In Labrador retrievers, coat color can be golden, black or chocolate. Golden ani
ID: 29627 • Letter: I
Question
In Labrador retrievers, coat color can be golden, black or chocolate. Golden animals result from recessive epistasis, and have an e/e genotype. Black and chocolate labs have a dominant E allele and are either B- (if black) or bb (if chocolate). The nose color of golden labs can be black or chocolate, depending on the alleles at B/b. Two dihybrid black labs are mated. What fraction of offspring is expected to be golden with a black nose?Explanation / Answer
Melanin is the substance that gives a dog's hair its color. There are two distinct types of melanin in the dog --- eumelanin and phaeomelanin. Eumelanin is, in the absence of other modifying genes, black or dark brown. Phaeomelanin is, in its unmodified form, a yellowish color. Melanin is produced by cells called melanocytes. These are found in the skin, hair bulbs (from which the hairs grow) and other places. Melanocytes within the hair follicles cause melanin to be added to the hair as it grows. However, melanin is not added at a constant 'rate'. At the very tip of the hair, eumelanin production is usually most intense, resulting in the darker tip. A protein called the Agouti protein has a major effect on the amount of melanin injected into the growing hair. The Agouti protein causes a banding effect on the hair: it causes a fairly sudden change from the production of eumelanin (black/brown pigment) to phaeomelanin (red/yellow pigment). An example of this coloration would be like the color of a wild rabbit. The term 'Agouti' actually refers to a South American rodent that exemplifies this type of hair. The Extension Locus - E This refers to the extension of eumelanin over the dog's body. The dominant form, "E", is normal extension. The recessive form, "e", is non-extension. When a dog is homozygous for non-extension (e/e), its coat will be entirely red/yellow (phaeomelanin based). All dogs that have a brown (chocolate) coat will have at least one "E" allele, because of the production of eumelanin. The way to tell the difference between an Agouti red/yellow and an Extension (e/e) red/yellow dog -- is the Agouti red/yellow almost always have some black/brown hair in the coat (usually around the ears and tail) and the Extension (e/e) dog won't. Another way is the Agouti red/yellow must have at least one ("A^y") allele and can carry at most one other agouti allele, the Extension (e/e) can carry any two Agouti alleles (not necessarily "A^y"). DOMINANT BLACK -- "K" The dominant form of black: completely dominates all formation of phaeomelanin pigment. In the past, dominant black had been placed at the head of the Agouti series (symbol "A^s"). Now, it has been proven to be part of a separate series, the "K" series, and not at the Agouti locus at all. Dominant black (K) is epistatic to whatever is found at the Agouti locus (simply means that it causes the Agouti allele to act differently from what it normally would), however; "e/e" is dominant to "K" at the E locus. When "K" is in the dominant form, "K/K" or "K/k", there would be no expression from the A Locus and the color is dependant on what is at the E Locus. When "K" is in the homozygous recessive form "k/k", the coat color will depend on what is located on the "E" and "A" Locus. Dominant "K" codes for both dominant black and brindle in decreasing order of dominance: K -- dominant black (does not allow the A Locus alleles to be expressed) k^br -- brindle (expressed when A Locus alleles are expressed) k -- normal (allows the A Locus alleles to be expressed) A dog that is: K/K or K/k -- dominant black; dominant black carrying recessive black k^br/k^br -- brindled k^br/k -- brindled, carrying recessive black k/k -- 'normal' (recessive black) Brindling is 'stripes' of eumelanin-based (can be modified by the genes at the B and D Locus, so the color could be black, blue, chocolate or fawn) hairs in areas that are otherwise phaeomelanin based. In order to produce the brindle color, at least one parent MUST be a brindle. Brindle is dominant to its absence, so only one copy is needed. If a person has a brindle colored pup for sale and there are no brindle colors anywhere in the pedigree, then the sire that is reported on the registration papers --- genetically can not be the (true) sire. There is an exception to this if the dog is "e/e", he can be a carrier of brindle. It is thought that the three loci E, K and A act together as follows: If the dog is "e/e" at the E locus, and at the K locus, it is "K", "k^br" or "k", its coat will be entirely red/yellow (phaeomelanin based); If the dog is E/E or E/e at the E locus, and at the K locus, it is "K/K" or "K/k", its coat will be entirely dominant black (eumelanin based) [**NOTE: the phenotypic color will depend on what is at the B, D, C and M Locus]; If the dog is E/E or E/e at the E locus, and at the K locus, it is "k^br/k^br" or "k^br/k" it will be brindled with the color of the phaeomelanin part of the brindling affected by the Agouti alleles present; If the dog is E/E or E/e at the E locus, and at the K locus, it is "k/k" the distribution of eumelanin and phaeomelanin will be determined solely by the Agouti alleles present. The Agouti Locus - A Simply, this is how the pigment is distributed on the dog's body and hair shaft. The Agouti locus controls the formation of the Agouti protein, that in turn is one of the mechanisms that controls the replacement of eumelanin with phaeomelanin in the growing hair. The alleles of the Agouti locus can affect not just whether or not the eumelanin -- phaeomelanin shift occurs, but also where on the dog's body this happens. Two promoters are generally associated with the "wild type" version of the agouti gene. Cycling Promoter Ventral Promoter The Cycling Promoter produces a banded hair with a black tip and yellow middle over the entire body. If only the action of this promoter is disrupted, the hair color on the dog's back will be black and its belly and inside of the legs will be yellow. This produces the black and tan color. The Ventral Promoter dictates that there will be only yellow color in the hair on the belly. The animal will have black banded hair on the dorsal (back) side and paler yellow hair on the ventral (belly) side. If only the action of this promoter is disrupted, the hair color on the dog will be banded over its entire body. This is said to be solid agouti color. If something inactivates the agouti protein, or if both promoters are disrupted, the animal will appear to be solid black. If a mutation occurs at one of these Promoters, this can cause the yellow to be expressed over most of the body. NOTE: In part of a series on Dog Coat Color Genetics by Sheila Schmutz, she states that recent studies show that the agouti signal peptide (ASIP) competes with melanocyte stimulating hormone (MSH), which produces eumelanin pigments, to bind on the melanocortin receptor and must sometimes win. Both the E allele and Em allele are responsive to agouti or melanocortin binding in dogs. However dogs that are ee have a mutation in MC1R and produce only phaeomelanin. The dog's agouti genotype doesn't affect its coat color, which will be some shade of cream, yellow or red. To further complicate things, agouti has 2 separate and somewhat distant promoters. Roughly, one seems to control ventral or belly color and the other dorsal or back color. The simplest way to "see" this is on a black and tan dog......the back is black from eumelanin pigment being made and the belly is tan or red from phaeomelanin pigment being made. The agouti gene has been mapped in the dog and DNA studies to determine which patterns are under the control of this gene in the dog are in progress. This gene undoubtedly has several alleles, but how many is still an open question. Some have been identified using DNA studies and tests for agouti phenotypes in some breeds may become available soon. Although several books attempt to state the dominance hierarchy of the agouti alleles, since no breed has all the alleles, it is not possible to know this for sure. Most books suggest that it is aw > ay > at > a. Breeding data and DNA data from our collaborative study with Dr. Greg Barsh's group at Stanford supports this. However the data confirm pairwise dominance/recessive relationships in different families.......not the entire hierarchy in one family. Decreasing in order of dominance: (**sable may be dominant over wolf in some breeders) "a^w", 'wolf' color - This is like "a^y" but the tan is replaced with a pale gray/cream color and the hairs usually have several bands of light and dark color, not just the black tip of sable. Example would be Keeshond, Siberian and Norwegian Elkhound. "a^y", 'sable' - also known as 'dominant yellow' or 'golden sable'. This results in an essentially red/yellow phenotype, but the hair tips are black (eumelanin). The extent of the eumelanin tip varies considerably from lighter sables (where just the ear tips are black, called "Clear Sables") to darker sables (where much of the body is dark, called "Shaded Sables"). "a^s", 'saddle' - Eumelanin is restricted to the back and side regions, somewhat like the black/tan ("a^t") allele (below). "a^t", 'tan points' - This is primarily a solid colored dog with tan (phaeomelanin) "points" above the eyes, muzzle, chest, stomach and lower legs. The hue can range from a pale biscuit to a rich ginger to a golden copper in color. Commonly seen in many breeds like hounds, Dobermans, Rottweilers and Kelpies. In breeds that have the Irish spotting, along with tan points, this is known as "tri" colored (Australian Shepherds and Border Collies). "a" - last of the Agouti series is recessive black. When a dog is homozygous for recessive black (a/a), there will be no red/yellow (phaeomelanin) in its coat (unless "e/e" is present, which is epistatic to the Agouti series). Examples of breeds that show to be recessive black are German Shepherd and Shetland Sheepdog. BLACK or BROWN (CHOCOLATE) - B GENE LOCUS: (pigment color) This gene, when in the homozygous recessive form, has a lightening effect on eumelanin (black-based colors) only. It has no effect on phaeomelanin (red-based colors). B/B or B/b - black b/b - brown It is believed that the Brown Locus codes for an enzyme, tyrosinase-related protein 1 (TYRP1), which catalyzes the final step in eumelanin production, changing the final intermediate brown pigment (dihydroxyindole) to black pigment. SO, ALL dogs start as BROWN and after the final step --- this directs the color to be black. When brown (b/b) is expressed, it means that the final step in eumelanin production has not been completed and the pigment remains brown. The brown color is not a genetic defect. When the alleles are in the homozygous or heterozygous dominant form of B/B or B/b, the color and pigment (nose, eye rims and lips) remains (or directs the color to be) black. When the alleles are in the homozygous recessive form (b/b), the color and pigment will be brown. This just means that the final step in eumelanin production of changing brown to black did not occur. Phaemelanin (yellow/red [e/e]) is not affected. BUT, in the e/e colored dog, if the dog is also b/b; they will be either red or yellow and will have brown pigment (nose, eye rims and lips). The pigment granules produced by "bb" are smaller, rounder in shape, and appear lighter than pigment granules in "B" dogs. The iris of the eye is also lightened. DILUTION - D GENE LOCUS: (dilution of pigment) Not found (has been bred out) in the ACD or ASTCD breed. This gene has an effect on both eumelanin and phaeomelanin. D/D or D/d - it allows for full color (black or chocolate). d/d - homozygous recessive form dilutes black (eumelanin) to blue, red to cream, and chocolate to a dull flat color (some call it milk chocolate color). COMBINATIONS OF B AND D IN EUMELANISTIC COATS: The effects of these 2 genes, when combined, form a range of 4 eumelanistic ('black-based') colors: The color of the pup/dog (Eumelanistic Color): B/B D/D or B/b D/d will be black in color B/B d/d or B/b d/d will be blue in color b/b D/D or b/b D/d will be brown/Chocolate (called red in Kelpies) b/b d/d will be flat or dull diluted brown/chocolate (called fawn in Kelpies). WHITE SPOTTING - S GENE: The "S" series alleles appear to be incompletely dominant. In dogs it is thought there are four alleles that deal with white spotting: "S" - 'solid/self color'. Most dogs that are homozygous for "S/S" have no white hair at all, or possible a tiny amount, like a white tail tip. "s^i" - 'irish spotting'. This involves white spotting on most parts of the coat, but not crossing the back beyond the withers. This color pattern is evident on the Border Collie, Australian Shepherd and other breeds that have the white collar. New research has proven that the white undersides of the Border Collie is dictated by a different gene."s^p" - 'piebald'. The white is more extensive than irish spotting, and often crosses the back. It is sometimes confused with the merle pattern. This coloration usually has large colored spots on the body. The white covers approximately 50% of the body.
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