Bicolour vs Tricolour Dog Genetics: What's the Actual Difference?

Bicolour and tricolour are terms that get used loosely across many breeds. Here is a precise explanation of what each means genetically and how they are produced.

In casual use, "bicolour" and "tricolour" just mean "two-coloured" and "three-coloured" respectively. But in the world of dog coat genetics, these terms have specific meanings that vary slightly by breed convention. Here is the breakdown.

Bicolour

Common usage: A dog with two colours — typically a dark primary colour (black, chocolate, blue) and white, with the white distributed in specific locations.

Genetic basis: Bicolour is typically produced by the S locus (piebald) — specifically the sp/S genotype (one copy of piebald), which produces white in standard locations:

• White blaze on the face (optional)
• White collar (optional)
• White chest
• White feet or socks
• White tail tip

Examples:

• Black-and-white Border Collie (no tan points)
• Chocolate-and-white Springer Spaniel
• Blue-and-white Australian Shepherd without tan points

Key distinction from tricolour: A bicolour dog has two colours — typically a solid base + white from the S locus. There are no tan point markings.

German Shepherd note: In German Shepherds, "bicolour" refers to a specific pattern where the dog is mostly black with tan only on the lower portions of the legs and paws. This is different from the standard saddle pattern. Bicolour GSDs are at/at with specific expression of the tan point pattern.

Tricolour

Common usage: A dog with three visible colours — typically black (or chocolate or blue), tan/copper, and white.

Genetic basis: Tricolour requires BOTH:

1. Tan point pattern (at/at at the A locus) — providing the dark body and tan markings
2. White markings (sp allele at the S locus) — providing the white chest, blaze, feet, etc.

The three colours are: dark base colour (eumelanin) + tan/copper points (phaeomelanin) + white (no pigment from S locus expression).

Examples:

• Tricolour Beagle: black saddle + tan points + white base
• Tricolour Bernese Mountain Dog: black + rust + white
• Tricolour Rough Collie ("Lassie"): black/sable + white collar + tan points
• Tricolour Corgi: black body + tan points + white markings
• Tricolour Australian Shepherd: black + copper + white

What About Merle-Tricolour?

In Australian Shepherds, Border Collies, and Shelties, you will encounter the term "blue merle tricolour" or "red merle tricolour." This adds the merle patterning (M locus) to the tricolour base:

• Blue merle tricolour: Merle (creating the mottled grey-blue pattern) on a black tricolour base — black areas become marbled grey and black, tan and white remain
• Red merle tricolour: Merle on a red (bb) tricolour base — red areas become marbled red and cream, tan and white remain

These dogs carry at/at, sp (for white), and at least one merle allele (Mm).

Summary Table

Summary

Bicolour dogs have two visible colours — usually a solid dark colour plus white from the S locus. Tricolour dogs have three — dark base, tan points (from at/at at the A locus), and white (from the S locus). Producing tricolour offspring requires both parents to contribute at least one at allele and at least one sp allele. DNA testing at the A locus and S locus predicts tricolour outcomes.