The basic colors and patterns of cat fur are defined by fewer than ten genes. Cats with white color in their coat are thought to have a mutant white-spotting gene that prevents the formation of coat color in patches over the cat’s body.
This gene has been investigated in several species, particularly mice, and is co-dominant to normal coat color as it prevents the migration of melanocytes into the developing hair follicles.
The genetics of this pattern are not as well understood in cats but at least some of the genes involved in melanocyte migration and survival may play a role similar as that in other animals.
Three genotypes are possible with the S (white spotting) gene, with capital S standing for a wild-type copy and lower-case s standing for the mutant.
- SS (two dominant alleles) results in high grades of white spotting (sometimes resulting in a solid-looking white cat or a white cat with just a few color hairs)
- Ss (one dominant, one recessive allele) results in medium grades of white spotting
- ss (two recessive alleles) results in solid color or low grades of white spotting (sometimes as little as a few white hairs)
The lack of tabby striping in bicolor cats is controlled by the agouti protein, which inhibits the production of melanin and thus prevents the formation of dark hair colors. In agouti cats the gene is turned on and off as the hair grows, producing hairs with alternating stripes yellow and black. In domestic cats, inactivation of the agouti gene by a deletion mutation causes an all-black coat color.