White spotting gene

The genetics of cat coat coloration, pattern, length (short, medium or long), and texture is a complex subject, and many different genes are involved.

Cat coat genetics can produce a variety of colors and coat patterns. These are physical properties and should not be confused with a breed of cat. Furthermore, cats may show the color and/or pattern particular to a certain breed without actually being of that breed. For example, cats may have point coloration, but not be Siamese.

A cat with Oo and white spotting genes is commonly called a calico. The reason for the patchwork effect in female cats heterozygous for the O gene (Oo) is X-inactivation – one or the other X chromosome in every cell in the embryo is randomly inactivated (see Barr body), and the gene in the other X chromosome is expressed.

For a cat to be tortoiseshell, calico, or one of the variants such as blue-cream or chocolate tortoiseshell, the cat must simultaneously express two alleles, O and o, which are located on the X chromosome. Males normally cannot do this, as they have only one X chromosome, and therefore only one allele, and so calico cats are normally only female. Male tortoiseshell or calico cats occur only if they have chromosomal abnormalities such as the genotype XXY (in which case they are sterile), chromosomal mosaicism (only portions of their cells have the genotype XXY, so these cats may be fertile), or chimerism (a single individual formed from two fused embryos, at least one of which was male). Approximately 1 in 3,000 calico/tortoiseshell cats are male. Chimericism (which may result in fertile male cats) appears to be the most common mechanism.

One can deduce that a grey male cat with a white bib and paws, but showing no tabby pattern:

Tabby cats (AA or Aa), normally have:

Most or all striping disappears in the chinchilla or shaded cat, but it is still possible to identify the cat as a tabby from these other features.

The genetics involved in producing the ideal tabby, tipped, shaded, or smoke cat is complex. Not only are there many interacting genes, but genes sometimes do not express themselves fully, or conflict with one another. For example, the melanin inhibitor gene sometimes does a poor job blocking pigment, resulting in an excessively gray undercoat, or in tarnishing (yellowish or rusty fur).

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Wikipedia