Dilution (blue and isabella)- The D series

The Dilution Gene

The dilution gene occurs on the D locus. It is recessive, so d is dilute and D is non-dilute, and in order for a dog to be dilute it must have the genotype dd. A dog which is Dd or DD will have normal pigment.

The dilution gene affects eumelanin (black and liver), although phaeomelanin (red) may be lightened slightly as well. When a dog has two copies of the d gene it impairs its ability to make full-coloured pigment, so the pigment it does produce is paler than on a normal dog. A black dog with the dilution gene becomes blue and a liver dog becomes isabella. A blue or isabella can have any coat pattern, but whatever they have, any black or liver in the coat will be turned to blue or isabella. It is genetically impossible for a blue dog to have any black in its coat, or for an isabella to have liver.

The main giveaway that a dog is a dilute is its nose colour. The coat may be entirely sable or recessive red, but if the dog has a blue nose, it is genetically blue.

Black nose (left) and blue nose (right).

Isabella, however, is slightly trickier. The dogs below show nicely-pigmented liver and isabella noses, but it’s common for both colours to have very light, even pink, noses, and it is also common for isabellas to have darkish noses which look liver. It is therefore very difficult to tell a liver from an isabella unless there is some liver/isabella in the coat.

Liver nose (left) and isabella nose (right).

The dilution gene also causes the eyes to become amber. The colour is likely to be paler than the amber eyes seen on liver dogs.

For more information on the effects of dilution on eye and nose colour, and further photo examples, see the eye and nose pages.

Breeds Carrying Dilution

Here are some of the breeds that carry dilution:

American Staffordshire Terrier	Great Dane	Pomeranian
Beagle	Greyhound	Rat Terrier
Bearded Collie	Harrier	Russian Toy Terrier
Border Collie	Italian Greyhound	Schipperke
Borzoi	Kelpie	Shar Pei
Canary Dog (Presa Canario)	Miniature Pinscher	Shih Tzu
Cane Corso	Mudi	Slovakian Pointer
Chesapeake Bay Retriever	Neapolitan Mastiff	Staffordshire Bull Terrier
Chihuahua	Newfoundland	Thai Ridgeback
Chow Chow	Otterhound	Tibetan Mastiff
Dobermann	Peruvian Inca Orchid	Weimaraner
Finnish Lapphund	Plott Hound	Whippet
Foxhound (American and English	Podengo Portugueso	Xoloitzcuintle
German Pinscher

Even within many of these breeds, the blue gene is very rare. Major exceptions are the Weimaraner and Slovakian pointer, which are the only breeds to come entirely in dilute and no other colour. The dilute gene is also notably common in Italian Greyhounds, Whippets, Tibetan Mastiffs and Neapolitan Mastiffs.

Blue

Black dogs become blue when they are dd on the D locus. Blues can range from silver to almost black, and it can be difficult to tell a blue from a black by just looking at photographs. However, when the dog is actually examined, it should be obvious that the nose is blue.
Brindle stripes, tipping on a sable, masks, black patches on merles, saddles, patches on a black piebald, and the black on a tan-pointed dog will all be turned to blue when a dog has the dilution gene. Any and all black on the dog is included. Sable tipping and merle patches may become difficult to see when they’re diluted.

The dogs below show blue in various patterns. I won’t explain the patterns here as they’re dealt with on their own pages, but hover your mouse over the photos to see a description of their colour.

Isabella

The same dilution gene that causes a black dog to become blue also causes a liver dog to become isabella (aka lilac), which is a pale greyish brown. Dilution and liver are both recessive and relatively rare, so isabella is a rarely seen colour. It is, however, the colour of the Weimaraner, and also occurs occasionally in a handful of other breeds (any of the breeds in the list above which carry liver will come in isabella, although if both liver and dilution are rare in the breed anyway then isabella dogs may be barely known or even never reported).
An isabella dog will have the genotype bbdd (homozygous for liver, homozygous for dilution).

The photos below show isabella dogs.

Isabella Border Collie photo by Cat of Dog Rad Design

Dilute Look-A-Likes

There are a few genes which can cause a dog to display a grey colour when in fact they’re not blue-pigmented, but black. Merle gives a bluish base coat, and the greying gene can also turn a dog grey.
The sure-fire way to tell a black from a blue is to look at the nose. If the dog looks blue but has a black nose, it is in fact black with the greying gene. This gene exists in Bearded Collies, Polish Lowland Sheepdogs, Bedlington Terriers, Old English Sheepdogs, Kerry Blue Terriers, Dandie Dinmonts and a few other breeds. Greying can also affect liver, so a liver dog could appear to be isabella.

True dilutes are sometimes known as “born blues”. This is because the dog will display the blue colour from birth, whereas a dog with greying will be born black.

The following dogs are not actually blues.

Quick Summary
No time to read the whole thing? Here’s the quick version!

The D locus controls the intensity of eumelanin in the coat (and also the eyes/nose/etc). There are just two known alleles – D and d (although there may be a number of different d alleles that are phenotypically the same – see the “Health Problems” page).

Eumelanin dilution is recessive, so D is non-dilute and d is dilute. Only a dd dog will actually be a dilute, and a Dd dog will be a carrier.

All eumelanin is affected on a dd dog. If the dog has any black or liver then it is not a true dilute. Generally the most failsafe way to tell a blue is by looking at the nose.

dd also affects liver as well as black. A liver dilute is a light grey/brown and is generally known as an isabella or lilac. This is the colour of the Weimaraner.

“Dog Coat Colour Genetics.” Dog Coat Colour Genetics. N.p., n.d. Web. 24 June 2014.