THE GREYING GENE
The greying gene occurs on the G locus. It is dominant, so G is greying and g is non-greying, and in order for a dog to express greying it only needs to have one copy of G (so its genotype can be Gg or GG). A dog which doesn't express the greying gene has the genotype gg.
The greying gene, like dilution, affects eumelanin (black and liver). However, unlike the dilution gene, it doesn't affect the nose or eye colour, it is progressive (so a dog with greying is born solid black or liver and becomes lighter as it gets older), and doesn't always affect the whole of the coat to the same extent (the shade of grey in different parts may vary, and some parts may even remain black). A dog with the dilution gene may have greying as well, but it's rare for the two genes to occur in the same breed.
Greying does not affect phaeomelanin (red). Sometimes breeds which have the greying gene also come in shades of cream, which suggests that the lightening of the two types of pigment is connected. However, it is not. Red is lightened by the chinchilla (C) series, and some breeds simply carry both greying and chinchilla.
One last interesting thing to note about greying is that while it affects dogs with tan markings (saddles, traditional tan etc) it does not affect masks. Kerry Blues are a striking example of this - their main coat may turn to silver while leaving the hair on their muzzle solid black. It is not known why this happens.
BREEDS CARRYING GREYING
The greying gene occurs in a small amount of breeds, the most notable of which are:
Old English Sheepdog
Bearded Collie
Tibetan Terrier
Polish Lowland Sheepdog
Bedlington Terrier
Dandie Dinmont Terrier
Basset Griffon Vendeen (Grand and Petit)
Poodle (Standard, Miniature and Toy)
Irish Wolfhound
Deerhound
Cesky Terrier
Kerry Blue Terrier
Havanese
Glen of Imaal Terrier
As you can see by this list, greying is found almost solely in long-haired breeds. This is probably because the hair lightens as it grows, so on a short-haired dog the hair doesn't get long enough to start greying before it is shed and a new hair starts to grow. Some short-haired breeds may well have the greying gene but it is just not visible. Greying seems to be particularly prominant on non-shedding breeds such as the Poodle and the Kerry Blue, presumably because each hair, if it's not shed, has much longer to turn grey.
Because greying is dominant, it appears regularly in mixed breeds (as only one parent needs to have the gene), particularly in the UK where Bearded Collie crosses are common, as well as various terrier crosses.
EXAMPLES OF GREYING
Dogs with greying may take years to turn from black to grey. These three Cesky Terriers show the progression of grey as a dog ages. The first dog
is very young, and is still black. The second dog is slightly older and is a darkish grey, and the last dog is almost completely silver. Notice
how the shortest areas of the coat (tail and ears) are the darkest, followed by the body, and the longest parts of the coat (belly and leg furnishings
and muzzle fur) are the lightest. The last picture was taken by June from the PKC forum.
This Kerry Blue Terrier has very lightly-coloured body fur, but its muzzle is still black. This is because it has a black mask (Em). This photo
was taken by June.
Greying in the Yorkshire Terrier (note how the tan is not affected), Bedlington Terrier, Dandie Dinmont and Polish Lowland Sheepdog. The first two photos here were taken by June.