B-Locus Genetics
Chocolate French Bulldog DNA
The complete guide to B-locus chocolate (b/b) and co-locus cocoa. How testable chocolate is produced, how it differs from cocoa, and how it builds into lilac and Isabella.
Model Chocolate PairingsWhat Is the B-Locus Chocolate Gene?
The B-locus controls brown pigment production in French Bulldogs through the TYRP1 gene. Two recessive alleles (b/b) transform black eumelanin into rich chocolate brown.
B Allele (Dominant Black)
The capital B allele is dominant and produces normal black pigment. Dogs with B/B or B/b show black noses, paw pads, and coat shading. They do not express chocolate coloring.
b Allele (Recessive Chocolate)
The lowercase b allele is recessive. A dog needs two copies (b/b) to show chocolate. This allele changes all black pigment to brown — coat, nose leather, and paw pads all become chocolate colored.
B-Locus Genotypes Explained
Every French Bulldog has one of three possible B-locus genotypes. Only b/b produces the chocolate phenotype.
B/B
Non-Chocolate
CommonFull black pigment. No chocolate possible from this dog alone unless it carries hidden modifiers.
B/b
Chocolate Carrier
CarrierLooks black but carries one chocolate allele. Can produce chocolate puppies when bred to another carrier or chocolate dog.
b/b
Chocolate Expressed
ExpressedThe dog shows rich chocolate brown coat, nose, and pads. Foundation for lilac when combined with d/d.
Testable vs. Non-Testable Chocolate
Understanding the difference between B-locus testable chocolate and other brown-producing genes is critical for accurate breeding predictions.
Testable Chocolate (B-Locus)
- Detected by all major DNA labs (Embark, AG, DDC)
- Gene: TYRP1 on chromosome 11
- Produces rich, uniform brown pigment
- Inheritance is Mendelian and predictable
- Builds into lilac and Isabella with other loci
Non-Testable / Cocoa (Co-Locus)
- May test as B/B on older DNA panels
- Actually co/co at the cocoa locus (HPS3 gene)
- Produces a lighter, more cocoa-toned brown
- Requires updated DNA panels for detection
- Combines with b/b to create Isabella
Model Chocolate & Cocoa Pairings
Our DNA Calculator covers both B-locus and co-locus chocolate. Enter your dogs' genotypes and predict chocolate, lilac, and Isabella outcomes with precision.
Open the DNA CalculatorFrequently Asked Questions
What makes a French Bulldog chocolate?
A chocolate French Bulldog is produced by the B-locus gene. The dog must carry two copies of the recessive chocolate allele (b/b). This gene changes black eumelanin pigment to rich brown. A b/b dog shows chocolate coloring on the coat, nose, and paw pads instead of black.
What is the difference between B-locus and co-locus chocolate?
B-locus chocolate (b/b) changes all black pigment to brown throughout the dog's coat, nose, and pads. It is the classic 'chocolate' Frenchie look. Co-locus cocoa (co/co) is a newer discovery that also affects brown pigment but produces a slightly lighter, more cocoa-toned shade. When both are present (b/b + co/co), the result is Isabella — an even lighter champagne-brown color.
Can a DNA test detect B-locus chocolate?
Yes. Major DNA testing labs including Embark, Animal Genetics, and DDC all test for the B-locus (tyrosinase-related protein 1, or TYRP1). The test identifies whether a dog is B/B (non-chocolate), B/b (carrier), or b/b (chocolate expressed). This is considered 'testable chocolate' and is fully verifiable through DNA panels.
What is 'non-testable' chocolate in Frenchies?
Some breeders use the term 'non-testable chocolate' for brown-looking dogs that test as B/B (non-chocolate) on standard DNA panels. This may be caused by other genes, modifiers, or the co-locus cocoa gene. However, reputable breeders should verify through updated DNA panels that include the co-locus (cocoa) test. A dog that looks chocolate but tests B/B may actually be co/co.
How much does a chocolate French Bulldog cost?
Chocolate French Bulldogs typically cost between $5,000 and $9,000. Pet-quality chocolates start around $5,000, while breed-quality dogs with full health clearances and verified b/b genotype range from $7,000 to $9,000. Chocolate is rare because the b/b genotype is recessive and requires both parents to carry or express the chocolate gene.
Can chocolate Frenchies have blue eyes?
Yes, chocolate Frenchies can have lighter eyes including hazel, amber, or even blue. The B-locus gene affects pigment production throughout the body, including the iris. However, eye color is also influenced by other genes and modifiers. Blue eyes in chocolate Frenchies are considered desirable by some buyers but are not guaranteed by the b/b genotype alone.