The ridge is caused by mutation (duplication of stretch of DNA sequence including several genes). The mutation is called simply a ridge gene and shows dominant inheritance.
We have developed the genetic test, which distinguishes dominant homozygotes (R/R - two ridge genes) from heterozygotes (R/r - one ridge gene). Using our genetic test, a breeder may predict birth of ridgeless puppies (see Table below).
Tested gene and mutation: 133 kb duplication including genes (FGF3, FGF4, FGF19 a ORAOV1)
Breed: Rhodesian Ridgeback, Thai ridgeback