In mammals, sex is determined by two sex chromosomes, known as the X and the Y chromosomes. Genes located on either the X or the Y chromosome are known as "sex-linked" genes. Genes on any chromosomes other than the X or Y are known as autosomal genes. The Karyotype: A Visualization of the Chromosomes Normal female mammals have two X chromosomes. Normal males have one X and one Y chromosome. This can be seen in this human male karyotype: The X and Y chromosomes appear at the bottom right corner of the image. If this were a female, the two sex chromosomes would both be relatively larger X chromosomes. As you can see, compared to the X chromosome, the Y chromosome is small and carries fewer genes.

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.

There are two possible (normal) male genotypes:

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:

Here's an overview:

This is why calico cats are almost invariably female.

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:

See more here:
The Genetics of Calico Cats - Department of Biology

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