"What Causes the Brindle Coat Pattern?
Just like other coat colors and patterns, the brindle color pattern is a genetic trait, caused by a particular combination of genes.
There are a handful of different places (loci) along your dog’s DNA strand that determine her color pattern. These are referred to as gene series, and they are labelled by a letter.
The mutation for the brindle trait is located at the K locus. There are three different variations of genes (called alleles) at this locus. One makes the dog all black, one essentially defaults to other alleles, and, as you may have guessed, the other one makes the dogs brindle. Brindle is dominant over the yellow (default) coloration, but recessive to the black gene."