Dominant black in dogs
The Dominant Black Gene:
Black is the "default" eumelanin colour for dogs. A dog that isn't homozygous for liver (bb) or for dilution (dd) will have black eumelanin. This means that it will have a black nose and, usually, brown eyes (eumelanin affects eye colour too), and any eumelanin in its coat will be black. Eumelanin is one of two types of pigment that occur in dogs. The other is phaeomelanin, which doesn't affect the eyes or nose and is only visible in the coat. It produces the colour "red", which is anything from deep Irish Setter red to light cream. Phaeomelanin doesn't "naturally" occur in the coat - it only appears if the dog has particular genes which allow it to occur.
Shades of eumelanin pigment - black, liver/chocolate, blue and isabella/lilac. In some breeds, liver-coloured dogs are referred to as "red", however I will only use "red" to refer to phaeomelanin (below), to avoid confusion.
Shades of phaeomelanin pigment (cream to red). Referring to phaeomelanin as red or tan.
There are two basic choices for a dog's markings - solid (no red/tan markings, just eumelanin) or non-solid (red/tan markings of any sort). Whether a dog has a solid eumelanin (black) coat or a coat with red/tan markings (caused by phaeomelanin) depends almost entirely on the K locus. K consists of three alleles:
K - dominant black (solid black, no red). Sometimes referred to as KB, although I'll use K here for simplicity.
kbr - brindle but for now all we need to know is that it acts as a k allele, but just adds brindle on top of any red markings)
k - recessive non-black (will still have black nose pigment and may have black markings, but may also have red markings too)
Because black is dominant, a dog with even just one K gene will be solid black. A dog with two k genes (i.e. homozygous for k) will be able to show tan markings. These tan markings are determined by another locus, A (agouti). So basically, a genotype of kk allows a dog to show whatever it has on the A locus. A Kk or KK dog may be genetically tan-pointed or sable on the A locus, but won't be able to show those markings because of its dominant black allele/s. Dominant black dominates the whole of the A locus, but it can be modified by other genes, such as liver, dilution, greying and merle. All of these will alter the way a dominant black dog looks, but the one thing they cannot do is add phaeomelanin (red) to the coat. The only way phaeomelanin can be added to the coat of a dog with the dominant black gene is through the e gene (E locus) - recessive red. This turns a dominant black dog (or indeed, any dog) into a solid red dog with black nose pigment.
This all probably sounds very confusing at this point, and if you have no background in genetics you may not understand everything. Don't worry - we'll deal with the genes that can affect black on different pages a bit later on. For now, all you really need to know is that Kk and KK on the K locus produce a solid black dog. That black coat can then be changed to liver, blue, isabella or merle. A kk dog may have some black in its coat, but it won't usually be solid black. The stuff about eumelanin and phaeomelanin is particularly confusing, but I've made sure to use those terms all throughout this site because if you can get a handle on them, you'll be able to understand dog genetics easily.
The Recessive Black Gene:
Most solid black dogs have the dominant black gene described above, but there is also another, much rarer type of black called recessive black. Recessive black only occurs in a handful of breeds, including the German Shepherd Dog, Shetland Sheepdog, Schipperke and Puli. Some breeds, such as the Belgian Shepherd Dog, are thought to carry both recessive and dominant black. Most of the breeds that carry recessive black are herding/pastoral types, implying that it originally occurred as a mutation in a common ancestor (most likely a primitive European herding dog).
Recessive black is thought to be on the A locus. It is denoted by a, and is generally put right at the bottom of the A locus because it is recessive to every other A locus gene (sable Ay, agouti aw, tan points at). This means that if a dog has just one a allele, it will not be solid black (but sable, tan-pointed, etc), as it needs two a alleles for the recessive gene to work.
If you're wondering why the dog shown above is white, it's because Samoyeds have also been proven to have the recessive black gene. However, here it is combined with other alleles, which dilute or remove the pigment.
Recessive black is, aesthetically, no different to dominant black. The only difference is in the breeding - a solid black puppy could be born from two parents who are non-solid black if they both carry (without expressing) one copy of the recessive black gene, whereas a dominant black pup could only be born if one or both of its parents are also dominant blacks. Another important aspect of recessive black is that it is on a different locus to dominant black. This makes it the only way that a dog can still be solid black if it is kk (non-solid black) on the K locus.
The gene causing dominant black in dogs is known as CBD103 (Canine Beta Defensin 103). When this was discovered, it was the first time that this gene had been shown to have an effect on coat/skin colour. The human version of this gene, DEFB103a, is involved in immunity, however there doesn't seem to be any link between the K locus and health in dogs.
If you've studied the genetics of any other domestic animals then you'll probably be aware that black in almost all other mammals is recessive, making dogs more-or-less unique. Cattle also have a dominant black gene, but theirs is located on the E locus (MC1R). K-locus black is found in dogs only.
The discovery of this DEFB103 gene will allow DNA testing to determine which dogs are homozygous for black, and which are heterozygous, in breeds that have black and fawn or black-and-tan varieties, such as Pug, Miniature Schnauzer, Briard, Japanese Chin, Italian Greyhound, etc..
K Locus Testing:
Animal Genetics currently offers a test for the K Locus to determine how many copies of the dominant "KB" allele a dog carries. Dogs can be DNA tested at ANY age.
Animal Genetics offers DNA testing for the dominant KB allele. The genetic test verifies the presence of the dominant mutation and presents results as one of the following:
The dog carries two copies of the dominant "KB" allele. The dog will not have fawn offspring. The dog will always pass on a copy of the "KB" allele to all offspring.
One copy of the dominant black allele was detected. The agouti phenotype will be altered and in some breeds can result in brindle. The dog could pass on this allele, or either the brindle or fawn allele, to any offspring.
The dog does not carry the dominant black mutation. The dog's coat color will be determined by the agouti gene, and may pass on brindle or non-brindle.
No time to read the whole thing? Here's the quick version!
The K locus in dogs has three alleles: K (dominant black), kbr (brindle) and k (non-black).
Dominant black dogs (Kk or KK) are solid eumelanin all over, with or without white markings. The eumelanin can be modified by other genes to liver, blue, isabella or even merle, but there can't be any red/tan (phaeomelanin) in the coat except for bronzing, which is discolouration or bleaching.
kk dogs are "non-blacks". A kk dog will display whatever it has on its A locus (sable, tan points or agouti), which will usually be a mix of eumelanin and phaeomelanin. A kbrkor kbrkbr dog will also display whatever it has on its A locus, but any phaeomelanin in the coat will be brindled.
Most black dogs have the dominant black gene, but there's also another, much rarer gene that can cause solid black too - recessive black (a on the A locus).
Seal and ghost tan are caused when the A locus seems to "show through" on a dominant black dog, possibly caused by a faulty K allele.