A Locus · ASIP

A Locus: Agouti

Controls how black and red pigment are distributed across the body — producing sable, wild agouti, tan points (phantom), or recessive black. Only visible when the K and E loci allow it.

Alleles

Ay: Sable / fawn: Mostly red coat with varying numbers of black-tipped (sable) hairs. Most dominant at this locus. Expression ranges from lightly tipped (clear sable) to heavily shaded (dark sable). Common in fawn French Bulldogs, sable Shih Tzus, sable doodles.
aw: Wild agouti: Banded hairs alternating light and dark — the 'wolf sable' or 'grizzle' look. Each hair has bands of eumelanin and phaeomelanin. Rare in toy breeds; more common in northern breeds and terriers.
at: Tan points: Solid body with tan/gold markings over the eyes, muzzle, cheeks, chest, legs, and vent. The classic black-and-tan or tricolor pattern (when combined with white). Known as 'phantom' in Poodles and doodle breeds.
a: Recessive black: Solid black pigment across the entire coat — only visible as a/a (two copies). Most recessive allele at this locus. Distinct from dominant black (KB) at the K locus.

What the A locus does

The A locus (ASIP — Agouti Signaling Protein) manages the timing and placement of eumelanin versus phaeomelanin within each hair follicle and across the body. It is what converts 'black pigment is permitted' into an actual pattern — whether a uniform red wash (sable), crisp point markings (tan points), or banded individual hairs (agouti).

The dominance order is Ay > aw > at > a. Ay is most dominant; a is most recessive.

When the A locus is visible

The A locus is only visible when two conditions are both met: the dog is ky/ky at the K locus (no dominant black covering it) AND the dog is not e/e at the E locus (black pigment is allowed in the coat at all).

If either KB or e/e is present, the beautiful agouti patterns are hidden — but the alleles are still inherited and still passed to offspring. A solid-looking black dog (KB/ky) carries an A locus genotype that its ky/ky puppies will express.

Sable (Ay) — from clear fawn to dark sable

Ay produces a coat where phaeomelanin dominates but eumelanin is deposited at the tips of guard hairs, creating the shaded 'sable' look. How much dark tipping appears varies with modifier genes and age — a puppy can look very dark sable and clear to mostly golden as an adult, or vice versa.

Em mask on a sable dog (Ay + Em) produces the classic fawn-and-mask look: warm golden body, dark muzzle, often dark ears. This is the most common French Bulldog color combination.

Tan points (at) — phantom, black-and-tan, tricolor

The at allele produces a solid body color with distinct tan markings at specific anatomical locations: above both eyes, on the muzzle and cheeks, on the throat and chest, inside and below the ears, on all four legs and feet, and around the vent. The body is solid eumelanin (whatever B and D loci produce); the tan areas are phaeomelanin.

In doodle breeds this pattern is called 'phantom' — it is genetically identical to the black-and-tan of a Doberman or the classic Yorkie pattern. When combined with S locus white spotting (sp/sp), tan-point dogs become tricolor.

Traditional Yorkies are at/at, which is why the standard blue-and-tan or black-and-gold pattern is the breed norm. The 'blue' in adult Yorkies is dilute (d/d) black acting on the body eumelanin.

Recessive black (a/a)

a/a produces a solid black coat across the entire body — distinct from KB dominant black because it requires two copies. Dogs with a/a can be mistaken for KB dominant black dogs, but their ky/ky offspring will express A locus patterns whereas KB dogs suppress them.

In breeds where recessive black occurs (some Shetland Sheepdogs, some German Shepherds), a/a looks visually identical to KB/KB but behaves differently in breeding programs.

A Locus FAQ

What is the difference between sable and tan points?

Sable (Ay) is a mostly-red coat with varying numbers of black-tipped guard hairs scattered across the body — the shading can be light or heavy but has no distinct boundaries. Tan points (at) is a completely different pattern: a solid-colored body with sharply defined tan or gold markings at specific locations (above the eyes, muzzle, chest, legs, and vent). Both require ky/ky at the K locus to be visible.

What is a phantom dog?

'Phantom' is the name used in Poodles, Doodles, and many hybrid breeds for the tan-point (at/at or at/a) pattern — a solid base color with tan markings on the face, chest, and legs. It is genetically identical to the 'black and tan' pattern of Dobermans or Yorkies. The name varies by breed community but the genetics are the same.

Can you tell by looking at a dog whether it carries tan points?

Only if the dog is ky/ky at the K locus AND not e/e at E — then at/at or at/a expresses visibly as phantom or tan-point patterning. If the dog is KB/ky (solid black overriding the A locus), or e/e (red covering everything), tan points are invisible in the coat. The only reliable way to know a solid or red dog's A locus genotype is DNA testing.

What causes phantom Goldendoodles and phantom Poodles?

Phantom Goldendoodles and Poodles carry at least one at allele at the A locus and are ky/ky at the K locus (so A locus patterns can show). The base color depends on B and D — black phantom is most common, but chocolate phantom (b/b), blue phantom (d/d), and even lilac phantom (b/b + d/d) are all possible. Two at alleles (at/at) always produce phantom if the K and E loci permit.

Can two sable parents produce a tan-point puppy?

Only if both parents carry at as a hidden allele (Ay/at genotype). Two Ay/at parents have a 25% chance of producing an at/at puppy, which — if ky/ky — will show tan points. Two Ay/Ay parents (homozygous sable) cannot produce tan points. This is why DNA testing is needed — a sable dog with a hidden at allele looks identical to a homozygous sable.

What is wild agouti in dogs?

Wild agouti (aw) produces hairs that are banded — alternating between dark (eumelanin) and light (phaeomelanin) zones along each hair shaft. The overall impression is a grizzled, wolf-like coat. It is subordinate to Ay but dominant over at and a. Wild agouti is rare in the toy and companion breeds we work with but is common in northern breeds, primitive dogs, and some terriers.

What A locus genotype do Yorkies carry?

Traditional Yorkshire Terriers are at/at at the A locus — homozygous tan points. This produces the classic black-and-gold puppy coat that clears to steel-blue (dilute black, d/d) and tan in adulthood. All-gold Yorkies without tan points are typically e/e (recessive red), which overrides and hides the tan-point pattern. DNA testing distinguishes these two genetically different 'gold' Yorkies.

Why does my dog's sable coat change color as it grows?

Sable (Ay) expression is highly sensitive to age, hormones, and modifier genes. Many sable puppies are born dark — heavily tipped with black — and clear to mostly golden or fawn as adults. This happens because the balance of ASIP signaling shifts during coat development. A puppy that looks almost black-and-tan at birth can be a very clear sable by 12 months.