Trait expression is not intuitive. Trait testing, although not usually as critical as disorder testing, can be quite confusing to interpret, with many tests available, and many interacting with each other to produce a unique result. Recessive, dominant, and semi-dominant traits are all common, as is something called epistasis, in which one gene prevents or masks the expression of another gene. If you need a refresher on terms, please visit our post on genetics terminology.
There are two pigments in dogs, eumelanin, responsible for the dark pigments of black and brown, and phaeomelanin, responsible for red, cream, and yellow pigments. Red and black pigments are not subject to averaging of the colors of the parents, but are predetermined by a handful of genes and their alleles.
Because of epistasis and recessive inheritance, some recessive traits appear as surprises in a few puppies in a litter and can be mistaken as the result of questionable breeding, rather than being recognized as the result of complex genetic interactions. Recessive traits are often present in greater numbers and for a longer time in an invisible carrier state than is realized in the absence of genetic testing. Trait results for individuals are typically written as two letters for each locus, one from each parent, e.g. Ee, BB, dd. Below, we’ll discuss some of the most important color trait loci. There are over 50 traits tested for by Wisdom’s testing, so additional information can be found by reading report details.
In dogs there are both dominant and recessive forms of black, and they reside in different genes. The most common cause of black by far is dominant black, or K. Other variants at this locus are Kbr (brindle) and ky (non-dominant black, or no effect black). Dominant black will mask brindle, as well as any red and black patterns of the E or A locus (discussed below) if present. It will, however, be hidden by the presence of two copies of recessive red (ee), discussed under E Locus, and can also be altered to dilute or brown/chocolate. Because the type of allele that causes brindle is not easily tested, and similar to dominant black, dogs with brindle or dominant black are reported together. A dog that is heterozygous dominant black (Kky, KKbr) will not display many of the potential colors it could pass on to offspring. A dog that is homozygous dominant black (KK), unless carrying clear red (e), will always produce black or chocolate dogs, as it will always donate K.
The Brown locus variants determine whether the dog will have black or brown pigmentation. Brown, also called liver or chocolate, is recessive (bb), and prevents the development of black pigment by a defective enzyme in the eumelanin pigment pathway. If even one copy of the dominant allele is present (BB or Bb), the dog will be able to produce black pigment, and no brown will be visible. Brown replaces black pigment wherever it would have occurred, including the eyes, nose, coat, nails, etc. Brown can occur in color patterns with red pigments or can be diluted by the dilution gene to a color called Isabella or lilac.
The Agouti locus is named for a particular color pattern called agouti, common to wild animals like wolves, in which the hair is alternately banded in red and yellow, which serves as excellent camouflage. The Agouti locus has recently been found to be caused by two genes working in tandem within the Agouti gene, a ventral promoter (VP), and a hair cycle promoter (HCP), with several types of sable and other color patterns identified. Historically, Agouti has been reported based on a different marker within the Agouti gene, which is less sensitive but still accurately detects red coat patterns in most breeds. Agouti causes the red and black patterns of sable (ay), agouti (aw), black and tan (at), and recessive black (a). Wisdom currently uses the traditional system of reporting of Agouti.
As mentioned, the Extension, or E locus, contains many alleles that govern red coat color patterns in dogs, similar to Agouti. The most recessive E alleles, e1, e2, and e3 are responsible for solid red coat coloring, often called “clear red,” in a number of breeds. Interestingly, although e1-e3 are recessive, and therefore will not be seen if more dominant red alleles are also present, if two copies are present, they will hide all other black and red patterning in the coat, including dominant black. Many common sporting dog breeds, such as Labrador Retrievers and Golden Retrievers, have Agouti patterns that are never seen because of the presence of both dominant black (K) and clear red (ee).
Dilution, like brown/chocolate, affects pigment production everywhere in the body. Like brown, dilution is recessive, and therefore the dog must have two copies, one from each parent, to show dilution (dd). Dogs that are Dd or DD (no dilution) will show no sign of dilution at all. When dilution is present, the pigment is “clumped” instead of being evenly distributed, leading to the appearance of a lighter color. Black dogs become “blue” or “gray,” chocolate dogs become “Isabella” or “lilac,” and red dogs have a creamy, “champagne” colored coat, which can be subtle to distinguish. Eye color and nose leather are also changed with dilution, and dilution is present from birth. There are many forms of dilution, some which clump hair pigment too much, and cause the hair to break off easily, and some which are only present in certain ancient breeds. There are likely other dilution alleles yet to be discovered.
There are four main causes of white coloring in dogs. White in dogs is actually not a color, but the absence of pigment. Albinism is recessive and uncommon, and therefore rarely seen. Albino dogs have pink noses and pale blue, hazel, or red colored eyes, and an entirely white coat. A second variant called merle interferes with pigment production, resulting in a patchwork coat color for heterozygous dogs, but for homozygous merle dogs, an almost entirely white coat, often with deafness and eye abnormalities.
A more common cause of white coloring, which is not a true white but appears as such, is red coat coloring modified by intensity genes, resulting in very low levels of red pigment in the coat, such as in the Poodle and Maltese. These dogs tend to have normal dark eye, nail, and nose color. Lastly, and most commonly, white spotting produces what is best understood as a localized albinism, in which pigment is not expressed in certain portions of the body. If expressed in the eye region, the eye will be blue, if over the nose, the nose will be partially or wholly pink, and if in the haircoat, the skin will be pink and the coat white. Any patterning or color of the coat will not be seen in that region. “Tri-color” dogs typically show a red and black coat pattern with white spotting. White spotting is poorly understood, as some breeds show “extreme white” with two copies of the White Spotting gene, and some show only a moderate amount of white, even with two copies. There are likely breed-based variants that modify expression of white spotting.
Genetic testing should never be considered the only measure of a dog’s merit to a breeding program. It should be viewed as one of many screening tools, and always seasoned with a breeder’s good sense of holistic goals for their kennel. Each breeder will have slightly different sensibilities about what they prefer in their dogs, which in an ideal world will encourage the preservation of the full variety of the breed to be enjoyed for generations to come. Some breeds are managed from within western registries, others are working landraces used for particular purposes in a specific region. The process of research and discovery is ongoing, and Wisdom Panel™ is honored to partner with breeders around the world in delivering the latest in genetic science in service of the preservation and enjoyment of our dogs, now and in the future.
