For all the cat lovers out there, cats come in many colors. Two basic pigments to be discussed here are orange and black. There is one curious, rule for cats with patches of orange and black; they are all supposed to be female. The genetics behind this fact is as interesting as the phenomenon itself.

Chromosomes are passed from parent to child. For many animals, including us, the X and Y chromosomes determine gender. For most mammals, the presence of two X chromosomes indicates a female, while the presence of one X and one Y chromosome indicates a male. For a cat, the alleles (types of genes) that determine orange or black color are located on the X chromosome. A female cat inherits two X chromosomes from each parent. A male inherits only one X from his mother and a Y from his father. For the male, there should be only two possible outcomes for color:

X (orange) Y – an orange male cat or X (black) Y – a black male cat.

A female inherits an X from each parent. She can receive three possible outcomes for color:
X (orange) X (orange) – a female cat with orange coloring, X (black) X (black) – a female cat with black coloring, or X (orange) X (black) – This cat’s coat has patches of orange and black. This example is not so straight forward.

What happens with X(o) X(b)? With two X chromosomes, only one is actually used. The other one is actually “shut down.” This is called X inactivation. One of the X chromosomes will “super coil” into a structure called a Barr body (like a zip drive on a computer). This process is called lyonization. This happens very early in embryo development. When there are different alleles on each X (known as a heterozygous genotype, or in this case – one X with orange, the other with black), the X that gets “turned off” is random in each individual cell. This is what causes the patchiness of the cat’s coat. Each patch is made from cells that descend from that first cell that lyonized to be a certain color. These “patchy” cats are called “calico”. Since males only have one X chromosome, they cannot be calico.

So, how does this explain “Eddie,” the male tortoiseshell calico kitten, born in England? As with almost every rule, there is always an exception. Apparently, in very rare cases (some say the odds are 1 in 3,000), a male cat can inherit a Y chromosome and two X chromosomes, making his genotype XXY. If this male is lucky enough to inherit X (o) X (b) Y, then he will be calico.

Unfortunately for Eddie, and all the other male calico cats out there, the extra X chromosome interferes with the production of viable sperm. Therefore this leaves the poor kitties sterile. In humans, this disorder would be called Klinefelter Syndrome.

Eddie seems to be a perfectly normal, male calico cat.

You can find more on this rare case at: