A handful of breeds share a unique feature compared to other dogs - they have a ridge of fur that grows in the opposite direction along their backs. For some this ridge defines them. After all what would a Rhodesian Ridgeback be without one? Although, like coated dogs appearing in a hairless litter it happens. While this ridgelessness is considered a fault, their presence helps keep the gene pool stable. On the opposite side there are breeds where the ridge pops up only occasionally.
There are five breeds where the ridge occurs: the Rhodesian Ridgeback (South Africa), Thai Ridgeback (Thailand), Phu Quoc Ridgeback (Vietnam), Africanis (South Africa), and Combai (India). Of these five only the Rhodesian Ridgeback is recognized by all major kennel clubs although the Thai Ridgeback has Fédération Cynologique Internationale recognition. The Phu Quoc Ridgeback, Africanis, and Combai have yet to gain any large following outside their respective homelands.
So what causes the ridge and where did it begin? Like all other characteristics, these ridges are caused genetically. Dogs carry either one or two of the ridge genes and then pass it onto their offspring. This gene is dominant, and because the Rhodesian, Thai, and Phu Quoc Ridgebacks have been selectively bred for generations to have the ridge it is passed quite easily from one generation to the next. However, the Africanis and Combai have traditionally been bred for their working ability rather than appearance and over time this means fewer dogs are born with the ridge.