D-Locus

A mutation at the Dilution-Locus can cause a color dilution of black or brown pigment. If a dog is homozygous recessive (d/d) at the D-Locus he can only produce eumelanin that is either blue (diluted black) or lilac/isabella (diluted brown).

What is Color Dilution?

The melanosomes found in a dog’s pigment cells produce and store pigment before they are distributed into the growing hair. But in dogs with color dilution, these pigment organelles can not be distributed evenly.

Instead, melanosomes clump together which creates large empty cavities inside the hair shaft. This causes the hair to absorb and reflect less light so a dog’s coat color will look pale and diluted.

Color dilution at the D-Locus oftentimes only affects eumelanin (although it is believed to also lighten the shade of phaeomelanin to some degree or n some families of dogs).

Diluted black pigment looks blue, but is also called ash-colored, silvery grey or slate. Diluted brown pigment is most often described as sandy, mousy grey, lilac, isabella or fawn.

Dogs with eumelanin dilution will also have diluted skin and nose pigmentation. Blue dogs will have a blue nose and blue footpads, eye rims, lips, skin spots or nails. And lilac dogs will in turn have all their skin and nose pigment turned to lilac.

The eyes of dogs with color dilution are often more bluish or greenish as puppies and will regularly stay a light yellow, green or yellow color in adult dogs.

Dilution is present at birth and is very obvious in puppies when compared to non-diluted littermates. However, nose or skin pigment on an adult dog with color dilution can sometimes still be very dark and in some cases hard to distinguish from a non-diluted color.

The d Allele

Eumelanin dilution is caused by loss-of-function variations in the MLPH gene (Melanophilin) on dog chromosome 25. Melanophilin is needed to transport pigment organelles called melanosomes inside of which pigment is produced and stored.

If there is less MLPH this results in defective transport of melanosomes.

This causes melanosomes to clump together inside pigment cells before these clumps are transported into the growing hair. But melanosome clumping also occurs in the pigment cells found in the skin[4].

So without a fully functional MLPH gene and altered melanosome transport, a dog can’t properly distribute eumelanin from pigment cells into its hair shafts. This causes the pigment to look diluted compared to the wild type.

Color Dilution Blue Thai Ridgeback

For now, we have found three different mutations that impair MLPH function (d1, d2, d3).

The d1 allele is the most common variant and was found in a variety of different breeds like American Staffordshire TerrierBeagleDoberman, German PinscherLarge MunsterlanderMiniature Pinscher or Rhodesian Ridgeback[1].

The d2 was found in Chow Chows, Sloughis and Thai Ridgebacks[2]. The d3 allele was detected in other breeds like Pumi, Mudi, Shih Tzu, Italian Greyhound, Chihuahua, Pekingese, Tibetan Mastiff, Yorkshire Terrier and Shetland Sheepdog, but also in some village dogs, wolves and wolfdogs[3].

The wild type allele D for non-diluted pigment is dominant over any recessive d allele:

D > d1 d2 d3

When will a dog be blue or lilac?

If a dog has at least one functional MLPH gene (D/-) he will express normal pigment.

Eumelanin color dilution is an autosomal recessive trait and will only be visible in the phenotype if a d allele is inherited from both parents (d/d).

What color a dog with color dilution will have depends on his B-Locus. Diluted black dogs (B/- d/d) will be born with blue pigment and diluted brown dogs (b/b d/d) will be born with lilac pigment.

All the different d alleles cause color dilution in any combination, e.g. d1/d1 or d2/d3 or d1/d2. It doesn’t matter what broke the MLPH gene, a dog with any d/d combination has two fault alleles.

So technically, every d allele is recessive to the wild type D allele.

D > d

But to actually have areas with blue or lilac color a dog must of course have a pattern with at least some eumelanin visible in his coat.

A dog that is all-white due to extreme piebald or on a dog that can only produce phaeomelanin there is no eumelanin visible in the coat that could have been diluted.

In this case, the nose color, eye color or skin color can be an indicator if a dog might be blue or lilac. But this can be a little bit tricky since even diluted skin pigmentation can still be rather dark. Sometimes the only thing you can do to be sure is testing.

D Locus Calculator

This simple tool can help you predict different D Locus combinations:

Color Dilution Alopecia

Some dogs with diluted pigment are affected by color dilution alopecia (CDA) which causes hair fractures, hair loss and skin problems in arteas with a diluted coat color[4].

CDA is only seen in blue or lilac dogs and only affects areas with diluted pigment.

But although CDA can only be observed in dogs with a blue or lilac coat the color dilution itself seems not to be the underlying cause why some dogs develop CDA, dilution just enables Color Dilution Alopecia.

There are plenty of dogs with diluted pigment that don’t suffer from CDA. But whatever causes CDA obviously needs areas with diluted pigment to cause the typical symptoms.

Since color dilution is sought after in color-bred dogs it is introduced into many dog breeds on purpose, e.g. blue Labrador Retrievers. This can potentially create new problems since no one can predict if a breed also carries whatever causes CDA in dogs with diluted pigment.

Blue and Lilac Examples

Phaeomelanin sometimes seems to be a little lighter than expected in dogs with color dilution. But all in all, dilution does not affect phaeomelanin.

Dilution only turns black pigment to blue and brown pigment to lilac, so the actual color depends on a dog’s B-Locus. Since color dilution affects all the eumelanin in any given pattern it can create a variety of distinct phenotypes:

Solid Coat Colors

Dogs with dominant black or recessive black can have a solid eumelanin-pigmented coat.

A solid blue coat can be seen in dog breeds like Mudi, Greyhound, Whippet, American Staffordshire Terrier, Great Dane, Newfoundland or Thai Ridgeback, etc.

A solid lilac coat can be found in Weimaraners and Slovakian Pointer. It is also known to occasionally occur in some other breeds, eg. Chihuahua, Shar Pei or Chesapeake Bay Retrievers, etc.

Nose & Eye Color

In blue dogs, all eumelanin will be blue. So even the nose, lips, eye rims or footpads will be blue. The same goes for lilac dogs where all eumelanin on a dog will turn lilac.

Dogs with color dilution typically have lighter eyes than dogs without color dilution. Puppy eye colors range from sky blue or green-bluish to yellow-green and will darken to pale amber or yellow-green in adults.

This also applies to dog where eumelanin is not expressed in the coat. The will still have diluted nose and sking pigment and a lighter eye color.

Color Dilution White Dog Blue Nose

Dilution and White Spotting

White spotting from things like piebald or whitehead can hide some of the color in a dog’s coat. Ticking or roan can sometimes develop inside white markings giving some of the color in these areas back.

Here are some examples of blue-white or lilac-white dogs:

Dilution and Sable

In tipped and shaded sable patterns, the dog has eumelanin-pigmented hair tips along his back.

If a dog produces blue pigment then all the hair tips will be blue. And if a dog is lilac all of his hair tips will be lilac. But of course, blue gives a far better contrast on yellow or red base color than lilac.

Dilution and Brindle

The pattern in brindle is created by vertical stripes of eumelanin on a yellow or red base color. These stripes will also have a diluted color if a dog is blue or lilac.

Again, lilac brindle often is barely visible because of the poor contrast between lilac and typical phaeomelanin shades.

But blue brindle is a typical coloration in Cane Corso, Whippet, Greyhound, Neapolitan Mastiff or Staffordshire Bull Terrier.

Dilution and Tan Points

A black-and-tan pattern will turn to blue-and-tan while liver-and-tan will turn to lilac-and-tan when affected by color dilution.

This coloration can be found in Chihuahuas, Russian Toys or Dobermans (which call their lilac-tan points fawn).

Dilution and Saddle Tan

Since a dark saddle gets its color from eumelanin it will be affected by color dilution.

Color Dilution Blue Saddle Beagle

Dilution and Masks

Masks express as an overlay of eumelanin on a dog’s muzzle. Blue masks or lilac masks are often a little smutty and not that easy to spot at first glance.

Dilution and Merle

Black dogs with a merle pattern are often called “blue merle” because of their greyish looks. But if a truly blue dog is also merle then all of the pigmented patches will be blue.

If a merle dog has black patches anywhere on his body he is in fact black merle and not diluted merle. In Border Collies a diluted black merle is called slate merle.

Dilution and Agouti

The combination of blue or lilac pigment with a wolf grey coat is quite rather.

The best chance to spot one might be in color-bred German Shepherd Dogs. As agouti is called “sable” in GSD they call their dilute pattern blue sable and lilac sable, but these terms should not be confused with a true sable pattern.

Dog Breeds with Color Dilution

Although eumelanin dilution is a rare trait it can be found in a variety of dog breeds.

In some of these breeds, it occurs at a very low frequency or is considered a faulty non-standard color (e.g. Malinois, Australian Shepherd, Rhodesian Ridgeback). In others, it was introduced on purpose to produce color-bred puppies (e.g. French Bulldog, Labrador Retrievers).

But in many other dogs blue or lilac coat is a commonly accepted coat color:

  • American Pit Bull Terrier
  • American Staffordshire Terrier
  • Australian Kelpie
  • Bearded Collie
  • Beagle
  • Border Collie
  • Cane Corso
  • Chihuahua
  • Chinese Crested Dog
  • Chow Chow
  • Doberman
  • Great Dane
  • Greyhound
  • Italian Greyhound
  • Mudi
  • Neapolitan Mastiff
  • Newfoundland
  • Pumi
  • Russian Toy
  • Shar Pei
  • Shih Tzu
  • Slovakian Pointer
  • Staffordshire Bull Terrier
  • Terrier Brasileiro
  • Thai Ridgeback
  • Tibetan Mastiff
  • Weimaraner
  • Whippet

Dilute Look-Alikes

Some other coat colors can resemble a dog with diluted pigment:

  • Progressive greying usually isn’t present at birth but can fade a dogs pigment to a greyish color in black dogs or a beige color in brown dogs. But the nose pigment will keep its darker shade.
  • Atypical merle can give a very uniform and faint pattern even on a black or brown dog.
  • Fever Coat is a phenomenon that causes a greyish or grizzled color on a dogs back. It likely is a result of some problems during pregnancy and the dogs in most cases turn back to their normal full color after some time.
  • Ancient domino and grizzle can give dogs with a genetically dominant black coat a greyish phenotype.
  • Dogs with heavy ticking or roan patterns are sometimes called blue. But if an affected dog has black pigment anywhere in it’s coat or on it’s body he is a black dog.
  • There is a rare disease in Rough Collies called cyclic neutropenia or grey collie syndrome. This blood cell disorder causes a variety of symptoms and a severly limited life expectancy.

D-Locus Testing

If you want to test your dog’s D-Locus genotype you should consider what company to test with.

Not all companies test for all the different variants (d1, d2, d3). It is also very likely that further research will find some more d alleles[3].

Learn More

Links

[1] Cord Drögemüller, Ute Philipp, Bianca Haase, Anne-Rose Günzel-Apel, Tosso Leeb. A Noncoding Melanophilin Gene (MLPH) SNP at the Splice Donor of Exon 1 Represents a Candidate Causal Mutation for Coat Color Dilution in DogsJournal of Heredity, Volume 98, Issue 5, July/August 2007, Pages 468–473. https://doi.org/10.1093/jhered/esm021

[2] Bauer, A., Kehl, A., Jagannathan, V., Leeb, T. : A novel MLPH variant in dogs with coat colour dilution. Anim Genet 49:94-97, 2018. Pubmed reference: 29349785. http://dx.doi.org/10.1111/age.12632

[3] Van Buren, S.L., Minor, K.M., Grahn, R.A., Mickelson, J.R., Grahn, J.C., Malvick, J., Colangelo, J.R., Mueller, E., Kuehnlein, P., Kehl, A. : A Third MLPH Variant Causing Coat Color Dilution in Dogs. Genes (Basel) 11:, 2020. Pubmed reference: 32531980. http://dx.doi.org/10.3390/genes11060639

[4] Philipp, U., Hamann, H., Mecklenburg, L., Nishino, S., Mignot, E., Günzel-Apel, A. R., Schmutz, S. M., & Leeb, T. (2005). Polymorphisms within the canine MLPH gene are associated with dilute coat color in dogsBMC genetics6, 34. https://doi.org/10.1186/1471-2156-6-34