Star Wise asked me a question about the genetics of the cat Venus, pictured above and in this post on her blog. The cat appears to have two different faces, split right down the middle. She has generated a lot of wonder about what could cause such a strange fur pattern.
The name ‘chimera’ is code for a cat that is biologically two individuals each with their own genetic code. So the idea is that the left half of the head is one, and the right the other side. Is she a mixed up pairing of two kittens?
All tortoiseshell cats have two colors (aside from white) – an ‘orange’ and a ‘black’. The intensity of the colors can change due to other genes. For example, my cat Rhea is a dilute tortie, so the black is faded to grey/blue, and the orange is diluted to creme. All this is normal, and doesn’t require anything exotic. The orange and black is typical of a normal, undiluted tortoiseshell cat.
The Straight Line Divide
Cats, like most animals (including us) have bilateral symmetry. Both of us are designed to be symmetric about a line down the middle of the body..thus the left side of the body is an image of the right side, flipped.
You can see the same “dividing line” on Rhea, my cat. You also see that the face is divided into just three sections, one on the left, two on the right. When the white blaze runs out, the dividing line runs right down the center of the forehead.
This linear divide is not uncommon at all with torties and calicos. My other tortie cat had a line on the center of the forehead and nose as well. If you use an Image Search for tortoiseshell cats, you will commonly see the spots of color divided on the centerline of the body, and often of similar size on either side.
And with Rhea, you can see another feature of bilateral symmetry – the stripes in the grey and creme regions match those on the other side, aside from color.
How are the Color Spots Formed?
The reason female torties have the two colors is because of two features of cat genetics. The first is how a cat ‘decides’ to have black hair or orange hair.
You might think that somewhere on the cat there is a single gene that determines hair color. This is not the case. All cats have a chromosome that determines the color of their black hair. The grains of black pigment have different shapes according to the gene, ranging form normal (Black) to a distorted shape (Chocolate) and so on.
However, there is a second gene that tells the cat to replace this pigment with a different, orange pigment. This gene is on the X, or sex determining chromosome. Male cats only have one of these genes, female cats have two.
But since cats have developed to only need one active X gene, the two copies in females are a problem. So during development all cells ‘decide’ to turn off one X gene and use the other. This “X Inactivation” happens in all females, including humans. So when a female with one X chromosome that selects Orange and one that does not will have a visible notice of the patchwork nature of this process.
It is a observed fact that the White Spotting gene tends to make tortie cats have larger spots. It probably manages this by making the determination happen earlier. If the selection is made when the cells forming the head have just a few cells, there can only be a few spots. After the cells divide and there are thousands of cells, then there can the many regions and you can have very fine divisions – like Venus has on her paws. But notice that the toes are solid colors, which implies that the selection there was made before the toes grew out.
So to me, Venus is a tortie where the Inactivation in the head region was very early, and the few regions – perhaps just one – then all on each side decided to be the same color, and the other side decided the opposite. You can see from Rhea how close she is to having the same fifty-fifty split.
Stripes and Solids
But what about the stripes on the orange side? Surely this requires different genes! Well, no.
The Tabby Pattern gene that determines stripe patterns on cats exists on all cats. However, another gene called Agouti can suppress the tabby pattern and produce a solid color cat. You can see that Venus is solid on her black side.
The Agouti gene has one limitation, though. It doesn’t work on the orange pigment that is produced by the Orange gene. So even if the genes are telling the cat to suppress the stripes, this will fail to work on the orange patches. And this is exactly what you see on Venus. Notice that the orange stripes also exist in the chest, not split on the midline.
That One Blue Eye
So what is the only very rare feature on Venus? It is that one blue eye. Blue eyes are not normal for tortie cats. They are not normal for orange cats.
Some cats do have odd eyes – usually in white cats or cats with white spotting. This makes sense, as the color of the eyes are linked generally to the hair color in the area where the eyes develop. The more pigment, the more likely the eyes will be yellow or orange. Less pigment makes green, no pigment makes blue eyes. So if something, like these genes, interferes with the pigment when the eye forms, it becomes blue. A similar process with pigment when the ear forms leads to deafness, thus the common observation that blue eyes and deafness often come together.
But sometimes you get odd eyes without white hair anywhere nearby. This rare result gives a blue eye where it normally never happens. The picture above might show this, if it hasn’t been altered. If you do an image search for ‘odd eye cat’ you can find other images that look more realistic of cats without white with and odd eye.
This seems to have happened on the orange side of Venus – somehow that eye got no pigment and developed into a blue eye.
This still doesn’t require any odd genes or a chimera to happen. It could just be an error in development, so if you cloned Venus that eye would not be blue.
I don’t see the need for any major genetic factors to make a cat like Venus. All it took was some quirks in development and the standard tortie gene set.