D Locus · MLPH

D Locus: Dilute

Lightens eumelanin pigment: black becomes blue/grey and chocolate becomes lilac/isabella. Also potentially lightens phaeomelanin. Two copies of the recessive d allele are required.

Alleles

D: Full pigment (dominant): Normal, undiluted eumelanin. D is dominant — one copy prevents dilution. D/D is clear; D/d is a dilute carrier that looks full-pigmented but can produce dilute offspring.
d: Dilute (recessive): Two variants (d1, d2). Any two d copies — same or different variant — produce the dilute phenotype: black becomes blue/grey, chocolate becomes lilac/isabella. One copy alone produces no visible dilution.

What the D locus does

MLPH (melanophilin) controls how melanin-containing granules (melanosomes) are distributed within hair shafts. With full D function (D/D or D/d), granules distribute evenly — producing deep, saturated eumelanin color. With d/d, granules clump together unevenly, so light scatters differently — the eye perceives the color as lighter and cooler.

The result: black → blue or steel grey; chocolate → lilac or isabella; deep red can become a paler cream. The dilution effect is strongest on eumelanin, but some data suggests phaeomelanin can also be subtly affected in d/d dogs.

Blue, lilac, and isabella — what's the difference?

Blue: A dog that is black-based (B/B or B/b — no chocolate) + d/d. The coat appears steely grey to slate blue. Nose and paw pads are blue-grey. This is the color of blue French Bulldogs, 'blue' Yorkies (actually dilute black), blue Toy Poodles.

Lilac (Isabella): A dog that is both chocolate (b/b) AND dilute (d/d). The brown eumelanin is further lightened to a soft silvery-lavender. Nose and pads appear pale lavender-pink. Sometimes called 'isabella' in French Bulldogs. One of the rarest and most sought-after color combinations.

Dilute affects only eumelanin in the coat — so a d/d dog that is also e/e (recessive red) will still look red or cream in the coat but may have a diluted nose color.

Health note — Color Dilution Alopecia (CDA)

Color Dilution Alopecia (CDA) is a hereditary follicular dysplasia that can affect some d/d dogs. The same pigment granule clumping that dilutes coat color can disrupt normal hair follicle development, leading to hair thinning, patchy hair loss, recurrent bacterial folliculitis, and dry or flaky skin — typically in the diluted areas.

CDA does not affect every dilute dog — prevalence varies significantly by breed and bloodline. It is not currently predictable by a single DNA test (the d/d genotype identifies dilutes, but which d/d dogs will develop CDA requires further genetic research). It is a real consideration, not a guarantee.

This is exactly why we prioritize health and structure over rare colors at Star Struck Puppies. Dilute puppies from our lines come from parents chosen for temperament, health test results, and structure first — color is secondary. We are transparent about CDA risk with all families.

Dilute in specific breeds

French Bulldogs: Blue (d/d black) and lilac (d/d + b/b) are two of the most sought-after Frenchie colors. Both require two copies of d — so both parents must carry d. CDA is documented in French Bulldogs and should be discussed with buyers.

Yorkies: The traditional 'steel blue' in Yorkshire Terriers is from the dilute allele acting on black (at/at) eumelanin. Many Yorkies are D/d (carriers) or d/d. The blue-and-tan pattern results from at/at (tan points) + d/d (dilute black body).

Toy Poodles: Blue (dilute black) and café-au-lait (dilute chocolate) Poodles are d/d. The breed's progressive greying gene (M locus-independent) interacts with dilute, which is why Poodle color genetics can be layered and complex.

D Locus FAQ

How do you breed a blue French Bulldog?

Blue is the dilute (d/d) form of black. To produce blue puppies both parents must carry at least one dilute allele (D/d or d/d), and the puppy must inherit two d alleles (d/d). The pairing mathematics: two D/d parents = 25% d/d blue puppies; one d/d + one D/d = 50% d/d; two d/d = 100% blue. Both parents must also be black-based (not b/b chocolate) — otherwise d/d + b/b = lilac, not blue. Always DNA test before breeding for dilute.

What is lilac or isabella in dogs?

Lilac (isabella) is double dilution: a dog is simultaneously chocolate (b/b at the B locus) AND dilute (d/d at the D locus). The brown eumelanin is further lightened to a soft silvery lavender. Nose and paw pads appear pale lavender-pink. Producing lilac requires both parents to carry b (chocolate) AND d (dilute). Without DNA testing, breeding for lilac is guesswork.

What is Color Dilution Alopecia (CDA)?

Color Dilution Alopecia is a hereditary skin condition that can affect some d/d (dilute) dogs. The same melanin granule clumping that creates the blue or lilac coat color can also disrupt normal hair follicle structure. Affected dogs may develop thinning hair, patchy bald spots, and recurrent skin infections — particularly over the diluted coat areas. Not all d/d dogs develop CDA; it varies by bloodline and individual. There is currently no predictive DNA test for whether a specific dilute dog will develop CDA.

Are all blue or lilac dogs unhealthy?

No — the d/d genotype itself is not a disease. Many blue and lilac dogs live perfectly healthy lives with normal coats. CDA is a possibility, not a certainty, and prevalence varies significantly by breed and lineage. Health-conscious breeders who use DNA testing, prioritize structure over color, and communicate transparently about CDA risk can produce blue and lilac dogs with acceptable health profiles.

Can a dilute (d/d) dog have a black nose?

No. A d/d dilute dog's nose pigment is also diluted — a blue/grey nose in black-based dilutes, or a lavender-pink nose in chocolate dilutes (lilac/isabella). If a dog has a black nose, it is not d/d. This makes nose color one of the easiest ways to spot a dilute dog — though DNA testing still provides the definitive genotype.

What is the difference between blue and grey in dogs?

In canine genetics, 'blue' and 'grey' are often used interchangeably for d/d black-based dilute dogs. The coat appears a steely grey to slate tone. 'Silver' is sometimes used for Poodles with this color, often combined with progressive greying genes. Regardless of the name used, the underlying genetics is d/d dilute acting on black eumelanin.

How does the D locus affect Yorkies?

The traditional steel-blue adult Yorkie coat is caused by d/d (dilute) acting on black eumelanin (the at/at tan-point body pattern). Yorkshire Terriers are a historically dilute-heavy breed — many carry D/d (carrier) or are d/d. The blue-and-tan standard pattern is the visual result of at/at + d/d + B/B (or B/b). Parti Yorkies with blue in their base are also d/d.

Do d1 and d2 dilute variants behave differently?

Both d1 and d2 are loss-of-function mutations in MLPH that produce dilution — any combination of the two (d1/d1, d1/d2, d2/d2) produces a dilute dog with the same visual effect. There is no known difference in phenotype between the variants. A dog that is D1/d2 (one clear and one dilute variant) is a dilute carrier — it looks full-pigmented but can pass d to offspring.