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’re probably 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.
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.
Bonus Fact: Catnip, which is a perennial herb in the mint
family, contains a chemical called "nepetalactone" that is released when catnip is crushed.
When cats get a whiff of nepetalactone, most will start rubbing themselves against it,
playing around with it, sometimes eating it, and generally will act quite bizarrely.
It is hypothesized that this chemical mimics certain feline pheromones, specifically their
theoretical facial pheromones (it isn't known whether these actually exist, but many researchers
think so, which is why cats probably like to rub their faces on various things).
Once cats have been exposed to the nepetalactone for a few minutes, it loses its effect on
them and they will usually no longer be interested in it for about an hour or two, at which point
the chemical will start to kick in again as they breathe it in and they will once again
begin acting bizarrely around it for a few minutes.
Despite the apparent drug-like effect, it isn't thought that catnip is in any way harmful
to cats, nor is it thought that it is addictive.
Most researches think that nepetalactone simply triggers something in their brains that causes
them to want to rub up against this particular smell, not unlike what dogs often do when
they encounter certain smells.
That being said, cats can "overdose" on catnip, such as if they find a giant bag of it and
rub it all over.
This may result in vomiting, diarrhea, an almost catatonic… state, drooling, twitching,
and other bizarre behavior.
Unlike a person who's overdosed on a drug, though, once the cat is off the ‘nip, she
will relatively quickly go back to normal with no residual effects.
One interesting thing to note, however, is that not all cats respond to catnip.
Whatever genetic quirk that causes them to respond to nepetalactone is inherited, and
only about 70% of cats show a behaviorally difference around catnip.
Further, cats under the age of a few weeks old also are not attracted to catnip and some
even show an aversion to it.
Not only are very young cats sometimes averse to catnip, but so are cockroaches, mosquitoes,
flies, and termites.
In fact, nepetalactone extract has been shown to be ten times more effective at repelling
mosquitoes than DEET (N-Diethyl-meta-toluamide), which is the chemical traditionally used in
mosquito repellant.
However, it should be noted that when put on human skin, nepetalactone's effectiveness
as a mosquito repellant decreases quite a bit, so it's more suitable as a mosquito repellent
when sprayed on clothing or the like.