Genetics, heritability and interviews with the experts
74% of breeders and owners who returned their recent CCA health surveys had one or more dogs affected with luxating patellas. There was an average of almost 3 dogs affected for every one survey returned. In a recent study of small breed dogs in Austria, 76.9% of chihuahuas examined had some degree of luxation4. Luxating patellas are an important health issue that needs to be addressed in our breed. It is imperative that breeders strive to breed sound patellas and the only way to do that is to know the facts and to make responsible choices in breeding programs.
Luxating patellas are genetic in origin1, 2, 3. It is easy to rationalize that they are brought on by injury or exercise, but this is very rarely the case. According to Gert Breur, DVM, PhD, DACVS Purdue University and
Dr. Rory Todhunter, Cornell University Clinical Sciences professor, If an injury precipitates luxation of the patellas, it is highly likely there was a genetic predisposition to begin with (See full interviews below).
As responsible breeders we should not make excuses for breeding dogs with luxating patellas at the expense of the health of our breed’s gene pool. They are not easily “bred out” because of their genetic nature. Luxating patellas are polygenic threshold traits1,2.
A polygenic trait is one that is produced by multiple genes3. In the case of luxating patellas, the polygenes control the shallowness of the groove in the femur, the development of the guiding bony ridges and the strength or attachment of the ligaments governing movement of the patella. All of these factors are determined polygenically and contribute towards the onset of the defect2.
Several genes also influence a threshold trait, but the trait will not be expressed unless a critical number of those genes is present1,2. This means that a totally normal dog may be carrying a very high number of undesirable genes for a trait, but not quite the critical number needed for the trait to be expressed1.
Threshold traits develop only when the additive effects of genes exceed this critical number. If a puppy has luxating patellas, he or she inherited the critical number of genes from both parents that were needed to express the trait. If the “magic” number is ten, the puppy could have received four from the dam and six from the sire, or even just one from the dam and nine from the sire. Both parents are not necessarily equally at fault.
Dogs or bitches that have any degree of luxating patellas are carrying a higher number of the necessary genes which they will pass onto their offspring. It is not “bred out” if the puppies turn out to be physically normal- phenotypically they may be, but genotypically they may still be much more likely to reproduce luxating patellas.
This is why affected Chihuahuas should be removed from breeding programs. Siblings of affected chihuahuas are also at a higher risk and should be monitored and offspring followed up on1. Sires or dams that produce a higher than average number of offspring with luxating patellas should also be removed from breeding programs.
According to Robinson, “Family selection is more effective than individual selection in dealing with polygenic anomalies…Family selection is the most effective means of dealing with threshold characters.2“
A dog with normal patellas whose dam had luxating patellas will likely be carrying a much higher number of the genes needed to produce luxating patellas than a dog from two normal parents. If a dog with this background is linebred on it may produce an extraordinary number of offspring with luxating patellas even though the dog in question is phenotypically normal. If a dog with this background became a popular sire, it could do irreparable damage to the gene pool. Any dog or bitch with luxating patellas should never be bred.
Interviews with Dr. Rory Todhunter of Cornell and Dr. Gert Breur, Purdue:
Dr. Rory Todhunter, Cornell University Clinical Sciences professor
- Is it possible for a chihuahua to develop a luxating patella solely due to injury? [Todhunter, Rory] Anything is possible but almost always will be genetic in origin
- What is the likelihood that a grade 1 patella in only 1 knee (other knee is normal) was caused by injury if the chihuahua was witnessed to have a fallen off a bed or a couch? [Todhunter, Rory] Possible but highly likely had genetic predisposition
- If it was likely caused by injury, would you still strongly suspect a genetic predisposition? [Todhunter, Rory] Yes
- Would you ever recommend breeding a chihuahua with two grade 1 luxating patellas or a grade 1 where a likely injury was not witnessed? Grade 2? [Todhunter, Rory] No
- Are X-rays beneficial in determining whether a luxating patella was caused by injury or genetic predisposition? [Todhunter, Rory] No
Gert Breur, DVM, PhD, DACVS Purdue University
- Is it possible for a chihuahua to develop a luxating patella due to injury? [Breur, Gert J] I assume you are referring to a young mature to mature dog. My answer is “Yes”. However, I think that there has to bea pre-existing bony malformation that causes poor tracking of thepatella. Without the injury the dog manages to “keep the patella in”. The injury just disturbs this fragile balance and facilitates the luxation.
- What is the likelihood that a grade 1 patella in only 1 knee (otherknee is normal) was caused by injury if the chihuahua was witnessed tohave a fallen off a bed or a couch? [Breur, Gert J] Two possibilities I think.1) the underlying bony deformity is only in one limb (this indeed happens) or2) see Q1.
- If it was likely caused by injury, would you still suspect a genetic predisposition? [Breur, Gert J] Yes, a genetic predisposition for the bone deformity or the cause of the bony deformity.
- Would you recommend that a dog with a single grade 1 patella, otherpatella normal, be bred to a non affected mate if he is otherwise healthy and sound? [Breur, Gert J] Presuming that it indeed is agenetic problem, the answer is that to most effectively eliminate the trait from the breeding stock, the dog should not be bred.
- Are X-rays beneficial in determining whether a luxating patella wascaused by injury or genetic predisposition? [Breur, Gert J] I don’t think so, but radiographs will tell you about the underlying bony abnormality. However, there will be dogs
- with bony deformity that do not have luxating patellas (yet),
- that have subclinical patella luxation (i.e. they have the bony deformity and luxating patellas but no clinical problems) and
- dogs that have bony deformity and patella luxation)
- Orlandi PhD, Claudia ABC’s Of Dog Breeding 2005: 68, 136,137
- Robinson, Roy FI Biol. Genetics For Dog Breeders 2nd Ed. 1990: 62, 199, 224
- Isabell, Jackie Genetics An Introduction for Dog Breeders 2002: 85, 86