Male cats can not carry the tortoiseshell markings—that is a genetic fact. This is based on very scientific principals, mainly that male cats only have one X chromosome. And, cats have this weird DNA twist: their sex chromosomes are directly linked to their fur color! (For more on this see this article)
It is important to recognize that multi-patterned calicos and tortoiseshell cats are already a result of a mutation process. This mutation occurs during the formation of the embryo. In order for this to transpire a female must start off with the Xb (black chromosome) and an Xo (orange chromosome) combination. While the embryo is developing a random switch between the Xb and Xo occurs resulting in a patchy coat pigmentation.
This means that a male tortoiseshell would actually be a double-mutant since he would need to inherit both the Xb and Xo chromosomes PLUS one Y (for him to be male). Quite a feat! Yet, mutations can occur but they are rare and the male is rendered sterile.
Now, imagine the excitement when a British veterinarian came across one of these wonders during a routine day in her practice. A client brought three tortoiseshell kittens to Dr. Karen Home’s clinic in Harpenden, Herfordshire for routine vaccinations. While examining the kittens, Dr. Home was happily surprised to discover that one of them was a male!
She immediately recognized that this kitten was indeed something special and adopted the little guy into her family of five cats, four dogs, and three children. They named him Eddie, after Eddie Izzard, the comedian because as Dr. Home said, “…. he is essentially a boy dressed as a girl.”
Why Does Color Affect the Sex of a Cat? Or Vice-Versa?
You always knew your cat was special, unusual, and certainly unique. But there are even more fascinating facts about the cat that have stumped people for some time. I am referring to what most cat lovers already know: cats of a certain color will be predominantly one sex or another. This has been common knowledge but back in the “dark ages” no one actually knew why. Happily, advances in the field of genetics have uncovered this feline mystery.
It all begins with a very unique genetic twist embedded into the chromosome code of the feline. This is an oversimplification but for the sake of this explanation lets just state that certain chromosomes determine gender (male or female). For example, females have two X chromosomes (XX) and males have only one X chromosome and one Y (XY). Now here comes the twist: a cat’s X chromosome coding is directly linked to the color of its coat! This unique coding is called a sex-linked gene.
Again this is an oversimplification, the coat color in all cats is an X-linked gene for black and orange-brown colors. So the color-coding would be Xb (for black) and/or Xo (for orange-brown). A female cat has two X chromosomes so she is able to “display” two colors at the same time: black AND brown-orange—and all the variations in between (the white color is a separate gene). A female cat with two Xb Xb chromosomes would be black. A female cat with two XoXo chromosomes would be orange-brown.
A male cat on the other hand will only receive his color designation from the one X chromosome and this chromosome would render a monochromatic coat. (The Y chromosome is a “blank.”) For a male to display BOTH red and black he would have to have two of the X chromosomes described above PLUS one Y (for him to be male). This could happen but it would be a mutation (abnormality) that would render him sterile.
The explanation gets a little more complicated when it comes to multi-patterned calicos and tortoiseshell cats (multi-colors of orange, brown, black, gray, cream, or white without patches). These coat colors and patterns are the result of a mutation that occurs during the forming of the embryo. For this to transpire the female will need start off with the Xb Xo combination. During embryonic development some cells randomly—through the process of mitosis (cell division)—will turn off the Xb or Xo gene. This in turn will produce patchy coat pigmentation that results in either a tortoiseshell or patterned calico cat.
As for the orange tabby cat being predominantly male that is true. However female cats can also be orange. It’s mostly a matter of probability. As explained above a male cat has only one color gene and that is located in his X chromosome. So, whatever color that X chromosome has attached to it is what he will be. The X chromosome inherited by the male will either be Xo (brown-orange) or Xb (black)—keep in mind that the white color is an entirely different gene. Therefore the odds are that male cats will be either orange/white; solid orange; brown/dark-brown; brown/dark-brown/white; black/white; solid black; or a dilution of black resulting in gray/white; or solid gray.
The female orange tabby has received two Xo chromosomes. And this does happen, producing females with varying degrees of ginger colors that can be accompanied by white markings. However, she is more likely to receive two different types of X chromosomes such as the XoXb combination. And because of the double X sex-linked color chromosome it is safe to say that female cats will display more color combinations than males.