Cat Coat Genetics - Genes Involved in Red, Black, Brown, and Diluted Colors

Genes Involved in Red, Black, Brown, and Diluted Colors

• The sex-linked red gene, O, determines whether there will be red variations to fur color. This gene is located on the X chromosome. In cats with red variants of fur tone, phaeomelanin (red pigment) completely replaces eumelanin (black or brown pigment).
• Males have only one X chromosome, so only have one allele of this gene. O results in red variations, and o results in non-orange fur.
• Since females have two X chromosomes, they have two alleles of this gene. OO results in red toned fur, oo results in non-orange fur, and Oo results in a tortoiseshell cat, in which some parts of the fur are red variants and others areas non-red.

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.

• Rufous polygenes, as yet unidentified, can affect the richness of the orange/red gene's expression.

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 chimericism (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.

• The browning gene B/b/bl codes for tyrosinase related protein-1, an enzyme involved in the metabolic pathway for eumelanin pigment production. Its dominant form, B, will produce black color. Recessive variants are b, producing brown (or chocolate), and bl producing light brown or cinnamon.
• Barrington Brown is a recessive browning gene that dilutes black to mahogany, brown to light brown and chocolate to pale coffee. It is different from the browning gene and has only been observed in laboratory cats.
• The Dense pigment gene, D/d, codes for melanophilin, a protein involved in the transportation and deposition of pigment into a growing hair. When a cat has two of the recessive d alleles (Maltese dilution), black fur becomes "blue" (appearing gray), chocolate fur becomes "lilac" (appearing light brown), cinnamon fur becomes fawn, and orange fur becomes cream.
• Dilution modifier gene, Dm, "caramelizes" the dilute colors as a dominant trait. The existence of this phenomenon as a discrete gene is a controversial subject among feline enthusiasts.
• A mutation at the extension locus E/e (the melanocortin receptor) changes black pigment to amber or light amber. This phenomenon was first identified in Norwegian Forest Cats.
• A modifying factor has also been hypothesized in shaded silver and chinchilla Persians whose fur turns pale golden in adulthood, due to low levels of phaeomelanin production. These cats resemble shaded or tipped goldens, but are genetically shaded or tipped silvers. This is probably related to the phenomenon known as "tarnishing" in silvers.

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

• has the o variant of the orange gene on its only X chromosome (because the grey color corresponds to black, not orange)
• has at least one S variant of the white Spotting gene (because it has the white bib and paws)
• has two w alleles (because it expresses a fur color)
• has the dominant B allele (because its fur color is a shade of black rather than brown)
• has two d (dilute) alleles (because its fur is grey, rather than black).