To address coat color genetics in dogs, it pays to start from the beginning. Your dog’s body consists of trillions of cells. Most of these cells have a nucleus inside them, each of which contains 78 chromosomes. These chromosomes are made with deoxyribonucleic acid (DNA). DNA is then made up of nucleotides. In each cell, your dog will have about three billion base pairs of nucleotides. The unique pattern that these nucleotides form is what makes a gene special! So, while one gene might encode hair color into a hair shaft, another gene might be responsible for producing enzymes to digest food. For every gene, the code behind it is incredibly precise. One single mistake in the gene’s DNA sequence can have disastrous effects.
In short, your dog’s DNA determines their coat color. Also, each hair follicle at the base of your pup’s hairs contains cellular material that’s rich in DNA. Along with this, melanocytes surround each hair follicle. There are two types of melanin that melanocytes are responsible for: eumelanin (brown-black), and phaeomelanin (red-yellow). Depending on the underlying genetic influence, a melanocyte produces either type of melanin.
Alleles and Loci
In genetics, a locus is a fixed position on a chromosome where a specific gene resides. Each gene resides at a locus on a chromosome in two copies, one copy for each gene inherited from each parent. These copies, however, aren’t necessarily identical. When the copies of a gene are different from one another, they’re known as alleles. To understand dog coat color genetics, it’s important to get to know each locus, first.
A (agouti) locus
The A-locus is governed by the agouti signaling protein (ASIP) gene. This gene interacts with another gene known as MC1R to control red and black pigment switching in dogs. The ASIP gene governs four different alleles. These are ay = Fawn/sable, aw = Wild sable, t = Black-and-tan, and a = Recessive black. These four alleles work in a hierarchy and ay is the most dominant of them all.
E (extension) locus
The E locus is governed by the MC1R gene. It has three possible forms: black (E), melanistic mask (Em), and yellow/red (ee). The “E” allele allows a dog to produce eumelanin or black pigment. If a dog carries two copies of the E allele (E/E), it can produce black pigment. However, a mutation of the MC1R gene can cause a dog’s cells to only produce phaeomelanin in place of eumelanin. This mutation is represented as the “e” allele.
If a dog carries one copy of “E” and one copy of “e”, they can produce black pigment. The “E/e” dog passes on E to half of its offspring, and e to the other half, the latter of which can produce a yellow/red coat if inherited with another copy of e from the other parent. Because the “e” allele is recessive, a dog must have two copies of it to express the yellow or red coat color. A dog with e/e expression will have a coat that is white, cream, yellow, apricot, or red in color.
K (dominant black) locus
The K locus consists of three alleles. The first K locus allele is KB, or dominant black. This allele reduces or eliminates the expression of the A locus. This mutation is dominant, so your dog only needs one copy of KB to affect the expression of the A locus. A KB/KB or KB/n dog is solid black in color. The next allele is the “brindling” allele, written as “Kbr“. The Kbr allele allows the A locus to come through but causes brindling of the agouti patterns. However, this allele is recessive to the dominant black allele.
So, if your dog’s genotype is KB/Kbr, they will only be solid black and not brindle. The last allele is ky, or recessive non-black. This allele allows the agouti gene to come through without any brindling. So, a dog with two copies of ky will show whatever it has on the A locus, but will still have black nose pigment and may have some black markings, too. In contrast, a dog with both KB and ky will not be able to show any colors from the A locus.
B (brown) locus
The B locus is the home of the liver coat color. This gene affects eumelanin (black pigment). If a dog has a liver coat, their nose is typically brown or pink, and the eyes amber or light brown. The liver gene itself is recessive, so “b” represents liver, and “B” is non-liver, or black. A genotype of B/B or B/b would create a black dog. So, in order for a dog to have a liver coat, it must have the genotype b/b. If your dog has this genotype, it’s genetically impossible for them to have black or gray hair in their coat. The D locus commonly influences the B locus, leading to dilute versions of the liver coat.
D (dilute) locus
The D locus is responsible for lightening the coat. This gene is recessive, so “d” represents dilute, and “D” represents non-dilute. In order for your dog to have a dilute coat, it must have the “d/d” genotype. A dog that is “D/d” or “DD” will have a normal coat. The dilution gene mostly affects eumelanin (black and liver pigment), but phaeomelanin (red) can lighten, too. So, when a dog has “d/d” alleles, a black dog becomes blue or slate. A liver dog becomes isabella or lilac. The eyes will also lighten to amber, which is typically paler than the amber eyes seen in liver dogs.
M (merle) locus
The merle gene is responsible for diluting random sections of the coat to a lighter color, especially in black, liver, blue, or isabella dogs. There is a large number of different merle alleles that affect the coat in different ways. In the merle gene, there is an extra portion of DNA in the dog’s genome. This is its SINE insertion. The longer the insertion, the greater the effect on the dog’s coat.
So, for example, the harlequin merle gene (Mh) has a length of 269-280. In contrast, the standard merle gene (M) is 265-268, the atypical merle is 247-254, and the cryptic merle is 200-230. So, while the cryptic merle insertion isn’t long enough to affect the coat, harlequin merle dilutes gray areas to white or light gray. Because this is the longest insertion, the risk of serious eye and ear defects is highest with this form of merle. It is not safe to breed two merle dogs together for this reason.
H (harlequin) locus
The harlequin gene occurs on the H Locus of the Great Dane. This gene is a modifier. This means that, when inherited alongside another gene, it affects the way that the gene appears. If a dog inherits the modifier but not the gene that it modifies, there is no obvious change to the gene. So, when a Great Dane inherits both the harlequin gene and the merle gene, the areas between their dark patches become pure white. Sometimes, gray ticking or patches will develop, too. This means that a blue (black) merle becomes white with black patches, as the gray in its coat turns to white.
The modifier also affects phaeomelanin (red), so even if a dog is sable, their coat will become “fawnequin” with tan and black patches on a white base. Unfortunately, the harlequin gene may be a dominant embryonic lethal gene. This means that all “H/H” puppies die before birth, leaving only “H/h” puppies to be born.
S (spotting) locus
Most white spots on dogs are the result of genes on the S locus. White spotting can occur with any coat color and will take over both eumelanin (brown/black) and phaeomelanin (yellow/red). The white spotting gene stops the cells from producing skin pigment, causing white areas in the coat. So far, only two alleles are known to exist on the S locus. These are S, which produces no or very little white, and sp, which produces piebald patterns. A third allele might exist on the S locus, but it has not yet been proven. This allele is “extreme white” sw.
Eumelanin and Pheomelanin
There are two types of pigment that influence the color of your pup’s coat. These pigments are eumelanin and phaeomelanin. Most dogs’ coats contain both eumelanin and phaeomelanin. In these cases, the A locus determines how the two pigments mix in the coat. But what exactly are these pigments?
Eumelanin is responsible for black pigment. However, other genes can turn eumelanin into other colors. These colors include liver (brown), blue (gray), and isabella. If a dog has a gene that turns its eumelanin into another color, the entire coat color changes. This is because the gene changes the production of the eumelanin, meaning that none of the dog’s cells can produce the original pigment.
Phaeomelanin is responsible for red pigment. However, the name “red” applies to every pigment from deep red to cream, encompassing colors like yellow and orange, too. Unlike eumelanin, phaeomelanin does not occur in the nose or eyes. So, any gene affecting the phaeomelanin in the coat will not affect the eyes or nose.
What is color dilution alopecia? (CDA)
Color dilution alopecia (CDA) is a genetic recessive inherited condition that causes patches of hair thinning or loss and may also include flaky and/or itchy skin. The condition is associated with individuals who have what is called a “dilute” color and is most commonly seen in dogs with a blue or fawn coat. These puppies are born with a normal-looking hair coat, and the clinical signs of CDA may begin to manifest at six months of age or older. While the disorder has been commonly described in blue Dobermans, it has been recognized in other breeds as well, including:
• Chow Chow
• Great Dane
• Irish Setter
• Italian Greyhound
• Standard Poodle
• Yorkshire Terrier
• Bernese Mountain Dog
• Shetland Sheepdog
• Boston Terrier
Color-dilute individuals carry a recessive color gene (dd) and demonstrate blue, blueish grey, lavender, or flesh-colored lips, noses, and eyelids. Deeply colored individuals carry either DD or Dd genes and demonstrate either black or liver noses, lips, and eyelids.
What causes my dog to lose hair?
The actual cause of CDA is poorly understood. Dogs with CDA tend to have abnormalities in the hair follicles themselves, causing them to self-destruct, making it impossible for them to grow new hairs.
Can color dilution alopecia affect my dog's overall health status?
No. Other than her overall appearance, her health is not at risk. That said, there may be some skin-specific issues that will emerge and need to be treated. Your dog may develop scaly skin in balding areas. She may also develop small bumps or even pustules associated with a bacterial skin infection. Some dogs with CDA will experience itching that may need to be managed.