What is the brindle pitbull’s bloodline?

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The American Pit Bull Terrier (note that “Pit Bull” is two words, both capitalized when used as a proper breed name) derived from Bull-and-Terrier dogs (also the source of the American Staffordshire Terrier, Staffordshire Bull Terrier, Boston Terrier, Bull Terrier, Miniature Bull Terrier, and American Bulldog).

The brindle color was a common color in Bulldogs of the era when the breed was developed, and also can occur in several of the terrier breeds that may have been used in development of the Bull-and-Terrier type dogs (and then their later formalization into breeds). *Brindle coloring is controlled by the K locus and has middle dominance between dominant solid color and normal expression of the agouti gene (which controls where the two pigment types found in dogs occur in dogs that display both). Dogs with a brindle and a not-solid-colored parent are often brindle and a breed derived from an often-brindle breed is likely to have a lot of brindle individuals unless they are being actively selected against.

*This is, of course, a simplification of canine color genetics. There are other modifiers like the masking gene, and brindle still doesn’t “dominate” the agouti pattern so much as modify it by causing the tan/yellow/red portions to be striped with black/blue/chocolate/liver, so a sable + brindle dog is brindle everywhere, but a -and-tan pattern + brindle dog would show brindling only on the tan points.