The Genetics of Calico Cats

The Karyotype: A Visualization of the Chromosomes

The exact genes carried on the X chromosome varies among species. In humans, for example, the gene coding for normal clotting factors and the gene coding for normal cone photoreceptor pigment are located on the X chromosome. Abnormal mutant forms of these genes can result in hemophilia (a potentially fatal disorder in which the blood fails to clot) in the former case, and red-green color blindness in the latter.

Cats: One X-linked Gene with Two Alleles for Coat Color

Female Genotypes

Male Genotypes

There are two possible (normal) male genotypes:



or

Expression of Coat Color in Males

Expression of Coat Color in Females

At a certain point in the embryonic development of every female mammal (including cats), one of the two X chromosomes in each cell inactivates by supercoiling into a structure known as a Barr Body. This irreversible process is known as Lyonization; it leaves only ONE active X chromosome in each cell of the female embryo. Only the alleles on the active (uncoiled) X chromosome are expressed.

Lyonization is random in each cell: there's no way to predict which of the two X chromosomes will become inactivated. Hence, any given cell of a heterozygous female could end up as either of the following:

A heterozygous cat will be a patchwork of these two types of cells. Lyonization takes place relatively early in development, when the cat is still a blastula, and all the cells descended from a blastomere with a particular X chromosome inactivated as a Barr Body will also have the same Barr Body inactivated. That means that all the skin tissues that arise from a cell like the left one will express black fur, and all the skin tissue that arise from a cell like the right one will express orange fur.

Hence:

In a patch of skin in which the X chromosome carrying the B allele is inactivated, that patch of skin will express only the b allele (black fur).

In a patch of skin in which the X chromosome carrying the b allele is inactivated, that patch of skin will express only the B allele (orange fur).

Here's an overview:

This is why calico cats are almost invariably female.

Can Male Cats Ever be Tortoiseshell?

Real, Live Cats

A calico cat is a tortoiseshell expressing an additional genetic condition known as piebalding. A piebald animal has patches of white (i.e., unpigmented) skin/fur. This is controlled by a different locus (S) than the black/orange fur colors.

The patches may be relatively large, or rather small and interwoven:

Larger patches may be caused by: