The Ridgeback Gene

 You don't have to have Rhodesian Ridgeback in a mixed breed dog for it to inherit a ridge! There is some association of the ridge gene with a dermoid sinus or cyst - essentially they can get a small tube in their skin that goes from the surface of the skin as far as the spinal cord. Not a common thing but it can happen.

 This dog inherited a ridge but you can see from the chart below there is no Rhodesian Ridgeback in his ancestry at all.

The green cluster on the chart above is the genetic signature of the Rhodesian Ridgeback, and yet the tiny blue spot in the middle of the red cluster is where this dog's genetic ancestry lies.  The Rhodesian Ridgeback ancestry may be many many generations back and cannot be traced.

Basically the ridge is a dominant trait in genetics. If one of the parents carries the ridge gene (even though it might not have a ridge itself) it is possible that a ridge will be passed to any of the puppies in a litter.