Brindle coloration is caused by different alleles at the “K locus”, which is probably a gene called ASIP that controls pigment switching (Ciampolini, 2013). There are three alleles of the K locus: K^B, k^br, and k^y (Kerns, 2007). The K^B allele is dominant over the other two alleles and produces solid black color. k^br produces the brindle color pattern and is dominant over the k^y allele. This means that dogs with the genotype k^brk^br or k^brk^y will have the brindle color pattern. Dogs with the genotype k^yk^y are yellow in color.

Figure : This boxer shows the brindle color pattern, which looks sort of like tiger stripes. (Credit:  Steve Henderson Location: Memphis, TN)

The K locus and MC1R (which controls the difference between dark eumelanin and light pheomelanin production) have an epistatic relationship. If a dog has two recessive alleles for MC1R and is therefore unable to make eumelanin, the dog will appear yellow regardless of its genotype at the K locus.

References

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Ciampolini R, Cecchi F, Spaterna A, Bramante A, Bardet SM, Oulmouden A. 2013. Characterization of different 5′-untranslated exons of the ASIP gene in black-and-tan Doberman Pinscher and brindle Boxer dogs. Anim Genet. 44(1):114-7.

Kerns JA, Cargill EJ, Clark LA, Candille SI, Berryere TG, Olivier M, Lust G, Todhunter RJ, Schmutz SM, Murphy KE, Barsh GS. 2007. Linkage and segregation analysis of black and brindle coat color in domestic dogs. Genetics. 176(3):1679-89.