Greyhounds appear to have a pattern of white spots similar to the "Tetrarch spots" seen on some grey horses.
Elvis Brown posted this very interesting Greyhound to the blog’s Facebook page, and was kind enough to give me permission to share him here. Mr. Brown says he owned the dog from age three, and that he had always had the white spots, though at that time he was black. He is thirteen in this picture, and the greying is due to age. Mr. Brown also mentioned that someone from the Greyhound society had previously seen an Irish-bred Greyhound with a similar pattern.
These are somewhat reminiscent of Tetrarch Spots – sometimes called chubari spotting – in horses. Those take their name from the famous Thoroughbred, The Tetrarch, who was well-known for the unusual white spots on his coat. His daughter Mumtaz Mahal and (to a lesser extent) granddaughter Mumtaz Begum were similarly marked.
This kind of spotting in horses is associated with progressive greying, but progressive greying in dogs is different. In dogs grey is strongly associated with black pigment (eumelanin). It is most often seen in longhaired breeds, like the Bearded Collie, or in breeds that do not shed, like the Poodle and the Bedlington Terrier. Greyed dogs tend to be lightest where their hair is the longest, like on the topknot of some of the terriers, and darkest where the hair is short, like on the ears. This suggest that the hair loses pigment as it grows longer, rather than with each shed like a horse.
Another interesting aspect of greying in dogs is that while it lightens black pigment, if the gene for black masking is present, it does not alter the black there. That is why Kerry Blue Terriers are born black and turn blue-grey while their face remains dark. The black mask, which would not otherwise be visible on a black dog, is revealed by the greying.
Whatever caused the white ticking on this Greyhound, it does not sound like it is related to greying. In size and placement, the white spots actually look a bit more like Birdcatcher Spots, which are more common on red horses than black ones. Quite a few horse owners report those as increasing in number with age, so they could be considered progressive, though the various kinds of white ticking in horses is another under-researched topic. I have some images to post for that, as well as an update on a related horse from a previous post, for tomorrow.