Locus also known as "loci" is the position of a gene or mutation on a chromosome that contributes to an animals genetic traits. Each loci can have a variable number of alleles,however, not all breeds carry all the possible alleles at each locus. Each Locus has room for one of two more alternative forms of Alleles( gene variants). There are about 10 recognized "loci" of different color genes that determines the coat and skin color of a dog. Each can have a variable number of alleles that can influence the color outcome (dilution, pattern, dominate, or recessive). When a dog has two copies of the same allele they are said to be homozygous. When the two alleles they have are different, they are known as heterozygous (The Kennel Club UK 2020). As with all genetic traits, every animal inherits one copy of each allele from each of its parents (VetGen,2020) and the combination of alleles at each unique loci determine the final coat colors. The coat color genes (alleles) commonly found in a French Bulldog genetic panel are E, A, D, K, S, I, B, and/or C loci. The order of dominance to receptiveness of these genes for French Bulldogs are:
EmEM > EE > ee
DD > Dd > dd
K-Locus (Dominant Black)
K > Kbr > Ky
Ay > at > a
B-Locus (bb=testable chocolate,BnBn=non-testable chocolate)
BB > Bb> bb
CoCoa-Locus (Mutated Chocolate)
COCO > Co >co
I-Locus (Intensity Gene)
French Bulldogs coats come in a variety of colors. Cream, white , black, fawn (from light fawn to red fawn),pied, brindle are colors that are accepted by canine association like the AKC for show dog purpose. The more rare and popular colors are Blue, Isabella, Lilac, Merle, Chocolate, Blue fawn,Solid black, Sable, and Black with White. Rare color may be registered with a majority of kennel clubs, however, they are limited to specific competitions or not allowed to compete at all.
E-Locus ( Pattern Gene)
The E Loci also know has MC1-R is responsible the Melanistic Mask seen on breeds such as french bulldogs, boxers, and pugs etc. There are a total of 6 theorized types of E-Locus and three known. At its weakest the mask(Em) factor may produce black hair fringing the mouth, or a slightly smutty muzzle. At its strongest most of the head is black, and there is considerable blackening of chest and legs. In its strongest version, it can change a black and fawn/sable to a pseudo-black, with fawn/sable so restricted in its distribution that it may not be immediately apparent that the dog is not completely black.
E also helps to determine which areas of the coat can (and do) produce eumelanin (black/blue/liver/isabella) and which can (and do) produce phaeomelanin (red)(http://www.doggenetics.co.uk,2020). As previously mentioned, a dog only needs one copy of Em or E to express brindle. The following are the known E-Loci associated with French Bulldogs:
Em/Em = This dog carries two copies of E which allows for the production of black pigment (black mask). It replaces phaeomelanin (tan) with eumelanin (black). The distribution of the pigments on the rest of the face and on the body is determined by the K, A, and B Locus--these 3 locus together changes the visual appearance of brindling on a dog to nearly not being visible at all. The mask appears over the top of the A locus pattern in dogs that are heavily sabled.
E/E = 2 copies of black
E/e = 2 copies of Normal extension allowing all areas of the coat to produce both pigment type patterns expressed as per the A and K Locus. (brindling cannot be hidden)
e/e = 2 copies of red/yellow are present. Dog is red/yellow/cream (brindle will be hidden until next generation if dog also carries Kbr/Ky)
The Melanistic Masks can appear on any dog that is genetically sable, tan-pointed, saddled or agouti ( ex. AW- Locus =wolf grey). That means that the presence of a mask on a dog with the E-Loci heavily depends on the A and K gene series. In order to display a mask, a dog cannot be dominant black (Kb/Kb, Kb/Kbr, Kb/ky) on the K locus, because dominant black stops the A series from being expressed, and obviously, a black mask on a black dog is not going to be visible anyway. If a dog is brindle or non-black on the K locus, they can express the A locus. All of A-Locus genes can show masks except for recessive black (again, a black mask isn't going to be visible on a black dog, even if it is technically there!). (http://www.doggenetics.co.uk,2020).
How to read this VetGen Chart (VetGen.com): EmEm>KbrKy (Patterning locus>brindling) >dd (alters intensity of pigment--black areas altered to blue and A locus (champagne ) or or if bb Brown (isabella), Aya (background color determination), BB (determines color of dark areas--brindled pattern areas for example)
Note: A single copy of Ay (specifically sable or heavily sabled dogs) and I Locus (InIn) can prevent brindling from being seen--so while genetically the dog is brindle, visually they may not appear that way (i.e. not a true blue or lilac genetically). In addition, Kbr sometimes is not present in all dogs in general despite carrying Kbr because the D Locus will dilute boththe A Locus and D Locus (VetGen, 2021). For more information on champagne coloration that can occur as a result of D Locus dilution please visit or VetGen.com.
In the chart below you can see how the E Loci interacts with the B locus to determine coat color. The chart presents various outcomes of the B and E Loci combinations. Vertical (dog 1 is BBEE) when breed to horizontal (dog 2 is BBEE) will results in all black puppies.
K-Locus ( also called dominant black/ Pattern gene)
The K-Locus is often the most debated allele amongst breeders. The K-Locus consists of three different alleles, or variants. These variants in order of dominance are KB- dominant black, Kbr-brindle, and Ky-no brindle. The activity involved at the K-Locus are a result of Mosaicism and occurs in all brindle dogs. Mosaicismis is a condition by which some cells express one allele (KB-dominant black) and some express the other. For example, a brindle dog may express Kbr cells in some areas and ky (allows expression of the 'A' locus) loci at the other causing brindling. The K gene is considered a visual gene since only one copy of Kbr is needed to be expressed in a dogs coat to be seen visually and no commercially available tests exist that are able to detect brindle (Kbr). Brindle dogs will usually be Kbr/Kbr with obvious brindle markings or and carriers having Kbr/Ky ( with obvious or light brindle marks near hind quarters and/or feet). Kbr/Ky dogs have a probability of passing the brindle gene to 50% of their offspring.
Since Kbr is recessive to KB, no KB allele can be present in a dog for it to be brindle (Journal of Veterinary MedicalScience, 2011). As a result, all brindle dogs are either Kbr/Kbr or Kbr/Ky (Ciampolini R, Cecchi F, Spaterna A, Bramante A, Bardet SM, Oulouden A. 2012. Animal Genetics 44:114-117, 2012). Dogs that are Kb/Ky will be black in color. Dogs that are Kbr/Ky will express there A-Locus gene in place of they Ky gene to create the brindling pattern (Mocsaicism). The A-Locus allows us to visually see the base color and brindle pattern for dogs that are Kbr/Ky. Everything happening at the A locus for dogs that are Kbr/Ky is hidden until the next generation/breeding, for dogs carrying the ee loci (VetGen,2020) in addition to Kbr/Ky. Please note that a single copy of Em- or E loci allows the expression of brindle and as a result, dogs carrying these alleles cannot be hidden carriers of Kbr. (AnimalGeneticsUk.com,2020).
Genetic results for coat color conducted by genetic labs such as UC Davis,Vet Gen, and GenSol Diagnostics will only report results stipulating that the K gene exist with additional notation on whether it is dominant (Kb or Kbr), recessive, or it may say Carrier/Affected. Recall that either Kb or Kbr can be dominant depending on the combination of K alleles at the K-loci, with Kb being more dominant than the others. To determine if a French Bulldog is a brindle carrier look at either the E, B or A gene, in combination with K loci results as a method for identifying brindle on a dog that does not visually show brindle or to determine the brindle pattern.
A French Bulldog that has one or two copies of the Kb (dominant black) allele, will only express his base coat color, which is determined by the B-Locus and E-Locus. If a dog is Kb/Kb, Kb/Ky, Kb/Kbr, the B-Locus will cause brown(chocolate) fur to appear in fur that would otherwise be black in all dogs (French Bulldog Included). The French Bulldog has an additional genetic loci that can make it chocolate. The allele that make this possible is the C-Loci (mutated gene) . While the B-Loci generally results in a all dog being (bb=chocolate on B-Loci), Cocoa (coco=Cocoa ) can make a French Bulldog chocolate as well.
As previously mention, there are no test capable of determining if a dog carries brindle. However, the E, B, and A loci are good indicators for determining if your French Bulldog is brindle or carries brindle in combination with a visual check, and knowledge of their parentage. VetGen provides a wonderful chart that simplifies how these genes interact to determine the final color of there coat, including if they are brindle.I recommend including the E, D, K, A, B locus testing from your select genetic organization in order to conduct a reverse lookup using this chart in order to determine if your french is brindle, a brindle carrier, or rare color.
KB/KB = This dog carries two copies of KB which prevents expression of the agouti gene (A locus) and allows for solid eumelanin (black pigment) production in pigmented areas of the dog. However, this dog’s coat color is also dependent on its genotypes at the E and B genes. This dog will pass on KB to 100% of its offspring.
Interpretation: No agouti expression allowed
KB/ky = This dog carries one copy of KB and one copy of ky which prevents expression of the agouti gene (A locus) and allows for solid eumelanin (black pigment) production in pigmented areas of the dog. However, this dog’s coat color is also dependent on its genotypes at the E and B genes. This dog will pass on KB to 50% of its offspring and ky to 50% of its offspring.
Interpretation: No agouti expression allowed (Carrier)
ky/ky = This dog carries two copies of ky which allows for the expression of the agouti gene (A locus) which can result in a variety of coat colors including sable/fawn, tricolor, tan points, black or brown. However, this dog’s coat color is dependent on its genotypes at the E, A and B genes. This dog will pass on ky to 100% of its offspring.
B-Locus ( Black/Normal Chocolate)
The B locus (TYRP1) is a gene modifier that causes a change in the production of the eumelanin. This dilutes black color pigment into a brown color;Red pigment (pheomelanin) is not affected. This mutated gene that converts black to brown is known as the "b" allele (Animal Genetics,2020). B-Locus testing can be conducted on any breed of dog to test for the brown color or black, including french bulldogs.The results would appear as b/b for chocolate or B/b for chocolate carrier. There are some limitations to this test as it cannot identify the normal (untestable) chocolate gene on this same loci, specifically BB. Hence, the normal chocolate gene B/B is call un-testable chocolate. French bulldogs that test b/b are called testable chocolate, meaning that geneticist can confirm that they are 100% certain they are chocolate dogs based upon their testing methods. Often french bulldogs that received dna coat color results B/B. B/B is what is considered the 'Normal' chocolate gene and is called un-testable chocolate in french bulldogs; un-testable chocolate has not been clearly understood by geneticist and as a result is listed as BB. In Early 2020, genticist at VetGen identified another gene that causes french bulldogs to be chocolate. They decided to call that gene CoCoa in order to not cause confustion with the chocolate colors on the B-Locus.
Prior to the availability of the Cocoa coat testing available through VetGen, breeders relied their breeding experience and genetic coat. If test results came back as B/B, they would conduct a 'Red' eye test with a video camera to determine if a french bulldogs was indeed chocolate. If a french bulldog with B/B on the B-locus eyes show red glare these dogs were visually confirmed as chocolate dogs.
B/B = This dog carries two copies of B at all three of the bc, bd, and bs loci making the overall B locus genotype of this dog B/B. This dog is not chocolate. This dog is a black and may be a carry un-testable chocolate.
B/b = This dog carries one copy of B and at least one copy of b at the bc, bd or bs locus making the overall B locus genotype, meaning this French Bulldog carries one copy of testable chocolate and one copy non-testable chocolate.The dog’s coat color will dependent on the E, K, and A genes.
b/b = This dog carries two copies of b at the bc, bd or bs locus making the overall B locus genotype of this dog b/b. This dog has two copies of testable chocolate and will have a lighter chocolate coat than the non-testable chocolate.
CoCoa-Locus (Normal/Primary chocolate gene)
Testing for the new CoCoa -Locus mutation is available through VetGen and some European Genetic companies only to date. The mutation was identified by way of whole genome sequencing in two “untestable chocolate” French Bulldogs (VetGen,2020). Nearly all lilac, lilac fawn, lilac brindle and lilac merle dogs will have two copies of this mutation.If your french bulldog test B/B through UC Davis, I suggest that you take advantage of the Cocoa test offered through VetGen as well. The cost of the test is $55.00 for the first test and $20.00 for each additional test. The CoCoa test the Cocoa-locus for the normal/primary chocolated gene, while B-locus tests for the mutated chocolate gene. A french bulldog that is BBon a UC Davis coat color testwill be co/co(visable brown) on the Cocoa test, thereby removing regardless of what you may find in your at home red eye test with a camera.
Co/Co = no Cocoa gene
Co/co = carries one copy Cocoa
co/co = two copies Cocoa ( visually brown)