Polygenic disorders, such as hip dysplasia, congenital heart defects, and epilepsy, present a significant challenge for dog breeders. Unlike single-gene disorders, these conditions arise from the complex interplay of multiple genes, often requiring them to be managed as “threshold traits.” This means that a combination of genetic factors must combine to cross a certain threshold, ultimately resulting in an affected individual. Even when phenotypically normal parents produce affected offspring, it indicates that both parents carry a genetic load that, when combined, contributes to the disorder.
Hip dysplasia serves as a prime example of a hereditary disease governed by polygenic inheritance. Genetically influenced anatomical defects and joint laxity can lead to lameness in affected dogs, potentially causing crippling secondary osteoarthritis. Radiographic evaluation, particularly pelvic X-rays, is the primary genetic test used for managing hip dysplasia. Organizations like the Orthopedic Foundation for Animals (OFA) maintain extensive registries for hip dysplasia, assessing conformation, joint laxity, and arthritic changes through extended-hip radiographs. The Institute for Genetic Disease Control (GDC) also manages a similar registry.
A different approach, the PennHIP method, focuses on measuring joint laxity. This technique involves taking a radiograph while applying a controlled force to the hips of an anesthetized dog to determine the maximum distractibility. By comparing a dog’s distractibility to a breed average and selecting for tighter hips, the aim is to reduce the incidence of hip dysplasia over time.
Both the OFA and PennHIP methods have their advantages and disadvantages. The OFA radiograph highlights anatomical abnormalities like shallow sockets and early arthritic changes, assessing laxity only in a naturally extended hip. The PennHIP method, conversely, measures maximum joint laxity, though a high distraction index doesn’t always predict the development of hip dysplasia. Both techniques can yield false positive and false negative results. However, radiographic findings at an early age are strongly correlated with later dysplasia development. While OFA offers preliminary evaluations at any age, permanent certification is not granted until two years of age.
Environmental factors also play a role in the expression of hip dysplasia. Overnutrition and excessive environmental trauma during critical ossification periods can promote the development of dysplasia. While limiting these environmental stresses is advisable for pet dogs, breeders should avoid both excessive protection and over-stressing their dogs’ development to prevent masking the expression of genes that contribute to dysplasia. Breeders should evaluate prospective breeding dogs raised under relatively uniform conditions that neither promote nor overly protect against hip dysplasia.
A key reason for slow progress in managing hip dysplasia is its historical treatment as a single-gene disorder with a carrier test. In reality, canine hip dysplasia is not influenced by a single “excellent hip gene,” despite what many breeders select for. In polygenic disorders, an individual’s phenotype doesn’t perfectly reflect its genotype. Breeders must dissect affected phenotypes into specific traits that more directly represent the underlying genes. These traits include clinical signs of lameness (particularly between six and eighteen months of age), palpable joint laxity under anesthesia, deep acetabula, rounded femoral heads, the absence of arthritic remodeling, deeply seated hips on an extended leg view, and radiographic distractibility.
Hip dysplasia in dogs is not caused by the same gene combinations in all individuals. A dog with laxity and subluxation but normal anatomy may have hip dysplasia resulting from different genes than a dog with malformed sockets but no subluxation. Selecting against the individual components of the syndrome may offer better control. For instance, if a desirable dog has shallow hip sockets, it should ideally be bred to a dog with deep hip sockets. Breeding two dogs with fair hips together could result in significantly worse hip conformation if they share detrimental traits, or it could lead to improvement if their respective strengths complement each other’s weaknesses. The goal is to select for a sufficient number of genes that influence normal development to rise above the threshold at which dysplasia manifests. While not all these factors guarantee a genetically normal dog, incorporating more of them increases the probability.
Another factor contributing to diminished progress against hip dysplasia and other polygenic disorders is the historical emphasis on selecting for generations of phenotypically normal parents and grandparents (depth of pedigree). For polygenic disorders, the phenotype of full siblings offers a more accurate representation of the gene pool present in the breeding individual. Therefore, the breadth of the pedigree is as crucial, if not more so, than its depth in managing polygenic diseases. Phenotypically normal dogs from litters with a high incidence of dysplasia are still likely to pass on genes that promote hip dysplasia. By emphasizing the selection for a breadth of phenotypically normal littermates of breeding dogs, as well as their parents, all breeds should observe a decrease in hip dysplasia. Furthermore, monitoring the offspring of breeding dogs for their frequency of passing on the disorder is essential.
Many polygenic disorders are triggered by a major recessive or dominant gene that must be present for an individual to be affected. This trigger gene can vary between breeds and even families within a breed, meaning a single genetic test may not be universally useful. Future advancements in molecular genetic research hold the promise of identifying these specific genes, which will allow for more effective control of polygenic disorders such as hip dysplasia, epilepsy, and cataracts.
In summary, effective management of polygenically inherited disorders requires a multifaceted approach: 1) identifying specific traits that more closely represent the genes being selected against, 2) standardizing environmental factors that can influence gene expression and potentially mask selective pressure, and 3) prioritizing the breadth of pedigree alongside its depth in breeding decisions.
