Have you ever seen a cat that looks like its face was split down the middle? Like half of its face looks one way, and the other half looks like a different cat? Sometimes this is called mosaic and others it’s a chimera.
A what and a what?! I know, I know, but don’t worry, I’ll break it down so it’s easier to understand.
Let’s take a quick walk into the world of feline genetics which doesn’t disappoint and is just as fascinating as the cats themselves.
When you think of a mosaic, you probably have images of the art form, with pieces of different colored glass creating a full image. While that’s not exactly what we’re talking about, it isn’t too far off.
A mosaic has one set of DNA from two parent cells. And the best way to explain how this presents itself is through reviewing the genetics of tortoiseshell cats.
The gene that causes orange or black fur is carried on the X chromosome. Female cats carry two X chromosomes (one from each parent), however, during embryonic development, one X chromosome is activated in each cell at random, causing the other to be deactivated.
So, for example, if a cat inherits an X with black fur from one parent and an X with orange fur from the other parent, the kitten will be a mosaic of orange and black fur aka a tortoiseshell. This happens because in some cells, the orange fur is active, and in others, the black fur is active. This cat is a mosaic.
Calicos come from the same genetics (one X chromosome with orange and one with black), however they also carry the white spotting gene which gives them random patches of white.
Because the patterns are completely random, sometimes, a cat can appear to have a face that’s half orange and half white, and it looks like it’s split down the middle. It’s just the way the cells were activated during development.
In chimeras, you may also see a line down the middle of a cat’s face or body, and it looks like two different cats formed one.
That’s because they did.
A chimera is the result of two fertilized eggs or two embryos joining to create one organism. Basically, two fraternal twins join to create one cat. That means the cat will have two sets of genetics. Some cells will have one genotype, and others will have a different one.
Where a mosaic has one set of DNA from two parent cells, a chimera has two sets of DNA from four parent cells!
Although people tend to think of well known cats with split faces, many of those cats are in fact mosaics. A chimera isn’t always visibly obvious – in fact, it may happen more than we realize. A kitten can be born showing only small visible signs of being a chimera that aren’t even noticeable. Internally, the cat may even have entire organs that possess different genotypes or even blood that’s two different types!
Calico & Tortie Males
You’ve likely heard how rare calico and tortie males are. The reason for that is that male cats have the chromosomes XY. Since orange and black are carried on the X (not on the Y), a male can only have orange or black fur, not both.
Unless he has an extra chromosome, which I like to call a genetic oopsie. Males with XXY where one X carries orange and the other X carries black will be tortie. And if they have the gene for white spotting, they’ll be a calico.
Now, scientists believe that tortie and calico males are actually chimeras, and it’s the combining of two embryos that cause the male to pick up an extra X.
But What About Cats Actually Born with Two Faces?
You may have seen cats born with two faces, and when I say that, I mean literally two faces. That is not related to chimera, Even though it was once thought to be some sort of conjoined twin, science now knows better. Those are called Janus cats, and the condition is an extremely rare congenital disorder known as diprosopus. During the very early stages of development, parts of the face may duplicate when there’s too much of a specific protein called sonic hedgehog or SHH (no, I’m not kidding). This protein controls many aspects of the brain, as well as facial features.
Most kittens and human babies with this condition are stillborn or die in a few days; however, there was a cat named Frank & Louie, with one brain, two faces, three eyes, and two noses that lived to be 15 years old. He’s known as the oldest Janus cat according to the Guinness Book of World Records.
Do I Have a Mosaic or a Chimera?
You can’t visibly say for certain that any cat is a mosaic or a chimera, especially when it comes to tortie or calico females. The random activation and inactivation of the orange and black fur (along with other color combos) in cells make it difficult to know if a pattern is simply random or if it’s from two embryos.
If you have a kitty with a split face and really want an answer, there are a number of companies that do DNA tests on cats and they’ll be able to answer your question with certainty.
Basepaws Basepaws.com, The Chimera Cat - Its Own Non-Identical Twin.
Healthline. Healthline.com, What Is Chimerism?.
Parker, Emily. Catological.com, Facts About Chimera Cats.
The Scientific Reporters, TheScientificReporters.com, Diprosopus (Craniofacial Duplication) – A Congenital Defect
Venzel, Stacey. WideOpenPets.com, Everything You Need to Know About the Chimera Cat.
Article by Elizabeth Ann 🙋♀️
Cat Behavior & Fostering Specialist