Why Calico Cats Are Almost Always Female

While any breed of cat can be born with calico fur, the vast majority of these cats are female, with only about one in three thousand calico cats born male according to the Humane Society.
So why are most calico cats female?
As you may or may not be aware, females have two X-chromosomes, meaning that they can only pass down an X-chromosome to their offspring. Males, on the other hand, have an X-chromosome and a Y-chromosome.
This allows them to pass down either an X-chromosome or a Y-chromosome to their offspring, determining the genetic gender. Thus, a female receives an X-chromosome from both of her parents while a male receives an X-chromosome from his mother and a Y-chromosome from his father. This is true for both humans and cats, along with many other animals.
Why is this so important to the topic at hand? Because in cats the X-chromosome determines most of the fur color (with the potential exception of white). A male offspring only receives an X-chromosome from his mother, so that alone determines his fur color. But females receive an X-chromosome from both the mother and father. Each cell only needs one X-chromosome, so early on when the feline embryo is developing, one of the two gets shut off, with the inactivated one supercoiling into something called a “Barr Body.”
The important thing here is that the same X-chromosome does not inactivate for each cell. One cell may shut off the X-chromosome from the mother while leaving the chromosome from the father. That cell then creates more cells, each of which will use the father’s X-chromosome to determine the fur color. Likewise, another cell may silence the X-chromosome from the father and instead use the chromosome from the mother.
So, for instance, if the female offspring receives the chromosome for black fur from both of its parents, she will have black fur. In the case of calico cats, the same process occurs. However, the offspring receives the chromosome for, for instance, black fur from one parent and orange fur from the other. One cell inactivates the chromosome for black fur resulting in orange fur. Other cells use the chromosome for black fur instead.
In both cases, these cells are replicated and the inactivated chromosome will always stay inactive. Those two colors then combine on the cat’s fur to create the orange and black patches of fur. If the cat only has these two colors, it is known as a tortoiseshell cat.
The tri-color, calico (or often called “tortoiseshell-and-white” outside of North America), with the presence of white fur, happens due to a gene unrelated to the X and Y chromosomes. This results in piebalding, where skin and fur that would normally be pigmented lacks pigmentation, resulting in a white color.
Flickr/Allison Meier
So, if a cat needs two X-chromosomes in order for its fur to be calico, how do male calico cats exist at all? A male cat can have tri-colored fur if he inherits an extra X-chromosome, making his genetic makeup XXY. In humans, this condition is known as Klinefelter Syndrome, which is surprisingly common at about 1-2 out of every 1000 live male births, with many who have the condition remaining ignorant of it.
In humans, as with cats, the individual in question is usually considered genetically male despite having two X chromosomes. Besides potential other health issues, the extra X-chromosome almost always causes male calico or tortoiseshell cats to be sterile.
In the exceptionally rare cases where the male is not sterile (about 1 in 1,000 of the already rare 1 in 3,000 male calico cats), attempting to breed him with a female calico or tortoiseshell will not produce male calico or tortoiseshell kittens at a higher than normal rate, nor would he be more likely to produce male calico or tortoiseshell kittens as he would almost always only pass on his Y chromosome to the male offspring, excepting potentially in the, once again, exceptionally rare XXY cat.
For this reason, and potential other health problems with the cat, even when these rare fertile male calico or tortoiseshell cats do pop up, they are almost never used for breeders as there are simply no advantages, and some disadvantages, to using them over more virile felines.
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