (all over that is).
If she inherits only one copy of the O gene she becomes partly ginger or as the cat fancy say, tortoiseshell.
That leaves the question as to how much of each color there is. In other words is she going to be a black tortoiseshell (much more black than orange) or the converse?
This last factor in the tortie recipe is controlled by X Chromosome Inactivation. In cats with 2 X Chromosomes (females), some of the cat's cells activate the O gene and other cells activate the O gene in the equivalent place on the other X Chromosome. As this process can only occur where there are 2 X Chromosomes (female genetic make up) only females can be tortoiseshell (except for the odd anomaly).
In the black tortoisehell cat the number of cells activating the O gene is less than in a cat that has more orange in her coat. That is why she is a black tortoiseshell cat.
Update: 17th December 2008 -- see some fine tortoiseshell show cats on this page plus some more info...: Tortoiseshell cats.
- Top Black Tortoiseshell cat Skittles copyright and by Gini~
- Middle Black Tortoiseshell copyright and by cygnoir
- Bottom tortoiseshell headband copyright and by LensENVY