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I majored in genetics as an undergraduate, so whenever I see a cat with unusual patterning or coloration, I can’t help but wonder what molecular wizardry made that possible. Cat fanciers have long wanted to unlock the mysteries of feline traits. While breeders began to puzzle out the patterns before DNA was determined to be the blueprint of heredity, there are still some mysteries to be solved. However, the more obvious traits have been mapped onto the feline genome, and there are even DNA tests for breeders or uncommonly curious pet owners who want to know what genes their cats carry. Today I want to cover the basics of coat color genetics, but I’ll start with a quick overview of genetics so no one gets lost. Feel free to skip it if you know you don’t need it.
Genetics: A Primer
DNA (deoxyribonucleic acid) is the molecule on which all the necessary information to build an organism is stored. Each unit of storage in the DNA is called a gene, and genes code for proteins. Proteins serve an incredibly wide variety of biological functions in the organism, including providing structure and metabolizing food into energy. There can be multiple different versions of a gene in the gene pool of a species. These different versions are called alleles.
Because an individual inherits half of their DNA from each of their parents, they have two copies of each gene. There are species for which this isn’t true, but it is for most vertebrates, including cats and humans. So each individual should have two copies of every gene, with a notable exception that we will get to later, but some alleles are dominant to others, so not every allele will be expressed. In the case of simple dominance, dominant alleles will be expressed if they are present. However, if the individual has inherited a recessive allele from both parents, they will express the recessive trait.
If you’re confused, I recommend looking up a more thorough overview of basic genetics before proceeding. There is much more to be said on the point, and I will do my best to explain as I go along, but it is only going to be more complicated from here. Now, let’s talk about what makes cats so very colorful!
Tom and the Monochromatic Fur Coat
Cats have two coat color pigments: eumelanin, which is black or brown, and pheomelanin, which is red or yellow. The primary coat color gene produces eumelanin. There are three alleles. The dominant allele, B, leads to black fur. The b allele is recessive to B and produces less pigment, leading to a brown coat, sometimes called “chocolate.” The b1 allele is recessive to both B and b, and it leads to a light brown or “cinnamon” coat.
The pheomelanin gene has a dominant allele, XO, that produces a red/orange coat color instead of or in addition to the color at the black/brown locus. The recessive allele, Xo, produces no reddish pigment. The pheomelanin gene is different in that it is located on the X-chromosome. In mammals, females have two X-chromosomes and males have one X and one Y, with some exceptions because biology is complicated.
So tom cats only have one gene for orange pigment, which they inherit from their mother. That is why toms are either ginger or black, but only very rarely both. But she-cats can have a XO and a Xo allele, causing them to have patches of both orange and black. This pattern is called tortoiseshell, or calico if they also have white patches. Tortoiseshell and calico toms can be born if they have two X-chromosomes and a Y or a similar chromosomal anomaly.
50 Shades of White
White patches are due to the piebald spotting gene, where the recessive, s, allele codes for no white. If a cat has just one dominant allele, S, less than half of their fur will be white, and the white patches are often restricted to the feet, face, chest, and underside. If they have two dominant alleles, more than half of the cat will be white. It’s possible that they may even be completely white.
There are two other ways that a cat may be white, genetically speaking. The dominant white gene, as the name suggests, produces solid white cats with at least one dominant W allele. These cats are likely to be deaf, especially those with blue eyes and odd eyes. Those with odd eyes tend to be deaf in the ear on the same side as the blue eye. Rarely, white cats may be albino. There are two alleles that cause albinism, both recessive and both defective versions of the allele coding for the tyrosine enzyme. Cats with two c alleles are true albinos and have a translucent white pelt with pink eyes. Two ca alleles produce a white coat with pale blue eyes. There is no association of deafness with albino cats.
A Dilute Shade of Pale
The dilution gene is capable of modifying the appearance of other colors. The dominant allele produces the normal coloration. Two recessive alleles causes the pigment granules to clump in the hair shaft and leave large areas without pigment. The result is a lighter, or diluted, coloring. Black cats become grey, known as “blue” in the breeding world. Chocolate becomes lilac, cinnamon becomes fawn, and ginger, or “red,” becomes cream.
This covers the basics, although certain breeds can have unique alleles, such as the amber allele at the orange locus in Norwegian Forest Cats. I will cover the genetics of coat patterns another week. Until then, enjoy this picture of my gorgeous blue-cream calico.
Published February 20, 2020
Updated June 20, 2022
