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Myxomatous Mitral Valve Disease
Myxomatous mitral valve disease (i.e., mitral valve endocardiosis) is primarily seen in older toy breeds
and small dogs. For some breeds (e.g., Norfolk Terrier, Cavalier King Charles Spaniel), it may lead to heart disease at an average age of 6.25 years. As this is beyond breeding age, some Cavalier King Charles spaniel clubs have established a generational breeding- control program in which dogs are only bred if both parents are free of a murmur and Doppler evidence of mitral regurgitation. When this program is applied, the frequency of this disorder has decreased.
Cranial Cruciate Ligament Rupture
Cranial cruciate ligament rupture is a common traumatic injury. Although it is not typically considered a hereditary disease, studies show increased risk in Rottweilers,
West Highland White Terriers, Golden Retrievers, Yorkshire Terriers, Staffordshire Bull Terriers and some mixed-breed dogs. Studies of cranial cruciate ligament rupture in Newfoundlands show 27% heritability. Genetic predisposing factors for rupture may include issues with ligament extracellular matrix metabolism, degeneration and/or in ammation. Other predisposing factors may involve biomechanical and conformational variations (e.g., bone length, sti e angulation, tibial plateau variation or narrowed distal femoral intercondylar notch).
Patellar Luxation
Patellar Luxation is a complexly inherited disorder that causes the kneecap to pop out of the trochlea either medially or laterally. It is more common in the small stature breeds, however several larger breeds also
have a high incidence of the disorder. Patella luxation
can be subclinical or can cause pain, instability and arthritic changes. Weight control is an important factor
in decreasing morbidity. Affected dogs usually progress from Grade 1 (patella laxity) to Grade 4 (permanent  xation out of the trochlea) over time. Breeders must use breadth and depth of pedigree normalcy to select against this disorder.
Hereditary Cancers
While all cancers are due to mutations present in tumor cells, some cancers are perceived to be spontaneous or environmental, and some due to inherited predisposing in uences. The most frequently diagnosed inherited canine cancers are; lymphoma/lymphosarcoma, hemangiosarcoma, mast cell tumor and osteosarcoma. Other lower frequency cancers with genetic predispositions are malignant melanoma, squamous cell carcinoma, mammary tumors, transitional cell carcinoma and histiocytic sarcoma.
Cancer susceptibility genes include tumor suppressor genes involved in cell surveillance (removing “non-self” cells with mutations) and oncogenes that can cause
An Urban Experience
malignant transformation of cells. Many abnormal genes as well as chromosomal anomalies are identi ed in tumor tissues, but most of these  ndings do not correlate
to “cancer causing genes” in the normal (pre-cancer) genotype. Current research has identi ed genetic markers or gene variants that are present at a higher frequency in dogs diagnosed with cancer. Time will tell whether these  ndings will translate to screening tests for genetic cancer susceptibility.
Retained testicles are seen frequently in mixed and purebred dogs with a reported 6.8% prevalence. It can present as unilateral or bilateral. The location of the retained testicle can be abdominal or subcutaneous. Testes should normally be present in the scrotum by 8 weeks of age. Cryptorchidism is a complexly inherited sex-limited trait where both males and females inherit the trait, but only males can express it (similar to milk production in females). Therefore, genetic control must include selective pressure against female close relatives of affected males. Late descending testicles (up to 14 weeks of age) are an expression of the disorder, so this trait should also be selected against.
Hypothyroidism is a frequently reported disorder
of purebred and mixed-breed dogs. It is caused
by an autoimmune thyroiditis where thyroglobulin autoantibodies (TgAA) slowly destroy the thyroid hormone producing tissue. Thyroid screening shows 6.3% of blood samples from mixed and purebred dogs testing positive for TgAA.
At question is what percentage of dogs with measurable TgAA progress to end-stage clinical hypothyroidism, as this has not been documented in any studies. Resting T4 measurements are a poor indicator of primary hypothyroidism, as many dogs with non-thyroid illness have low T4 levels. Compounding this is that there are many thyroid hormone responsive conditions such as; obesity, lethargy and poor hair coat. While autoimmune thyroiditis certainly does exist as a hereditary disease
in dogs, it may be an improperly or over-diagnosed disorder.
Inherited Cataracts
Many dogs develop slowly progressive cataracts as they get older, and it is likely that all spontaneous cataracts have hereditary components. Eye screening programs in Germany have shown a reduction in cataracts in Dachshunds from 8.7% in 1993 to 3.1% in 2006. A testable genetic mutation (HSA1) causes autosomal- recessive early-onset cataracts by 6 months with complete opacity by 2 years of age in the Boston Terrier, Staffordshire Bull Terrier and French Bulldog breeds. A different mutation in the same gene causes autosomal

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