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Introduction

References: Most of the information here is adapted from doggenetics.co.uk, animalgenetics.us or from a study at the University of Saskatchewan on “Coat Color Alleles in Dogs”, with some help from other personal relations (read: genetic gurus) and my own interpretations.

The following alleles have been ordered in the hierarchy of genes that impact the phenotype. We’ll investigate these in-depth(ish) as “hobbyist canine geneticists” as we examine each allele. Please note that the genes included are the officially mapped alleles in canine genome. There are still plenty of unexplored phenomena and theoretical suggestions for some patterns, but we’ll primarily focus on the official ones here. This particular guide to canine genetics will attempt to help explain how each allele changes the color of a dog's coat. For starters, let's do a little vocab review from high school biology.

Glossary

Genetics: The study of heredity and variations of inherited characteristics of an organism. Gene: Information about a trait. Ex. Black, Agouti, White Spotting. Locus: Where the gene is found. Ex. K, A, S. Allele: Each different type, or form, of a locus. Ex. K, Ay, sp. Genotype: A list of the alleles that make up the phenotype. Phenotype: The physical characteristics of an organism, as determined by the genotype. Homozygous: Alleles that are identical. Ex. KK, AyAy, sisi. Heterozygous: Alleles that are not identical. Ex. Kk, Ayat, Ssi. Generally indicates the parent will carry the chance of showing a recessive gene to its offspring. Dominant: The allele that is physically expressed. In genetics, this is often expressed as an uppercase allele (like “K”). Recessive: The allele that is not physically expressed, but carried to offspring. In genetics, this is expressed as a lowercase allele (like “k”). Incomplete dominance: The allele’s physical expression depends on whether the pairing is homozygous or heterozygous for a particular pattern. Eumelanin: Causes black, liver, blue, and isabella melanin (or pigment cell) on a dog's coat. Dominant to pheomelanin. Pheomelanin: Causes varying shades of red/tan/cream melanin (or pigment cell) on a dog's coat. Recessive to eumelanin.

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Extension (E locus)

The extension gene refers to the extension, or coverage, of eumelanin on a dog’s coat (if at all). It is usually referred to as “masking” but the scientific name is Extension.

EmEm, EmE, Eme : the dog will be some shade of red with a eumelanin “mask” marking on its muzzle and ears, occasionally reaching the chest and paws. See the K and A loci for exceptions. EE, Ee : this allele refers to “normal” extension; the dog’s base color can show both pheomelanin and eumelanin pigments and will be determined by the other eumelanin genes (particularly the K and A loci). ee : the dog will be some shade of red and no other eumelanin markings will show; the dog's skin color will still be determined by the eumelanin pigment. A dog with this gene is known as "recessive red.”

this dog shows a eumelanin mask because of the “Em_” allele!

each of these dogs are genetically “ee” but the C locus changes the shade!

Red Intensity (C locus)

The following alleles are theoretical, but it’s been included here because it is a probable explanation for the varying shades (or the intensity) of pheomelanin pigment in dogs. Even then, there are still differences and the descriptions below are generalized into three categories. It also provides an alternative way to consider dilution for pheomelanin in contrast to how the “Brown” and “Dilute” genes alter the appearance of eumelanin (we’ll get to those later). In other theories and studies, the C locus is associated with the gene SLC45A2, thought to be responsible for albinism in Dobermans and small breeds of dogs.

CC, Ccch, Cce : the dog will be a dark shade of red cchcch, cchce : the dog will be a creamy/tan shade of red (aka lemon or yellow) cece : the dog will be a milky/near white shade of red (aka platinum)

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Brown (B locus; aka Liver or Chocolate)

The brown gene is known as a dilution. It is one of two dilution genes in dogs and it allows a black coat to fade to a shade of brown, while also turning skin pigment into a pinkish shade of brown. A brown dilute dog is usually called “liver” or “chocolate.”

BB, Bb : this gene will have no effect on the dog's physical traits; they will be determined by the other genes. bb : the dog’s base coat and skin pigmentation will be brown. The coat may be hidden in part or completely by the E and A genes, but the skin will remain brown and the dogs’ eyes are amber or hazel.

this dog is isabella because it expresses both “bb” and this dog is blue-grey “dd” dilute genes! because of the recessive dilute gene “dd”!

this dog is brown because of the recessive brown dilute gene “bb”!

Dilute (D locus; aka Blue, Slate or Steel)

The dilute gene is the first of the two dilute genes discovered, and thus given the original name “dilute.” It allows a black coat to fade to a shade of grey, while also turning skin pigment into a blueish shade of grey. A dilute dog is often referred to as “blue.”

DD, Dd : this gene will have no effect on the dog's physical traits; they will be determined by the other genes. dd : the dog’s base coat and skin pigmentation will be blue-grey. The coat may be hidden in part or completely by the E and A genes, but the skin will remain blue-grey and the dogs’ eyes are grey or hazel.

Note: If a dog contains both “bb” and “dd” they are referred to as a “double dilute” or “isabella.” An isabella colored dog will generally appear as a muted beige.

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Black (K locus)

The black gene (called dominant black) refers to whether or not a solid black coat will “override” any tan markings. This gene’s effect on a dog’s coat is highly dependent on its interaction with other loci, particularly A, B, D, and E.

KK, KKbr, Kk : if the E locus is “E_,” the dog‘s coat will be determined by the B and D loci. If all of these lack an effect on the coat, the dog’s base coat and skin pigmentation will be black. If the E allele is “Em_,” the mask will be hidden by the dominant black. kk : this gene allows the A locus to “erase” areas of eumelanin pigment on the dog to show tan markings. The dog’s base coat will be determined by the B and D loci, but if these both lack an effect, the dog’s base coat and skin pigmentation will be black.

the first “inspection” is to see whether or not the E locus will override black; if the dog is E_, move to the next row!

the second “inspection” *this doesn’t account for checks for any dilutes; if the brindle or patterns caused dog is B_ and D_, it must by the A locus! be black*!

Brindle (K locus)

Brindle is a pattern modifier on the K locus that affects the coverage of eumelanin over red in a striped, tiger-like marking. This pattern may effect any eumelanin pigment and varies in density. Brindle is dominant over “kk,” but it may also be combined with the A locus to create interesting brindle patterns where it would normally appear solid tan.

KbrKbr, Kbrk : if the E locus is “E_,” the dog‘s coat will be determined by eumelanin pigment genes. However, rather than having a solid overlay on the dog’s coat, the eumelanin is broken up into a striped pattern called “brindle.”

brindle varies in density; “dark” or “reverse” brindle “light” brindle means more means more eumelanin pheomelanin and less and less pheomelanin eumelanin pigment! pigment!

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Agouti (A locus)

The agouti gene controls varying expressions of pheomelanin pigment on a dog that goes beyond the masked marking in the extension gene. A dog with any of these may also display a mask from the extension gene.

AyAy, Ayaw, Ayat, Aya : the dog will have a sable coat (sometimes called “fawn” when there is very little to no eumelanin pigment on the coat despite the genetics indicating the presence of sable). Sable is an overlay of eumelanin pigment on red and can range dramatically in coverage; think Lassie versus Pluto. awaw, awat, awa : this allele, while less dominant than sable, is the agouti allele the gene is named after; it stands for "wolf agouti" or "wild type sable"; it’s important to note that "sable" German Shepherds are often agouti and not genetically Ay, but aw. atat, ata : the dog will have a variation of tan points on a mostly eumelanin coat** aa : this allele refers to a dog having a recessive "regular" color (often referred to as recessive black); even if the dog is kk which allows tan markings to show, if it has aa, it will appear solid and the base color will be determined by the B/D/K loci.

Note: Linked to the at allele on the A locus is a gene called RALY. This gene determines the difference in the amount of tan that shows on a dog with the tan points allele (at_). The appearance of the RALY gene on the tan points allele causes the difference between the “tan points” and “saddle” tan markings. If only one of the markings shows up in the breed, then the tan points allele will only show that one marking (ie. Doberman Pinschers only have tan points and Beagles only have saddles on the tan points allele).

**creeping tan refers to a pattern that looks like a mix of tan points and saddle; the tan reaches farther on the dog's body (covers most of the face, legs, and reaches to the chest) than traditional tan points, but does not recede far enough to be distinguished as saddle. Creeping tan is usually “progressive” in appearance, meaning the dog may be born with more eumelanin pigment that recedes into tan as it ages.

agouti fur is often these dogs express two “banded,” or striped, giving it that “mixed” extremes of the tan points appearance! allele; the one on the left is classic tan points and on the right is a traditional saddle tan!

this dog is expressing a this dog is displaying minimal sable pattern classic agouti where the which stretches across the eumelanin is “mixed” with topline (or back) of the dog pheomelanin pigment in and its facial features, and some areas and is a clear also has a mask (Em_)! difference in others!

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Merle (M locus)

Like brindle, merle is a pattern overlay that causes patches of lightened pigment similar to an “incomplete” dilution. Merle only shows physically on eumelanin pigment, but it occurs on its own locus. Merle can occur genetically on non-eumelanin dogs, which makes it especially dangerous for breeders because a dog born with two dominant merle alleles, called “double merle,” is almost always born with health problems such as blindness, deafness, or underdeveloped features, and can even be stillborn. It's a highly discouraged practice; you should always breed a merle dog to a non-merle dog instead! In almost all cases, a merle dog will have blue eyes (or normal color eyes with blue areas) even if its difficult to see the spotted pattern on the coat, and this is the easiest way to identify merle dogs. Genetic testing is also encouraged in breeds with the possibility of displaying the merle gene.

MM : the dog will display “dilute” patches with large white areas and will probably have health defects. Mm : the dog will display “dilute” patches on its coat and have blue or multi-color eyes. mm : no merle will show on the dog and other genes will determine the coat color.

the dog on the left is an example of a normal merle; the one on the right is double merle!

the dog on the left is an example of a normal harlequin; the one on the right is double harlequin!

Harlequin (H locus)

Harlequin is a unique gene that does not display on its own. It requires the presence of merle (M_) to show; think of it as a “modifier on a modifier.” Harlequin acts the same way that merle does in the fact that it only affects eumelanin pigmented dogs. It is also dangerous if two copies are present.

[M_ + HH ]: lethal harlequin; the dog will be “lethal in utero,” meaning stillborn, or have severe health issues like a double merle if it survives gestation. [M_ + Hh] : harlequin spots (think smaller merle patches with a white base) will be displayed on the dog. [M_ + hh] or [m_ + H_] : no harlequin will display on the dog’s coat.

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White Spotting (S locus)

The white spotting gene sounds pretty much how it appears: this gene controls how much white is displayed on top of a dog’s other markings. No other gene physically appears on top of white. Think of it like a white blanket with holes, and certain alleles determine how many holes there will be, although the placement is mostly random.

SS : the dog will be “solid” as in there will be no white spotting on top of its other markings, but occasionally a genetically solid dog may have "residual” or “minimal" white (such as a small star on the chest or white toes), which is not genetic but just a harmless pigment-less causing mutation on certain extremities! Ssp : a small amount of white will show on the dog’s coat (generally less than half of the dog’s coat will appear white). This is generally referred to as “irish” white, and may be “collared” if the white wraps around the dog’s neck like a collar. spsp : a large amount of white will cover the dog’s other markings (generally more than half of the dog’s coat will appear white). This ranges from ”piebald” (between 51-75% of the coat is white) or “extreme piebald” (76-95%). Extreme piebald may also be referred to as “hooded” in certain breeds where the white completely covers the body except the head and occasionally a patch on the rump.

above: the progression of below: the coverage solid/minimal white (S_) difference between an to extreme coverage of average ticking and white (spsp)! roaning on white!

Ticking and Roaning (T locus)

If the white spotting gene resembles a blanket with large holes, then the ticking gene symbolizes really small punctures throughout the blanket in varying densities.

TT, Ttr, Tt : the dog will have a ticking pattern interacting with white areas on its coat; it may vary from a few spots to many, but there will still be clear white areas. trtr, trt : the dog will have such a heavy ticking pattern that is referred to as “roan” and usually clear white areas are difficult to detect, it will seem mingled. tt : any white on the dog’s coat will be clear of additional spotting.

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