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Basic Genetics of Chocolate (Liver) Coloration in the Canine.
by Fred Lanting
Copyright November, 2005 -
Most black, brown, and yellow colorations are due to the presence of melanin class pigments. The word melanin is derived from the Greek word for black, and is commonly used to refer to the two or three known chemicals similar to each other, which produce the above color families. It can be said that there are only two colors in the haircoat of the canine: black and yellow, all others being variations of those. Eumelanin gives the black and dark brown colors; phaeomelanin gives the light brown and yellow ones. Certain genes on various chromosome locations affect the expression of these two so that the dark melanin may appear as black or be diluted to blue or changed to a chocolate brown, and the various shades of gelbe may exist as yellow, tan, light to darker brown, cream, etc. These genes can be referred to as modifier genes. Now, we will be talking about these modifiers as well as genes with more obvious, large-scale effects, and you should remember that whichever one is being discussed, many or perhaps most genes have different versions. The versions are always on the same location on the chromosome, and are called alleles of that gene; modifier genes are distinctly different DNA segments and have their own loci (and their own alleles).
Each gene is an individual, not duplicated anywhere else on any other chromosome or locus, but is duplicated by splitting during cell division, and may be exactly mirrored on the partner-chromosome (the condition is called homozygosity). So, we have a gene that calls for dark color parts to be black, and it appears at a specific locus. We also have a version (allele) of that gene calling for the dark color to be liver-brown and a version calling for those areas to produce blue instead. Yet there is only one “reserved director’s seat” available for the gene that determines “dark pigment” (eumelanin) and no seats anywhere else on that or any other type of chromosome. If you don’t have a ticket, you can’t sit there and direct the performance, and you aren’t allowed to sit anywhere else, either.
As the coat grows out, melanocytes manufacture melanin which then passes into the cortex of the hairs, becoming keratinized along with the cytoplasm of the hair cell as it grows up from the root through the skin. Deposited in certain patterns, these melanins along with differences in body form help a dog identify other members of his breed. That red brown tipped burnt appearance of the coat that we see sometimes, especially noticeable in black dogs, is most-often a result of sun bleaching the melanin in the hair to some extent, made more obvious by a high-protein diet, but it could also be the action of a partially otherwise hidden recessive modifier gene for dilution or lighter than-normal pigment.
If B is present (as homozygous BB or heterozygous Bb), it specifies that the “dark pigment” controlled by the gene(s) at that chromosome locus must be black. But if “bb” is there instead, they require that the dark pigment be brown instead of black (we usually call it chocolate or liver). Capital letters are used when a trait is believed to be dominant or "stronger" than the other genes that could occupy that same locus. The world allele is used to refer to a form of a specific gene. For example, B and b are two possible alleles that could exist at the B locus in the color chromosome. One calls for black pigment in the saddle, the other calls for chocolate-brown in the same place. If both members of the pair are B, the saddle (and/or other dark areas) would be black; if both were b, it would be brown or liver colored. If the cell had one of each (Bb), the dominant one (B is higher on the map than b) would prevail, and the color there would be black.
In other words, if the gene we call B is present in a GSD, eumelanin will show up as black, even when paired with b, but if B is absent and only the double b is present, that "same" melanin will be expressed as brown in the “dark pigment” portions of the coat. That is, only the saddle and other areas that normally may be black in the German Shepherd (or several other breeds) will be liver brown instead. The causative gene will have no noticeable effect on the tan parts of the dog, since another gene affects the phaeomelanin that controls these tan parts. The eumelanin affects the nose, nails, and saddle (if the dog has a “saddle-pattern” gene at the A locus). Other genes on other loci determine the extent of black or brown or blue coverage, such as the size of the saddle and markings. In many breeds, the modifiers are so numerous or so variable, that the dogs are said to come in a great variety of colors; Greyhounds and Poodles are good examples. Dogs have 39 pairs of chromosomes in a set, and you mathematicians can calculate the number of possible arrangements or combinations as being well over a billion; the odds of getting even littermates that are close to looking like twins are also extremely remote. Foxes, with 19, have less phenotype variation for this reason as well as due to the fact that breeding choices are made differently.
The Black/Brown Series, B Locus Does not Affect the Other Areas of the Coat
In the German Shepherd Dog breed, only two alleles exist for the B locus. BB or Bb will produce black pigment, but bb will affect the production of eumelanin granules in such a way (more diffuse, different shapes, etc.) that instead of black, they show brown. This brown is not the same braun as appears on the tan parts, but rather is found where black is normally found in a BB or Bb dog, such as the saddle, nose, eye-rims, etc. It is variously referred to as liver, chocolate, or seal brown, depending on the custom in whatever breed or species is being discussed. German Shepherd Dog fanciers use the term liver most of the time.
Black is dominant over liver (B is stronger than b), but most dogs I've seen that carry the liver recessive have a lot of reddish coloration in the neck and croup areas, and much mixed in the saddle as well. Most do not have black on their faces, although there is a different gene calling for a dark mask. These genes on the B locus affect only the dark (eumelanin) pigment, not the yellow (phaeomelanin) pigment. In the case of sables, the phaeomelanin's effect is seen not only on feet, belly, and tan markings, but seems to be visible in the undercoat over the entire dog. But this yellow/tan pigment is overlaid and sometimes partially hidden by the darker eumelanin in the tips of the hairs. Sometimes, whether due to the ribbing or to rows of bundles of hairs, sables may have a slightly and more or less striped or stippled appearance; however, this should not be mistaken for brindling, which is vertical in appearance.
The German Shepherd Dog with liver nose and saddle is fairly uncommon, but other than appearing lighter, the liver and tans can be as good looking as B&Ts. One very attractive liver and tan bicolor male was whelped in a litter of B&Ts. His sire was a B&T bicolor whom I wouldn't have suspected of carrying the liver gene, so this was another example of how a dog can hide his genotype. Of course, the dam had to carry the recessive, too, but as with the sire, I don't remember anything unusual about her; she appeared to be a fairly normal, though not as deeply pigmented bitch of B&T saddle marked coloration. The liver gene can appear in any coat pattern whether sable, saddle, bicolor, or even solid, as B/b is independent of the A (pattern) locus. It is fully visible in the homozygous state, bb. If the liver and tan bicolor mentioned above were to be bred to another of the same pattern and color, and if each had the ebeb alleles on the E locus, they might produce a small percentage of atatbbebeb dogs that would be solid liver. Unless intentionally bred for, this is unlikely to happen since most GSD breeders are quick to hide their “mistakes”, not realizing it is a simple Mendelian trait and easier to control than polygenic traits. Pups with disqualifying faults are harder to sell, so such dogs or their parents don't often get bred. By the way, since we have a longer string of alleles in the above example, I should state here that some geneticists like to use slash marks between loci (example: Ay/as or Ay/Ay) to make these strings of abbreviations easier to read.
I don't think I've ever seen a homozygous liver sable GSD, unless it was one of those I mistook for fawn, but the condition can happen, and would produce a less distinct contrast of overlay to the tan ground color. Much of the appearance would depend on what genes are at the C, E, A, and perhaps others unidentified loci.
There have been reports that b may be a lethal or semi lethal gene, but there is no basis in fact for this old-wives’-tale. The presence of bb has not been shown to be lethal or have any deleterious effect on viability in the German Shepherd Dog or in other breeds, either. The liver dogs I have seen are as healthy as other, “normally-¬colored” dogs are.
While to my knowledge it hasn't happened yet, a GSD could be dihybrid for both bb and dd. Have you ever seen a dog of another breed that was homozygous for both liver and blue, one whose partial genotype was bbdd? Of course you have — the ghostly silver Weimeraner is an example. Another is the “Isabella” Dobe. Yes. it could happen in the German Shepherd Dog breed, as the genes are all present in the population, and if it did occur it could be a solid color dog (atatbbddebeb), a saddle (asbbddE), a bicolor (atatbbddE), or a sable (AybbddE).
Fred Lanting is an internationally respected show judge, approved by many registries as an all-breed judge, has judged numerous countries’ Sieger Shows and Landesgruppen events, and has many years experience with SV. He presents seminars and consults worldwide on such topics as Gait-&-Structure, HD and Other Orthopedic Disorders, Anatomy, Training Techniques, and The GSD. He conducts annual non-profit sightseeing tours of Europe, centered on the Sieger Show (biggest breed show in the world) and BSP.
All Things Canine -- consulting division, Willow Wood Services Phone: 256-498-3319 Fax: 256-498-3311 E-mail mr.gsd@netscape.com
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