Ridge Inheritance in the Rhodesian Ridgeback
“What’s that on your dog’s back?”
“Is he mad?”
“Did he have an operation?”
Ridgeback owners are accustomed to the curious reactions to the breed’s trademark characteristic, the ridge of backward-growing hair on its back.
One charming explanation for the presence of the ridge especially for children is “That’s where God sews them up when he’s done stuffing them.”
But for those who want a more precise understanding of how the ridge has been passed on through umpteen generations, stretching back centuries to the ridged native dog of the Khoikhoi people of South Africa, read on.
Geneticists studying the ridge have concluded that it is a simple autosomal dominant trait that is, only one parent needs to have a ridge in order for it to be passed down to offspring.
Ridgeless Rhodesian Ridgebacks don't carry the ridge gene at all, and cannot produce ridged offspring any more than a Chesapeake Bay retriever or Chihuahua can. Ridgeback breeders who are interested in ridge inheritance often forget that not having a ridge is the normative state in dogs. The ridge gene is a dominant mutation that makes our ridged dogs different.
If a Ridgeback has a ridge, he is either carrying two copies of the ridge gene (homozygous), or just one copy (heterozygous). Either way, the dog will have a ridge -- you can't tell if he has one or two copies from just looking at him. You might be able to figure out his genotype (that is, whether he is heterozygous or homozygous) retrospectively based on breeding history.
It is the breeding of heterozygous Ridgebacks that produces ridgelessness.
When two heterozygous dogs are bred together, each puppy has a 25 percent chance of being ridgeless; a 50 percent chance of being ridged and heterozygous (only having one copy of the ridge gene) and a 25 percent chance of being ridged and homozygous (having two copies of the ridge gene, and so never producing ridgeless no matter who they are bred to).
Homozygous dogs -- those with two copies of the gene -- will never produce ridgeless. That's because, when it comes time for homozgyous Ridgebacks to donate one of their two genes to offspring, all they have to pass on is a ridge gene. And because the ridge gene is dominant, no matter what the other parent contributes, the offspring will have a ridge.
When a homozygous Ridgeback is bred to a heterozygous Ridgeback, all the puppies will have ridges, but genetically they can be different. Each puppy has a 50 percent chance of being homozygous (having two copies of the gene, and never producing ridgeless) and a 50 percent chance of being heterozygous (having only one copy of the gene, and so potentially producing ridgeless if bred to another heterozygote).
Unfortunately, without a genetic marker test, we cannot tell the last two apart just by looking at their outward appearance (what scientists fancily call phenotype).
Have you ever heard of ridgeless-to-ridgeless breedings producing ridged puppies?
Periodically, people will talk about ridgeless-to-ridgeless breedings that supposedly produced ridged dogs. But they can never produce any identifying details no dates, not even the name of the breeder or the dogs involved. That’s because these stories are apocryphal they are folklore.
Breeding two ridgeless Rhodesian Ridgebacks together will result in an entire litter of ridgeless.
How are certain aspects of the ridge for example, number of crowns inherited?
There are many elements of the ridge that modify it, including length and width, number and placement of crowns, and shape of the fan. The mode of inheritance for these modifiers is not yet understood.
How can I determine retrospectively if my Ridgeback is heterozygous (carrying only one copy of the ridge gene) or homozygous (carrying two copies)?
If your Ridgeback has ever produced ridgeless puppies, he or she is heterozygous, as is the dog he or she was bred to.
If your dog has never produced ridgeless, that does not necessarily mean he or she is homozygous. Indeed, perhaps it is the mate that is! However, if your dog has never produced ridgeless -- and has been bred multiple times to other dogs that have produced ridgeless in other pairings, but not with yours -- there is a good chance that your dog is homozygous. But the only way to know for sure is with a genetic-marker test.
Is a genetic marker test for the ridge available?
Not yet, though we are hopeful that the research on ridge inheritance that is ongoing at the University of California at Davis will eventually lead to one. Currently, the researchers have isolated the region of the genome where the ridge mutation is located, but have not found the specific marker. To use a metaphor, they know the street that it lives on just not the precise house number!
So once a genetic marker is available, will the goal be to eliminate ridgelessness entirely from the Rhodesian Ridgeback population?
We hope not, for several reasons.
The Khoikhoi dogs of South Africa, from which our dogs derive the ridge trait, were not an all-ridged population. Because their breedings were not managed and directed by humans, a degree of ridgelessness in their breed population was normal. This arguably maintained an important balance in the gene pool: As we know from breeds that have tried to “fix,” or standarize, dominant mutations such as hairlessness in Chinese crested dogs and taillessness in Manx cats homozygosity can lead to unwanted results. (In both those cases, homozygous offspring are born dead.)
Ridgeback breeders and researchers have long known that there is a relationship between the presence of the ridge (or some element of it) and dermoid sinus, a congenital defect that is found in the world’s two ridged breeds -- Rhodesian and Thai ridgebacks. While we do not fully understand this correlation, some have posited that increased ridge homozygosity could have an impact on the prevalence of dermoids. While this is just educated conjecture, it does remind us that moderation in our dogs as well as our breeding practices is often the wisest approach.We are not suggesting that Ridgeback breeders increase the incidence of ridgeless puppies, but rather that they take a nuanced, textured look at our breed and its genetic history, and not rush to swing the pendulum in any one direction, lest we invite unforeseen and potentially disastrous -- results.
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