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
The dilution gene only occurs in particular breeds. These are:
Weimaraner
Miniature Pinscher
German Pinscher
Dobermann
Greyhound
Italian Greyhound
Whippet
Foxhound (American and English)
Beagle
Borzoi
Chart Polski
Harrier
Otterhound
Plott Hound
Podengo Portugueso
Thai Ridgeback
Border Collie
Bearded Collie
Kelpie
Finnish Lapphund
Mudi
American Staffordshire Terrier
Brazilian Terrier
Staffordshire Bull Terrier
Rat Terrier
Chihuahua
Lowchen
Pomeranian
Russian Toy Terrier
Shih Tzu
Chow Chow
Schipperke
Shar Pei
Peruvian Inca Orchid
Xoloitzcuintle
Great Dane
Neapolitan Mastiff
Newfoundland
Tibetan Mastiff
Cane Corso
Canary Dog (Presa Canario)
Slovakian Pointer
Chesapeake Bay Retriever
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 - they're dealt with on their own pages.
ISABELLA
The same dilution gene that causes a black dog to become blue also causes a liver dog to become isabella, 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. They are all solid-coloured except for the Miniature Pinscher, which is tan-pointed, and the Neapolitan Mastiff, which is probably brindle. The similarity of the diluted brindle stripes to the base colour mean that the stripes are barely visible, if at all.
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.
The following dogs are not actually blues. Notice their jet-black lip and nose pigment. The pictures were all taken by June from the PKC forum.
