The Ridge
The Rhodesian Ridgeback is distinguished by a dorsal ridge of hair that grows in the opposite direction to the rest of the coat. This ridge is associated with a genetic variant on chromosome 18. Genetic testing can determine whether the ridge allele is present in heterozygous form (R/r) or homozygous form (R/R). By assessing the genotypes of breeding partners, it is possible to estimate the likelihood of ridgeless offspring
Detailed description
A defining characteristic of the Rhodesian Ridgeback is the ridge: a clearly demarcated strip of hair along the spine that grows forward, opposite to the surrounding coat. Breed standards specify that the ridge should begin immediately behind the shoulders and extend centrally along the back to approximately the level of the hip bones, tapering slightly towards the rear. Ideally, the ridge measures 4–5 cm in width at the shoulder region and is sharply distinguishable from the adjacent coat. Within the ridge, two hair whorls (crowns) are typically present, positioned opposite one another in a symmetrical, mirror-image arrangement
The ridge phenotype is caused by a duplication of a 133 kb DNA segment on chromosome 18 that encompasses multiple fibroblast growth factor (FGF) family genes (FGF3, FGF4, FGF19, ORAOV1) and adjacent regions. This duplication represents an autosomal dominant variant with incomplete penetrance
Genotype overview
- r/r (homozygous wild type): ridgeless; the ridge variant is absent and cannot be transmitted to offspring
- R/r (heterozygous): one copy of the ridge variant is present; offspring inherit the variant with a 50% probability; approximately 95% of R/r dogs develop a ridge, while about 5% do not due to silencing of the dominant allele
- R/R (homozygous variant): two copies of the ridge variant are present; the variant is transmitted to all offspring
Genetic testing identifies whether an individual carries the ridge allele in heterozygous or homozygous form. This information enables breeders to anticipate whether ridgeless puppies may occur in a litter
There is evidence that deviations from the ideal ridge morphology—such as shortened or incomplete ridges or more than two crowns—may be more common in dogs with the R/R genotype, although the precise relationship between genotype and ridge variation remains under investigation
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